Downey Long Travel Review
Jul 1, 2008 | 05:35 AM
#41
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Joined: Feb 2004
Posts: 1,352
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From: Plainfield, IL
Hey ripster,
Great post really cleared things up about the bar stiffness and ithe of road uses. Downey sells an econo kit which is 2" rear springs and their 26 mm torsion bar they say not to lift the front beyond 1.5 inches. Would this kit be ok for a desert running situation?
I was reading an article about tbars, and they concluded with something to the effect that a smaller diamer tbar cranked up 1.5" would be very stiff, however a larger diameter bar cranked up 1.5" would not be as stiff, untill it was cranked further around 2". They liken it to a small tire inflated to its max. that would be a rough ride, howeverif you take the same volume of air and put it inside a larger tire, the ride would be much softer. So far ive read that the downey torsion bars are way to stiff etc. Is this because they are
just overcranking these things? Is the tire analogy sufficient to describe the spring actionin a torsion bar?
Great post really cleared things up about the bar stiffness and ithe of road uses. Downey sells an econo kit which is 2" rear springs and their 26 mm torsion bar they say not to lift the front beyond 1.5 inches. Would this kit be ok for a desert running situation?
I was reading an article about tbars, and they concluded with something to the effect that a smaller diamer tbar cranked up 1.5" would be very stiff, however a larger diameter bar cranked up 1.5" would not be as stiff, untill it was cranked further around 2". They liken it to a small tire inflated to its max. that would be a rough ride, howeverif you take the same volume of air and put it inside a larger tire, the ride would be much softer. So far ive read that the downey torsion bars are way to stiff etc. Is this because they are
just overcranking these things? Is the tire analogy sufficient to describe the spring actionin a torsion bar?
Did this article actually run the bars and measure their deflection? What they are claiming is pretty miuch mechanically impossible. The comprarison between tires is not really based in any sort of supportable scientific fact.
You have read that the Downey bars are too stiff because they just plain are. Here's the background. I'll leave out the math for now because it's really concepts we are after here. The math isn't hard but one thing at a time I say.
Springs for the most part work like this with a few exceptions that don't apply to our torsion bars or the vast majority of coil springs. You apply 1 unit for force and you get 1 unit of deflection. On a torsion bar, this force would be torque, or twisting force and it would result in a number of degress of twist. On a coil spring, you apply some force and it gets shorter or longer depending on the application. (shorter for our trucks)
So, with a softer t-bar (this all applies to coils too but we'll stick with t-bars for now), you will get more deflection for a given amount of force. Now where things get confusing is when people think about the fact that you have to turn the adjuster screw more times for a softer t-bar to support the weight of the truck. They equate that with "tighter" which it really is not, it's applying the same load as it would with a stiff bar which is enough to resist the twisting force applied by the a-arm from holding up the weight of the truck. It is true that the softer bar will deflect more under that same weight just as it will deflect more when you hit bumps or drive on uneven surfaces. The point is, for a given load, it simply deflects more. The Downey bars on the flip side, being stiff, simply deflect less. It's really that simple.
The tire analogy is now mixing in pressure which is a function of area and is not a good analogy because if that. Equal force over a larger area (equal pressure in a bigger tire) will hold more weight with out compressing than that amount of force over a smaller area (equal pressure smaller tire). This equates to the spring concept by saying a larger spring diameter made out of a given metal will defelect less under a given load than a small spring diameter of the same metal nder that same load. So, it applies but is not so straight forward and in fact shows that the springs properties are not at all as they appear to have been stated in the article.
Frank
Jul 1, 2008 | 08:35 PM
#42
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Joined: Apr 2008
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From: Los Angeles
Ripster,
Upon reading your response, it seems that you believe along with everyone that the downey tbars are very stiff. can this be corrected by a soft shock (i believe that the soft shock should dampen the quick aggressive movements of a stiff torsion bar)? In a long travel application that the downey and tc utilize, do you think a stock torsion bar would snap in torsional failure?
Another question about the downey setup this time for the rear shocks, downey recomends using an extra soft shock like the MV12 due to the fact that not much goes on back there.
