High gear clutch slippage

[Stewart Way]

Coming back from a test session a couple of years ago, a very experienced person commented that he couldnt understand why a 4 speed car would go thru the first 3 gears without clutch slippage and then slip in 4th. I said it was because the car makes more power in 4th than in the first 3 gears. He allowed as how that was the stupidest thing he had heard, the engine doesn't know what gear its.
My feeling is like on a dyno. The slower you bring the engine up at WOT the more power it makes. An engine at 1000rpm/sec test rate will not have as high a HP number as an engine tested at 600 or 300 rpm/sec. I have never seen it done but I would think the highest HP would ne seen at a steady state test. So in 4th gear the engine sees its slowest accel rate due to various resistances and therefore makes the most power.
True or false?
I bring this up because he make the same statement yesterday and I just let it go.

[69Cobra]

I don't know about the dyno thing but in my opinion when you go to high gear the clutch has more of a load on it than being in low gear. The less gear reduction the more stress on the clutch. For example if you have a 3.25 first gear ratio and a 1.94 second gear and you dump the clutch in first gear the clutch is almost out of the equation as far as load or slippage. If you dump the clutch in second gear you will slip the clutch because its seeing more load. At least that's my view on it.

X2 , when I was racing the firebird last year the same thing happened to me. Not enough counter weight, or not enough base pressure.

Originally Posted by 69Cobra

I don't know about the dyno thing but in my opinion when you go to high gear the clutch has more of a load on it than being in low gear. The less gear reduction the more stress on the clutch. For example if you have a 3.25 first gear ratio and a 1.94 second gear and you dump the clutch in first gear the clutch is almost out of the equation as far as load or slippage. If you dump the clutch in second gear you will slip the clutch because its seeing more load. At least that's my view on it.

[goinbroke2]

Nothing to do with engine hp.
It's resistance.
In lower gears there is gear multiplecation and less resistance on the input shaft of the tranny.

Same reason an old 6cyl pick up will go up a hill in 3rd without slippage but in 4th it breaks free. The engine isn't making anymore power, it's just reaching the coefficient of friction. Steep enough hill and it will slip in third too.

It's been well documented that slower rpm/second will increase hp on a dyno because of less torque loss to rotational forces. In other words, it's not eating so much torque to actually spin up the rotating assembly. (big blocks more effected than small blocks)
And while I agree with you, I don't think this is the case here for slippage. Not saying it can't be, I just don't think the clutch is adjusted to just the fine edge, where a couple lb'ft gained by the slower rpm increase would effect it. The only increase in torque would be from the difference of rotational speeds of 3rd and 4th.

I would say the clutch needs to be tuned to stop the slippage. (actually it does regardless, but it's because the resistance applied in 4th is more than the clutch can accomodate)

Where are the clutch guru's? They will be able to explain must more concise than I.

[Stewart Way]

First, I know you can adjust the clutch to stop the slippage in 4th. That wasn't the question, why it slips was. Maybe you guys are right. If you are using the clutch to shift or have a stocker type cluchless, where the engine is unloaded for an instant the load drops out the bottom (as would HP)so my theory may be flawed unless the slippage happens well after the gear change when the engine is climbing. If it happens at the change, it sounds like it could be the difference in resistance due to ratio changes, rolling resistance and aero effects.

But if you make a 1-2 gear change and the clutch has enough friction to hold it why doesn't it have enough friction to hold it on the 3-4 change. Same power applied to the clutch disc and same clamping force on the pressure plate. Goes back to my original theory. If you put more resistance to the engine being able to RPM it will produce more power and overcome the CofF of the clutch. Don't see how the same amount of power will overcome the Cof F in 3-4 shift and not the 1-2 or 2-3.

Here is my thoughts. You have a clutch set that will slip at 600hp. On your 1-2 change because the engine is allowed to climb at whatever rate it produces 550hp max. Then at the 2-3 change it climbs slower and makes 580hp. On the 3-4 change due to all the reasons we have stated the rate is slower yet and it goes above 600. Now you have slippage. Those numbers are likely larger than the differences would be in real life but you see my theory.

Again, my theory is only that and could be swiss cheese.

[magnumv8]

This falls back on the high school theory of "work".....if you put a load on a rope with a block and tackle pulley arrangement (say first gear) and then you(the engine) pull on the rope it is fairly easy to move the load ( the vehicle) .....but if you eliminate all the pulleys (high gear) it removes the mechanical advantage you had putting the full load of what you are trying to move on you (the engine) so it has to "work harder"....

