The physics of CV breakage
 

I've got a general question about breaking CVs and/or manual hub clutches. I have read specs before that it requires a certain amount of torque on the CV to break it, and I think these specs were generated by a company that made self-destructing hub clutches.

Can you experienced wheelers tell me exactly HOW a CV breaks? Is it due to all of the engine torque being applied to a wheel while it is in a flexed position? Or is it due to the torque applied on the CV when a tire hits something? Reason I ask is I am trying to develop a better sort of self-destructing hub to save the life of CVs. Don't argue practicalities with me :hillbill: because I know it's one of those things that might not even be worth doing.

So, physics of CV breakage. Ready? Go.

CV's are weakest when you are at full lock. You do have to apply power to break a CV. Things like a ledge that is undercut, where your tire has to move back before going up can break one. If you bounce in the air with a tire spinning and come down with torque still applied, you will probably break a CV. Excessive wheel spin will tend to cause premature death.

This sums it up.

yeah, nice job heath.

the funny thing is, i wheeled the crap out of my truck at paragon and all i did was poke a hole in a boot :laugh:

yep, turning a wheel to full lock and applying power is about the fastest way to break a CV, or a birfield for that matter.

depends on how much you have your rig built too. For most of us, the above mentioned will break the CVs. I have seen a truck grenade a cv with the tire straight, and climbing a small rock, but that was because the truck had 5.29's, 10.71 in the tcases, and a 3.4 conversion. Way too much torque for those axles.

If you do decide to built this "self destructing" hub, be careful how you do it. You can end up damaging the axle anyway, if you aren't careful.

Yeah, but that is also helped by the fact that when I was spotting you I was keeping an eye on those cv's. I never ran you up against a ledge with your tires turned all the way unless I had absolutely no choice, remember me always saying, "turn them back in a little, there now go" and even then I kept yelling, "EASY, EASY!" :laugh:

My idea is to have something inside the hub that will release the hub from the axle at a certain torque "breaking point." Here's what the difficulty may be...

When the tires are straight it may take 100 units of torque to bust that birfield. But when the tires are turned fully it may only take 10 units. However, when you're just accelerating in a straight line, your engine may provide 50 units or torque at most, so if the breaking point was set to 10 units, you could set it off just by accelerating moderately when your wheels are not turned.

Anyone see a way around this? Am I totally off track?

This is meant to be heard as constructive. :grouphug:

What if what you are trying to design has already been designed into the system by Toyota? Meaning, what if Toyota or others could improve the strength of the CV but chose not to, knowing that other parts would fail instead.

Even though a blown CV is a problem, it can be fixed on the trail. If the diff or other major components failed, the truck might really be dead in the water. Worse yet, other failures might lead to control issues or bad road manners. One can get home with that snapped CV.

That said, back to your idea. Although you have some things to figure out, I believe you would only be onto something if the part you design to fail is easier to replace or repair trailside than a CV.

What about a ratchet clutch of sorts. One could set the torque limit as they saw fit. When it "failed" the loud popping or ratcheting would tell the driver to re-think the line. Clutch would reset with power off, and one could try again.

Just some ideas. :great:

Yep, a ratcheting clutch is precisely what I had in mind. It would simply break free and freewheel when too much torque was applied.

But back to my other question - if the torque limit was set low enough to save a CV when it was at full articulation, would it be too low to even accelerate in 4WD? I suppose this might only be a testable thing.

I like your idea of being able to set the torque limit.

I also understand fully that the CV joint is the weakest link. The idea is the make an even weaker link that is reusable.

I'll put my thinking toque on (to borrow a phrase from Bob & Doug MacKenzie) and see what I can come up with.

In the realm of brainstorming....

Why not move your idea from the hub(s) to the front driveshaft. Probably at the transfer case end. Easier to get to, maintain, repair. Now, take this "torque clutch" and give it a bit of a brain, make it electronic.

This could now monitor 4H or 4L and allow torque to change accordingly. Going a step further, allow the "torque clutch" to understand how far the steering is left or right and adjust according to those loads.

Ta-Da!!! You'd never have to replace a CV again and could try hard and tricky lines all day with confidence.

Now, this system could work on any vehicle, Toy IFS, Toy straight, Dana... what ever, and just be told what the limits of the vehicle it's installed on are going to be.

Oh yeah, one would have to design a failsafe system for when the clutch released that it could re-engage quickly with the power off so the vehicle wouldn't loose all it's braking force and sail backwards down the hill. :eeek: And maybe add a rev limiter that would kick in- when the shaft failed so things didn't rev out of control.

Both especially true if you ran this "torque clutch" on both front and rear driveshafts.

:roll: :roll: :roll:

Think about a solid axle, everything is kept straight in alignment. Things tend to break less that way. Of course any moving parts like a u-joint or birf breaks easier. Less angle+motion=less breakage. The more leverage and the more torque also makes things easier to break.