"Fast hard off-roading requires tough high quality shocks up front (Bilsteins whenever possible), and tough ultra-soft shocks in the rear (MV-12’s whenever possible). That’s because your off road vehicle receives 9 times the pounding in the front (heavy engine) as it does in the rear (empty bed). So what happens when you install overly firm springs and/or shocks in the rear, then go out and hit the oh-my-goshes at desert speed? The overly firm rear suspension becomes tightly spring loaded during compression (bottoming), then violently unloads during rebound (suspension extension), sometimes lifting the rear tires off the ground. And then what happens when the rear tires have left the ground? Well, at that point you might as well let go of the steering wheel, you’ve lost your rudder and you’re no longer steering the vehicle! The vehicle can land completely sideways on the road at 20,40, or 60 mph of forward momentum - - absolutely horrifying! Ever wondered why Ivan Stewart says, “I sincerely hope my rear bumper drags every inch of the Baja 1000?” MV-12’s are the worlds best kept secret (insider information). Rick Doetsch designed them for actual desert racers who use them on their absolutely serious pre-runners. Rick does not advertise these shocks, but there’s nothing to prevent Downey from hooking you up with the “real deal” MV-12’s are:
Bigger than most (hold more oil)
Tougher than most (larger 18mm shaft)
They have NYLON SHAFT WIPERS above the oil seals (boots not required). Then they kick every other shock’s ass with ULTRA-SOFT VALVING - - - gentle giants!!!"
Your thoughts on this type of rear suspension?
Upon reading your response, it seems that you believe along with everyone that the downey tbars are very stiff. can this be corrected by a soft shock (i believe that the soft shock should dampen the quick aggressive movements of a stiff torsion bar)? In a long travel application that the downey and tc utilize, do you think a stock torsion bar would snap in torsional failure?
Another question about the downey setup this time for the rear shocks, downey recomends using an extra soft shock like the MV12 due to the fact that not much goes on back there.
"Fast hard off-roading requires tough high quality shocks up front (Bilsteins whenever possible), and tough ultra-soft shocks in the rear (MV-12’s whenever possible). That’s because your off road vehicle receives 9 times the pounding in the front (heavy engine) as it does in the rear (empty bed). So what happens when you install overly firm springs and/or shocks in the rear, then go out and hit the oh-my-goshes at desert speed? The overly firm rear suspension becomes tightly spring loaded during compression (bottoming), then violently unloads during rebound (suspension extension), sometimes lifting the rear tires off the ground. And then what happens when the rear tires have left the ground? Well, at that point you might as well let go of the steering wheel, you’ve lost your rudder and you’re no longer steering the vehicle! The vehicle can land completely sideways on the road at 20,40, or 60 mph of forward momentum - - absolutely horrifying! Ever wondered why Ivan Stewart says, “I sincerely hope my rear bumper drags every inch of the Baja 1000?” MV-12’s are the worlds best kept secret (insider information). Rick Doetsch designed them for actual desert racers who use them on their absolutely serious pre-runners. Rick does not advertise these shocks, but there’s nothing to prevent Downey from hooking you up with the “real deal” MV-12’s are:
Bigger than most (hold more oil)
Tougher than most (larger 18mm shaft)
They have NYLON SHAFT WIPERS above the oil seals (boots not required). Then they kick every other shock’s ass with ULTRA-SOFT VALVING - - - gentle giants!!!"
Your thoughts on this type of rear suspension?
Jul 2, 2008 | 04:57 AM
#43
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Joined: Feb 2004
Posts: 1,352
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From: Plainfield, IL
You can't really "correct" for a stiff spring with a soft shock. Choosing both correctly is important.
Spring force is a function of position, which is pretty intuitive. Viscous damping force, which is what a fluid filled shock provides, is a function of velocity. Pushing a wheel up slowly when crawling will load the spring but because the wheel is moving slowly the shock provides little resistance. Hit a big bump at speed and the forces become very high quickly overcoming the spring. However, because the wheel is being forced up so quickly, the shock provides substantial resistance.
So, when you jump your truck, you actually don't force the wheel up that fast on a typical landing, the spring really has to absorb that force. However, hit a whoop hard or land on the upside of another, now you are forcing the wheels up very quickly and shocks need to damp that motion.
Typically, a softer sprung longer travel with good shock damping is the best set up to keep the chassis settled over rough terrain. Now, how soft is soft? Well that's a matter of suspension tuning. We have so many options from fiber glass fenders to winch/bumper combos that can change the overall weight and distribution thereof significantly so there is no one size fits all answer to what soft is. However, as many have experienced, Downey's 26mm t-bars are very stiff. In fact, since the spring rate increases with diameter at a rate of the diameter raised to the 4th power, it's like 100% stiffer or close. I have to check on that but it's a lot stiff than stock even though it's just a few mm thicker.