An engine won't produce it's full power until it needs to, so even at full throttle maximum torque won't be achieved until the engine sees a full load where it is required to do the most "work"....most dyno rooms used by the OEM's in the old days where called "load cells" because that is the only way to get a true measure of an engines power....

So to answer your question it is because the engine has to work harder in 4th gear than it does in 1st,2nd, or 3rd is the reason for the clutch slippage....



D L Rambo....

[Stewart Way]

DL
Thats kind of what I was saying. It made more power due to loading but then I did a google of 4th gear clutch slippage and found a writeup under worldphaco.net. At first I thought this kind of shot my theory that the engine makes more power(works harder) in 4th or at least that that is the cause of the slippage. But this writeup uses the theory of CEP (constant engine power) which is not real world. The writer talks about when you accelerate but doing it with constant power. Without additional power how do you accelerate? Not sure his calculations are valid in the real world.

[Jeff Lee]

Originally Posted by Stewart Way

First, I know you can adjust the clutch to stop the slippage in 4th. That wasn't the question, why it slips was. Maybe you guys are right. If you are using the clutch to shift or have a stocker type cluchless, where the engine is unloaded for an instant the load drops out the bottom (as would HP)so my theory may be flawed unless the slippage happens well after the gear change when the engine is climbing. If it happens at the change, it sounds like it could be the difference in resistance due to ratio changes, rolling resistance and aero effects.

But if you make a 1-2 gear change and the clutch has enough friction to hold it why doesn't it have enough friction to hold it on the 3-4 change. Same power applied to the clutch disc and same clamping force on the pressure plate. Goes back to my original theory. If you put more resistance to the engine being able to RPM it will produce more power and overcome the CofF of the clutch. Don't see how the same amount of power will overcome the Cof F in 3-4 shift and not the 1-2 or 2-3.

Here is my thoughts. You have a clutch set that will slip at 600hp. On your 1-2 change because the engine is allowed to climb at whatever rate it produces 550hp max. Then at the 2-3 change it climbs slower and makes 580hp. On the 3-4 change due to all the reasons we have stated the rate is slower yet and it goes above 600. Now you have slippage. Those numbers are likely larger than the differences would be in real life but you see my theory.

Again, my theory is only that and could be swiss cheese.

You are assuming the same RPM drop on each gear change. It is not. Look at your friends transmission gear ratio's. Determine the percentage of RPM drop in every gear. Then you will be able to see why there is such a difference going into 4th as opposed to going to 2nd.

[69Cobra]

Correct. The gear ratio percentage drop is going to have a lot to do with this as well as rpm's. For example if you have a 1.50 3rd vs a 1.20 3rd gear. The 1.50 3rd gear is going to put a lot more stress on the clutch when going into high gear vs the 1.20 3rd gear. Therefor you are going to have to put extreme amounts of base and/or counterweights to keep from driving through the clutch with the 1.50 3rd and this is going to have you chasing your tail on the starting line to find a balance for the starting line and high gear. Hope this helps.

[Stewart Way]

Lets back up. First. we were not testing a clutch car, we were just talking in the toter on the way back from running an automatic. The man who asked the ? has run gassers, SS, NHRA and IHRA Pro Stock over a 50 year span. We know how to adjust a clutch (sort of, we think). If it slips going into high gear we know what to do to stop it. The question was what causes it. Is it more HP, more torque, more resistance at the higher MPH or a combination of them?
With the up hill example, pulling it into high gear drops the rpm putting more torque and likely less hp on the input shaft.
If you look at 3rd vs 4th ratios a closer ratio will get you a higher 4th gear rpm at the change for more hp but less torque at the shift. But the closer ratio gets you a higher MPH at the gear change for more of the resistances. The 1.50 vs 1.20 3rd gear ratio example talks about the added "stress" on the 1.50 vs 1.20 ratio. Where does this come from. The engine. It seems to me that since the clutch is between the engine and trans that any change in ratios causing more "stress" has to happen by the engine producing more power since thats the only thing making power.
DL Rambos pulley example would seem to show that as the resistance increases for what ever reason the power transmitted increases. Get rid of the mechanical advantage of a pulley or gear reduction, it takes more power.
All good thoughts. Thanks and keep them coming.