For the driveline idea, keep in mind that the torque is multiplied again by the gears in the diff.

If you've got the thing set to ratchet at the torque just below the breaking point of a turned cv, you'll be fine on the road. even straight. You have to be really pushing on that thing.

I'm all for pushing new ideas, and getting brand new stuff out there, but i honestly think this is a whole lot of time energy, and money spent on something that really isn't going to buy you a whole lot.

Here's why. I have 4.88's, a crawler, and 33's. I wheel a bunch. I'm locked up front, and i have never ever had a problem with a CV. I have destroyed my steering a couple times, but never a cv. I keep a spare with me, and i have the stud's removed. A couple of things help this. Probably the biggest is that my engine is putting out like 100 horses, and not a whole lot of torque. THe other is that i have big ol bumpstops. I realized that IFS isn't going to flex all over, and by keeping my CV's at pretty small angles, i keep them intact. I also know not to apply full throttle at full lock. But i never back off a line because i'm worried about an axle, only when i'm worried about the steering.

Figuring out how to do it is more fun than knowing it's useful...

I really like the idea of a driveshaft-mounted "torque limiter." It seems like two gears much like the side gear of a differential and a half of a lock-rite (would those be 'radial splines'?) with a compressive force on one side would do the trick. Perhaps behind the spring would be a hydraulic cylinder somehow tied to the functioning of the steering system, such that when the steering system has more pressure (is there more pressure in the hyrdaulic lines when the wheel is turned, or just turning?) it would compress the spring less. Then there would be less friction for one of the gears to overcome, and it would jump out of its current teeth and ratchet until the torque decreased.

The cool thing about this system is that it is like a ball & spring pressure release valve, it breaks the seal when its overloaded, but as soon as the pressure it back to normal, it continues to do its job. It wouldn't require having to get out of the truck and reset something, and it actually might not even screw up an approach if the torque limit was exceeded. For instance, you're climbing a rough hill and a tire hops over a large rock under power, the torque limiter ratchets a bit, which gives the tire about 30 degrees of rotation unlocked from the CV, then the ratchet binds again and you're back to normal.

That makes me realize, however, that a driveshaft-mounted torque-limiter might not be as desireable as one on each wheel. For two reasons: (1) it's not simply torque from the engine that causes CVs to bust, and a torque applied on the tire would still have to travel through the whole driveline before the limiter absorbed it in the driveshaft. (2) If one wheel exceeded the limit, you would lose motion in both front wheels while it is ratcheting. It would be better to have each wheel do its own thing.

Let's hear some more ideas - if I patent this thing and make a killing (which I probably wouldn't, even if it worked), I'll share the weath and maybe do a monthly SAS-my-ride giveaway :bigok:

... (why don't we do that already?)

Doesn't one of the manual hub companies already make something like this? I thought someone was talking about installing them. Not sure, but you might be able to use the idea towards the aisin hubs.

Warn manual hubs has a fuse in their setup. I don't know much about it, but it's supposed to be the weak link and a quick, easy fix.

Wow. What a complex, expensive "solution" to a (at worst) $70 problem - that's what CV's for 1st and 2nd gen trucks cost with a lifetime warranty (after core return).

Warn USED to have a special splined insert called the hub fuse that was supposed to break before the CV or axle. (Everyone talks about our CV's, but the Toy front end is stronger than a Dana 30 SFA in my experience) Guess what - they don't make them anymore 'cuz they couldnt make them work. The difference in breaking strength from when the wheels are straight versus when they are turned and articulated is too much. If you set it to fail when the wheels are turned, it will fail too easy when you're going straight.

The other thing to consider is that breaking a CV on a 1st or 2nd gen is not that big a deal because you can drive the truck without them (which is not true on Taco's and 3rd/4th gen 4Runners) if you have to.

Put on manual hubs, replace the studs with bolts, carry a spare CV, and don't 'wheel like an idiot and you'll be fine.

Dude... I understand that breaking a CV is not that big of a deal. But consider IF there was a safety mechanism, such that it would reliably preserve the life of your CVs and nothing would have to be replaced (like a fuse) when it does its thing. Wouldn't you pay like, $100 for it? Especially if it was a drop-in mod to an Aisin hub?

No.

'Cuz I have Warns. :lol:

(Actually, the Warns, even without the fuse, seem to take care of it pretty well)

Well when I patent it I won't send you a brochure :)

Making things break is almost always a waste in my mind.

I have trashed my steering and 4 CV's.

Swapping one on the trail became a half hour ordeal in good conditions.

If you have a hub "fused" in theory rather than as Warn intends, it will then need to break before the CV. This means your truck is now weaker because it breaks sooner.

I wheeled with a Tundra that had the fuses. After 3 in one trail, the novelty had worn out.

Make your truck as strong as possible for all conditions. When it breaks, it breaks.

Mine were breaking because of too much droop, gearing, HP, crawl-ability and crappy design. Not so much Physics as using a part beyond design parameters.