The stock torsion bar would not snap right away but it might fatigue over time. I'm not sure but I'll try to calculate that today if I have time. It would be nice to have idea there. BTW, if you are flexing two suspension to their limits and have identical length torsion bars but one has a larger diameter, the one with the larger diameter is experiencing more strain.
As for the rear shocks, this gets a little more complicated. The race truck has very little weight towards the rear. The bed is gutted and what's left is usually not made of metal anymore. It has tall tires that are aired down to absorb impact. The rear springs are now much softer because they don't need to be stiff anymore, there's much less weight to support. So, naturally you want a softer shock. You don't want to go so soft that the rear axle bounces uncontrollably or you'll have more control issues, but you want one that will let the axle travel upwards quickly to absorb impacts that the tires cannot. Since much of your damping is tire related spring forces, and the tires are very soft airded down, you again can get a way with a softer shock.
If you have a regular truck with steel bed that hauls anything ever on streets which hard street tire pressures, you might not want such a soft shock because it won't dampen motion when loaded and will let the tires bounce a lot. That can be dangerous on the road. However, you probably aren't wheeling a truck like that any where near as hard as a race truck so it's all good.
Frank
Spring force is a function of position, which is pretty intuitive. Viscous damping force, which is what a fluid filled shock provides, is a function of velocity. Pushing a wheel up slowly when crawling will load the spring but because the wheel is moving slowly the shock provides little resistance. Hit a big bump at speed and the forces become very high quickly overcoming the spring. However, because the wheel is being forced up so quickly, the shock provides substantial resistance.
So, when you jump your truck, you actually don't force the wheel up that fast on a typical landing, the spring really has to absorb that force. However, hit a whoop hard or land on the upside of another, now you are forcing the wheels up very quickly and shocks need to damp that motion.
Typically, a softer sprung longer travel with good shock damping is the best set up to keep the chassis settled over rough terrain. Now, how soft is soft? Well that's a matter of suspension tuning. We have so many options from fiber glass fenders to winch/bumper combos that can change the overall weight and distribution thereof significantly so there is no one size fits all answer to what soft is. However, as many have experienced, Downey's 26mm t-bars are very stiff. In fact, since the spring rate increases with diameter at a rate of the diameter raised to the 4th power, it's like 100% stiffer or close. I have to check on that but it's a lot stiff than stock even though it's just a few mm thicker.
The stock torsion bar would not snap right away but it might fatigue over time. I'm not sure but I'll try to calculate that today if I have time. It would be nice to have idea there. BTW, if you are flexing two suspension to their limits and have identical length torsion bars but one has a larger diameter, the one with the larger diameter is experiencing more strain.
As for the rear shocks, this gets a little more complicated. The race truck has very little weight towards the rear. The bed is gutted and what's left is usually not made of metal anymore. It has tall tires that are aired down to absorb impact. The rear springs are now much softer because they don't need to be stiff anymore, there's much less weight to support. So, naturally you want a softer shock. You don't want to go so soft that the rear axle bounces uncontrollably or you'll have more control issues, but you want one that will let the axle travel upwards quickly to absorb impacts that the tires cannot. Since much of your damping is tire related spring forces, and the tires are very soft airded down, you again can get a way with a softer shock.
If you have a regular truck with steel bed that hauls anything ever on streets which hard street tire pressures, you might not want such a soft shock because it won't dampen motion when loaded and will let the tires bounce a lot. That can be dangerous on the road. However, you probably aren't wheeling a truck like that any where near as hard as a race truck so it's all good.
Frank
Jul 2, 2008 | 12:52 PM
#44
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Joined: Apr 2008
Posts: 58
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From: Los Angeles
Ripster,
After reading your previous posts about the Tbars and shocks, I am assuming that you probably dont think the downey "pro desert" kit is a good idea Tbars rear springs and doetsch tech pre runner shocks all around. Now as we were saying with the stiffer torsion bars, I would assume that this would be a basic entry level kit for a go fast desert combo, however it would most likely ruin my on-road ride quality. Correct?
After reading your previous posts about the Tbars and shocks, I am assuming that you probably dont think the downey "pro desert" kit is a good idea Tbars rear springs and doetsch tech pre runner shocks all around. Now as we were saying with the stiffer torsion bars, I would assume that this would be a basic entry level kit for a go fast desert combo, however it would most likely ruin my on-road ride quality. Correct?
Jul 3, 2008 | 07:14 PM
#45
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Joined: Aug 2005
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From: Philly PA
These are some newbie questions,
1) Questions on the shock system: Does the remote reservoir provide any sort of extra dampening? If its there to dissipate heat, how hot do these things get? Is it possible to run a remote reservoir on something like this (if needed)?
2) Ewong, you pointed out an interesting point that these kits are a way to use up old stock. however even if its just using up old stock of rancho arms, what do you think about the additions? (megatravel ball joints, and retaining stock steering?) I think these additions keep the truck easier maintenance for a daily driver. (greasable zerk fittings and suppsely "highly perishable heim steerig?).
3) I was reading an article regarding torsion bars, and apparently there are progressive rate torsion bars i guess by using different cross sections throughout the length of the bar. have any of you guys seen something like this for our yotas?
4) The total chaos gen 1 kit uses torsion bars. So I called Total Chaos to see what they thought about the downey kit. They say that there kit rides so much better than downey and that there is no comparison. hence the name "caddy kit." However, I wonder if this kit is using torsion bars, wouldnt they ride similar? Assuming that a 2.5 remote reservoir shock is valved the same as the bilstein. (Can this be done?) neglecting heat dissipation, would they ride the same?
5) Yota82, I appreciate your input alot because you seem to have alot of experience with downey parts being that your father was sponsored by them. What do you think about Ewongs theory of the recycled arms and axles?
Thank you again to all who are reading this, I believe all your opinions and presentation of facts have really enlightened this thread.
1) Questions on the shock system: Does the remote reservoir provide any sort of extra dampening? If its there to dissipate heat, how hot do these things get? Is it possible to run a remote reservoir on something like this (if needed)?
2) Ewong, you pointed out an interesting point that these kits are a way to use up old stock. however even if its just using up old stock of rancho arms, what do you think about the additions? (megatravel ball joints, and retaining stock steering?) I think these additions keep the truck easier maintenance for a daily driver. (greasable zerk fittings and suppsely "highly perishable heim steerig?).
3) I was reading an article regarding torsion bars, and apparently there are progressive rate torsion bars i guess by using different cross sections throughout the length of the bar. have any of you guys seen something like this for our yotas?
4) The total chaos gen 1 kit uses torsion bars. So I called Total Chaos to see what they thought about the downey kit. They say that there kit rides so much better than downey and that there is no comparison. hence the name "caddy kit." However, I wonder if this kit is using torsion bars, wouldnt they ride similar? Assuming that a 2.5 remote reservoir shock is valved the same as the bilstein. (Can this be done?) neglecting heat dissipation, would they ride the same?
5) Yota82, I appreciate your input alot because you seem to have alot of experience with downey parts being that your father was sponsored by them. What do you think about Ewongs theory of the recycled arms and axles?
Thank you again to all who are reading this, I believe all your opinions and presentation of facts have really enlightened this thread.
a - allows more oil in a given length shock can by allowing the can to be longer "remotely"
b - potentially allows more compression range in a given size shock can by moving the mono tube oil nitrogen divider into the remote res
By having extra oil - you gain thermal mass - thus better heat capacity.
The remote res also in theory allowed better cooling.
Ive never seen it (I live on the EAST coast) but I have heard that dez racer shocks are hot enough to "brand" ya if you get too close post race...
2) I have no idea if ball joints are even a liming factor on the Toyota IFS - because I haven't cycled the suspension - but if you want to run "ball joints" instead of uniballs because of street use concerns - IMHO run the TC gen1 kit or fab up arms and run a KORE or CONE ball joint.
http://www.autofab.com/coneball.htm
BTW - the REASON that the TC kits and other use a longer A arm is that the longer A arm maintains STOCK angularity on the OEM ball joint... If you dont believe me then draw it out (geometry 101).
As far as greaseable a arms busings... well - if you want low maintenance - the OEM vucanized rubber has ALOT going for it
3) back in the day SAW used to make a two stage torsion bar. It was achieve by running a "sleeve" torsion and a tab that engaged the outer sleeve....
I havent seen em in ages.
BTW - I run super stiff 300M torsions.
My 4Runner SUCKS at rock crawling..not that I care
4) the ride is less depending on spring "type" (torsions or coil over) and MUCH MORE on suspension geometry...
For a given torsion - a LONGER lever arm will have more travel to engage the same amount of spring rate... thus the reasont he TC rides "softer" than Downey.
Shocks are really - as the Brits know - dampers and not "shock absorbers".
They can "fine tune" a suspension for a givne spring rate - but they should not be used to compensate for incorrectly spec'd spring rates
5) I was not claiming the arms are "recycled". The mid/late 90's Downey catalog CLEARLY stated that their "kit" was a combo of Rancho parts (A arm) and Downy parts (axles)
Jul 3, 2008 | 07:25 PM
#46
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Joined: Feb 2004
Posts: 1,352
Likes: 3
From: Plainfield, IL
Ripster,
After reading your previous posts about the Tbars and shocks, I am assuming that you probably dont think the downey "pro desert" kit is a good idea Tbars rear springs and doetsch tech pre runner shocks all around. Now as we were saying with the stiffer torsion bars, I would assume that this would be a basic entry level kit for a go fast desert combo, however it would most likely ruin my on-road ride quality. Correct?
After reading your previous posts about the Tbars and shocks, I am assuming that you probably dont think the downey "pro desert" kit is a good idea Tbars rear springs and doetsch tech pre runner shocks all around. Now as we were saying with the stiffer torsion bars, I would assume that this would be a basic entry level kit for a go fast desert combo, however it would most likely ruin my on-road ride quality. Correct?
I think their kit will actually do very well in the desert. I think it will do Ok crawling. I think if you kept your stock bars it would do much better crawling but still do rather well (maybe not as good as with their stiff bars) in the desert. Either way, your truck is going to perform better off road with it than without. Now, is it better than Total Chao or JD fab? Well you'd need to run those kits to compare and so far it doesn't sound like anyone one has run the Downey kit and one of the other two. Wanna be the first?
Frank
Jul 3, 2008 | 07:32 PM
#47
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Joined: Feb 2004
Posts: 1,352
Likes: 3
From: Plainfield, IL
Ok, here is what scares me about Downey. I copied this from their site.
he stock IFS front torsion bars are 22.8mm, which is adequate for stock 28” tires and wheels, or for normal highway use with heavier tires and wheels. Install a set of 31” tires (or larger) then go off road with your 22.8mm torsion bars and see what happens. The heavier the tire, the worse your off road experience will be.
* Downey’s 26mm Torsion Bars will eliminate most of the hard bottoming in off road terrain.
* Downey bars will hold your cranked-in lift without fatiguing.
* Downey bars will give a softer ride when lift is cranked-in because your stock bars become radically pre-loaded when cranked-in, while Downey’s 26mm bars are relatively relaxed at any ride height.
Referencing my previous post, what I have put in bold is absolutely false. I'm an engineer and that is just plain ridiculous. First off, stock bars hold the cranked in lift without fatiguing just fine (didn't bold that). Their stiffer bars are just plain stiffer. The preload is from the weight of the truck which is unchanged when you swap bars so their bars are loaded just as much as stock. They just flex less.
I would feel much more comfortable with Downey products if they hired an engineer to design their stuff and didn't make completely false statements like that. There are reasons to get stiffer bars which are completely legitimate but those are not some of them.
frank
he stock IFS front torsion bars are 22.8mm, which is adequate for stock 28” tires and wheels, or for normal highway use with heavier tires and wheels. Install a set of 31” tires (or larger) then go off road with your 22.8mm torsion bars and see what happens. The heavier the tire, the worse your off road experience will be.
* Downey’s 26mm Torsion Bars will eliminate most of the hard bottoming in off road terrain.
* Downey bars will hold your cranked-in lift without fatiguing.
* Downey bars will give a softer ride when lift is cranked-in because your stock bars become radically pre-loaded when cranked-in, while Downey’s 26mm bars are relatively relaxed at any ride height.
Referencing my previous post, what I have put in bold is absolutely false. I'm an engineer and that is just plain ridiculous. First off, stock bars hold the cranked in lift without fatiguing just fine (didn't bold that). Their stiffer bars are just plain stiffer. The preload is from the weight of the truck which is unchanged when you swap bars so their bars are loaded just as much as stock. They just flex less.
I would feel much more comfortable with Downey products if they hired an engineer to design their stuff and didn't make completely false statements like that. There are reasons to get stiffer bars which are completely legitimate but those are not some of them.
frank
Jul 5, 2008 | 08:04 AM
#48
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Joined: Aug 2005
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From: Philly PA
Ref Elripster's comments on Downey's ad copy...
This is NOT a disagreement with ElRipster!
I'm just pointing out how the "marketing" types might "justify" what they say...
A thicker torsion bar is in THEORY a "higher spring rate"
(Since spring rate is also dependent on material as well as length and diameter - the thicker = stiffer is a generalization and not a statement of fact)
A stock torsion bar on a stock truck has a given "ride" feeling as well as a baseline "lift".
Now we change things
-- go to a taller and heavier tire (stock 28" 225/75R15 to 31" 31x10.5)
-- add a stiffer torsion bar
-- Adjust the preload to a given RIDE HEIGHT of 1.25"
'lift" from OEM
So - if the STOCK bar and the STIFFER bar are just installed and set to an OEM ride height - the stiffer springs are - no duh - stiffer...
Now we start fiddling with the pre-load...
What this does (from a geometry point) is re-indexes the spline relationship between the ends of the bar.
So as we rotate the bar - the truck is lifted...
At issue - is that the amount of turning in degrees of rotation to the lift achieved....
A stiffer bar will respond with more "lift" for a given amount of pre-load as measured in degrees of twist at the adjusting end of the bar.
So - to get the same amount of "lift" - a softer bar will require more "rotation" than stiffer bar - the adjust bolt will have to be turned more.
To the uninitiated - more "adjusting" is not as good as "less adjusting".
Torsion bars work the SAME as coil springs do - as the wheel travels - the suspension geometry will twist the bars - they get stiffer (resist the twist). Stiffer bars by definition - resist twist at a higher rate than stock bars...
So - as ElRipster points out - to claim that a stiffer bar is "softer" is a load of hogwash.
What is "true" is that at a given amount of pre-load as measured as amount of threads adjusted - (which BTW means that the pre load IS NOT the same - DOH) the stiffer bar will have more "lift" - doh.
So - if I install stiffer bars and DO NOT PRELOAD THEM as much as stock bars I will have MORE ride height - no duh - they are stiffer...
Now I compare this another truck with stock bars.
I OVER CRANK the pre load...
What this means is that I compress the torsion bar twist against the upper top out snubber...
This takes up the "twist" of the bar....
Now spring rate on torsion bars can be thought of as "weight" against "twist".
BUT
We can also think of spring rate as wheel_travel_position (with zero being full droop) to resistance_force_expressed_as_weight
(i.e. how a coil spring works)
When we preload - we "take away" the initial values and move up the scale.
Lets look at an example.
Coil spring with 10" travel and a rate of 500 lbs/inch
so at 1" of compression - the spring holds up 500 lbs
at 2" of compression - the spring holds up 1000 lbs
What pre loading does is move us into the higher WEIGHT RESISTANCE range of the spring.
So - lets take a comparison.
Lets take a 500 lb/inch spring and NOT preload it
Lets compare that to "softer" 300 lb/inch spring BUT we preload that spring by oh 2" of compression...
And for yucks - lets compare to a to "softer" 300 lb/inch spring BUT we preload that spring by 3" of compression...
at "rest"
the 500 lb/inch spring (0") holds up 500 lbs
the 300 lb/inch spring (2") holds up 600 lbs
the 300 lb/inch spring (3") holds up 600 lbs
at one inch of compression
the 500 lb/inch spring (0") holds up 1000 lbs
the 300 lb/inch spring (2") holds up 0900 lbs
the 300 lb/inch spring (3") holds up 1200 lbs
so - which spring rides "softer" - the 500lb spring or the 300lb spring preloaded by 3"?????
So - what the marketing guys are saying is that a STIFFER bar has a lower weight holding capacity at a given ride height compared to a softer spring that has been over pre-loaded... gee really?
Its a stupid comparison..
This is NOT a disagreement with ElRipster!
I'm just pointing out how the "marketing" types might "justify" what they say...
A thicker torsion bar is in THEORY a "higher spring rate"
(Since spring rate is also dependent on material as well as length and diameter - the thicker = stiffer is a generalization and not a statement of fact)
A stock torsion bar on a stock truck has a given "ride" feeling as well as a baseline "lift".
Now we change things
-- go to a taller and heavier tire (stock 28" 225/75R15 to 31" 31x10.5)
-- add a stiffer torsion bar
-- Adjust the preload to a given RIDE HEIGHT of 1.25"
'lift" from OEM
So - if the STOCK bar and the STIFFER bar are just installed and set to an OEM ride height - the stiffer springs are - no duh - stiffer...
Now we start fiddling with the pre-load...
What this does (from a geometry point) is re-indexes the spline relationship between the ends of the bar.
So as we rotate the bar - the truck is lifted...
At issue - is that the amount of turning in degrees of rotation to the lift achieved....
A stiffer bar will respond with more "lift" for a given amount of pre-load as measured in degrees of twist at the adjusting end of the bar.
So - to get the same amount of "lift" - a softer bar will require more "rotation" than stiffer bar - the adjust bolt will have to be turned more.
To the uninitiated - more "adjusting" is not as good as "less adjusting".
Torsion bars work the SAME as coil springs do - as the wheel travels - the suspension geometry will twist the bars - they get stiffer (resist the twist). Stiffer bars by definition - resist twist at a higher rate than stock bars...
So - as ElRipster points out - to claim that a stiffer bar is "softer" is a load of hogwash.
What is "true" is that at a given amount of pre-load as measured as amount of threads adjusted - (which BTW means that the pre load IS NOT the same - DOH) the stiffer bar will have more "lift" - doh.
So - if I install stiffer bars and DO NOT PRELOAD THEM as much as stock bars I will have MORE ride height - no duh - they are stiffer...
Now I compare this another truck with stock bars.
I OVER CRANK the pre load...
What this means is that I compress the torsion bar twist against the upper top out snubber...
This takes up the "twist" of the bar....
Now spring rate on torsion bars can be thought of as "weight" against "twist".
BUT
We can also think of spring rate as wheel_travel_position (with zero being full droop) to resistance_force_expressed_as_weight
(i.e. how a coil spring works)
When we preload - we "take away" the initial values and move up the scale.
Lets look at an example.
Coil spring with 10" travel and a rate of 500 lbs/inch
so at 1" of compression - the spring holds up 500 lbs
at 2" of compression - the spring holds up 1000 lbs
What pre loading does is move us into the higher WEIGHT RESISTANCE range of the spring.
So - lets take a comparison.
Lets take a 500 lb/inch spring and NOT preload it
Lets compare that to "softer" 300 lb/inch spring BUT we preload that spring by oh 2" of compression...
And for yucks - lets compare to a to "softer" 300 lb/inch spring BUT we preload that spring by 3" of compression...
at "rest"
the 500 lb/inch spring (0") holds up 500 lbs
the 300 lb/inch spring (2") holds up 600 lbs
the 300 lb/inch spring (3") holds up 600 lbs
at one inch of compression
the 500 lb/inch spring (0") holds up 1000 lbs
the 300 lb/inch spring (2") holds up 0900 lbs
the 300 lb/inch spring (3") holds up 1200 lbs
so - which spring rides "softer" - the 500lb spring or the 300lb spring preloaded by 3"?????
So - what the marketing guys are saying is that a STIFFER bar has a lower weight holding capacity at a given ride height compared to a softer spring that has been over pre-loaded... gee really?
Its a stupid comparison..
Jul 6, 2008 | 04:39 AM
#49
Registered User
Joined: Feb 2004
Posts: 1,352
Likes: 3
From: Plainfield, IL
Ewong,
Regarding lift, the softer bar will require the same amount of rotation. It will require more adjusting to attain a given baseline ride height but after that it's just as simple as rotate the bar some rotate the a-arm some. Also, remember that preload goes down as you increase the ride height not up.
Therefore, a given amount it preload is in how many threads have been used up adjusting. It's the weight the the truck trying to rotate the upper a-arm.
This ties into the end of your post where you ask the big questions, which rides softer, the soft spring at preload or stiffer spring at what is actually the same preload (truck doesn't get heavier/lighter with new/old torsion bars). The answer is the soft bar. The problem with the comparison at the end if your post is you list different preload numbers when infact they are the same number.
What you have shown though is that if you hit the soft spring with 400lb of force Vs. and hard spring, regardless of where the spring is in travel, the soft spring will flex more and that is what makes for a softer ride.
Frank
Regarding lift, the softer bar will require the same amount of rotation. It will require more adjusting to attain a given baseline ride height but after that it's just as simple as rotate the bar some rotate the a-arm some. Also, remember that preload goes down as you increase the ride height not up.
Therefore, a given amount it preload is in how many threads have been used up adjusting. It's the weight the the truck trying to rotate the upper a-arm.
This ties into the end of your post where you ask the big questions, which rides softer, the soft spring at preload or stiffer spring at what is actually the same preload (truck doesn't get heavier/lighter with new/old torsion bars). The answer is the soft bar. The problem with the comparison at the end if your post is you list different preload numbers when infact they are the same number.
What you have shown though is that if you hit the soft spring with 400lb of force Vs. and hard spring, regardless of where the spring is in travel, the soft spring will flex more and that is what makes for a softer ride.
Frank
Jul 8, 2008 | 11:01 PM
#50
Thread Starter
Registered User
Joined: Apr 2008
Posts: 58
Likes: 0
From: Los Angeles
So I went back through my old strength of materials courses 
and found some equations for you guys please let me knowif my assumptions are incorrect:
A couple torsion and angle of twist formulae
Shearing stress:
S=Tc/J eq 1
T=torque (moment)
c = distance from centroid to extreme stress fiber of cross section
J = Poar moment of inertia
Angle of twist
for lack of a theta, ill use d = torsional displacement
d = TL/JG eq 2
T = Torque (moment)
L = length of shaft
J = Polar moment of inertia
G = modulus of rigidity (stiffness of material)
Lets assume that the lengths of all different bars are the same since they all fit on the same truck. We can assume that if we are riding the same course, with the same truck, the T experienced by the trucks at an point shoud be the same. I guess we dont know about the modulus of rigidity, since nothing was advertised for downey bars or stock bars. so I guess I will assume that the G for the stock bar and the G for a downey bar is the same.
assuming we go over a bump fast, for a comfy ride, we would like the displacement to be rather large? (please correct this assumption if this large displacement would be better)
By looking at eqn 2, We will see that if we want to analyze diplacement with the same torque and we use a larger diameter bar like downey (assuming T,L,G constants) we will receive less displacement. This does go against downey's advertising and and I agree that there marketing statement is misleading. (larger bar, less preload, softer ride statement).
However as far as fatigue, fatigue occurs when a material is repeatedly stressed we can agree that these torsion bars are constantly in motion twisting though the rotation is small, they will continuously rotate especially in an off road situation. you can see that the stress level of a larger bar given the same torque should be less than that of a narrow bar.
In conclusion, I have deduced that the larger bar given similar torque will displace less (regardless of preload) however, it will fatigue less than a smaller diameter bar.
Your thoughts?
and found some equations for you guys please let me knowif my assumptions are incorrect:A couple torsion and angle of twist formulae
Shearing stress:
S=Tc/J eq 1
T=torque (moment)
c = distance from centroid to extreme stress fiber of cross section
J = Poar moment of inertia
Angle of twist
for lack of a theta, ill use d = torsional displacement
d = TL/JG eq 2
T = Torque (moment)
L = length of shaft
J = Polar moment of inertia
G = modulus of rigidity (stiffness of material)
Lets assume that the lengths of all different bars are the same since they all fit on the same truck. We can assume that if we are riding the same course, with the same truck, the T experienced by the trucks at an point shoud be the same. I guess we dont know about the modulus of rigidity, since nothing was advertised for downey bars or stock bars. so I guess I will assume that the G for the stock bar and the G for a downey bar is the same.
assuming we go over a bump fast, for a comfy ride, we would like the displacement to be rather large? (please correct this assumption if this large displacement would be better)
By looking at eqn 2, We will see that if we want to analyze diplacement with the same torque and we use a larger diameter bar like downey (assuming T,L,G constants) we will receive less displacement. This does go against downey's advertising and and I agree that there marketing statement is misleading. (larger bar, less preload, softer ride statement).
However as far as fatigue, fatigue occurs when a material is repeatedly stressed we can agree that these torsion bars are constantly in motion twisting though the rotation is small, they will continuously rotate especially in an off road situation. you can see that the stress level of a larger bar given the same torque should be less than that of a narrow bar.
In conclusion, I have deduced that the larger bar given similar torque will displace less (regardless of preload) however, it will fatigue less than a smaller diameter bar.
Your thoughts?
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