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MotoIQ - Project Budget 400whp S197 Mustang Track Car

Norm Peterson

Corner Barstool Sitter
831
616
Exp. Type
HPDE
Exp. Level
5-10 Years
a few miles east of Philly
What I think I'm getting is that the Torsen/TrueTrac not only allows greater total drive traction, but distributes it in a manner that introduces an oversteerish component of yaw moment (the Trac-loc being somewhat insistent on making that an understeer yaw moment term). From that, I sort of get that you probably wouldn't want as much rear bar as you might with a Trac-loc, or alternatively that you might want relatively more front bar. Or both.

What I can't seem to figure out is what happens during trail-braking, particularly with respect to whether braking torques at the wheels tries to make these devices oppose the free differentiation that you'd get on low/neutral/off-throttle cornering.


Norm
 
Even under actual braking? That's the part I'm having difficulty with.


Norm
Even under braking.

And torsens don't introduce more oversteering yaw moment than traction loks. That thought (as well as the thought of torsens torque vectoring) are somewhat related to each other and both are not how torsens function.

A 3.0:1 TBR does not mean the outside tire is being driven 3X as fast. Think of it as the inside tires grip determines the overall grip and acceleration of the car.

If the inside tire can generate 0.3 Gs of acceleration before slipping, the outside tire will be sent 3X that amount of torque, or 0.9 GS (assuming the tire has that grip). For simiplicitys sake and the sake of understanding (at the cost of some accuracy) assume the car accelerates at 0.9 GS because the outside tire is the most heavily loaded tire and determines the majority of the cars acceleration.

Now let's say you're in the rain, and the inside tire only has 0.1 Gs of grip. The Torsen will only deliver 0.3 Gs of grip before the inside tire lights up and spins. So due to the overall decrease in grip, limited by the inside tires total tractive force, the car in this occasion will only accelerate at 0.3 Gs before lighting the inside tire up with wheel spin. The car accelerates at 1/3 the rate as the first example because the inside tires grip was reduced to 1/3.
 
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Billy, regarding the bushing choice, you said this..."The rear bushing appears to be identical between the old and new arm. Its very common to change this bushing to a smaller diameter and Delrin or polyurethane bushing to reduce unwanted alignment changes when cornering. This is a big problem on old, worn-out bushings but since our car is a dual purpose street/track car, we didn’t want the increased Noise Vibration & Harshness (NVH) of a stiff bushing and decided to use a new large diameter OEM style bushing because they hold up quite well and don’t affect the geometry much when new."

To get the RC improvement, the 302S control arm (M-3075-RA) w/extended ball joint would do the same but the FR bushing kit (M-5638-C) has cautionary words about increased NVH. Would the stock bushing you used on the project work with the 302S arm (without compromising location and intended geometry improvement)? Thx
 

Norm Peterson

Corner Barstool Sitter
831
616
Exp. Type
HPDE
Exp. Level
5-10 Years
a few miles east of Philly
Yes, because the change in torque is coming from the axle side of the diff.
Thanks.

It's beginning to make some sense, without me having full understanding quite yet. IOW, it's not yet my own "own knowledge" if that makes any sense.

Maybe I'm looking for an overrunning clutch analogy here?


Norm
 

Norm Peterson

Corner Barstool Sitter
831
616
Exp. Type
HPDE
Exp. Level
5-10 Years
a few miles east of Philly
Even under braking.
I think I'm beginning to get this side figured out.

And torsens don't introduce more oversteering yaw moment than traction loks. That thought (as well as the thought of torsens torque vectoring) are somewhat related to each other and both are not how torsens function.

A 3.0:1 TBR does not mean the outside tire is being driven 3X as fast. Think of it as the inside tires grip determines the overall grip and acceleration of the car.

If the inside tire can generate 0.3 Gs of acceleration before slipping, the outside tire will be sent 3X that amount of torque, or 0.9 GS (assuming the tire has that grip). For simiplicitys sake and the sake of understanding (at the cost of some accuracy) assume the car accelerates at 0.9 GS because the outside tire is the most heavily loaded tire and determines the majority of the cars acceleration.

Now let's say you're in the rain, and the inside tire only has 0.1 Gs of grip. The Torsen will only deliver 0.3 Gs of grip before the inside tire lights up and spins. So due to the overall decrease in grip, limited by the inside tires total tractive force, the car in this occasion will only accelerate at 0.3 Gs before lighting the inside tire up with wheel spin. The car accelerates at 1/3 the rate as the first example because the inside tires grip was reduced to 1/3.
I'm not looking at straight line behavior here, and I get that a torque bias ratio isn't an rpm thing.

What I'm missing is what happens to the unequal torque distribution assuming that you're getting that while cornering under some amount of throttle, let's say at least 'leading throttle'.

Does the fact that total torque is coming from the driveshaft essentially lock the differential up?

If the diff doesn't lock up, what happens to the unequal traction forces and why wouldn't they add a yaw moment in similar fashion to how a dragging rear brake in a car with an open diff might?

Or maybe for this I belong on the short bus.


Norm
 
Billy, regarding the bushing choice, you said this..."The rear bushing appears to be identical between the old and new arm. Its very common to change this bushing to a smaller diameter and Delrin or polyurethane bushing to reduce unwanted alignment changes when cornering. This is a big problem on old, worn-out bushings but since our car is a dual purpose street/track car, we didn’t want the increased Noise Vibration & Harshness (NVH) of a stiff bushing and decided to use a new large diameter OEM style bushing because they hold up quite well and don’t affect the geometry much when new."

To get the RC improvement, the 302S control arm (M-3075-RA) w/extended ball joint would do the same but the FR bushing kit (M-5638-C) has cautionary words about increased NVH. Would the stock bushing you used on the project work with the 302S arm (without compromising location and intended geometry improvement)? Thx
Yes
I think I'm beginning to get this side figured out.


I'm not looking at straight line behavior here, and I get that a torque bias ratio isn't an rpm thing.

What I'm missing is what happens to the unequal torque distribution assuming that you're getting that while cornering under some amount of throttle, let's say at least 'leading throttle'.

Does the fact that total torque is coming from the driveshaft essentially lock the differential up?

If the diff doesn't lock up, what happens to the unequal traction forces and why wouldn't they add a yaw moment in similar fashion to how a dragging rear brake in a car with an open diff might?

Or maybe for this I belong on the short bus.


Norm

Torsens don't lock. In open diffs and torsens, torque takes the path of least resistance. Torsens are LIMITED by their TBR in sending power to the tire with the most grip (determined by the grip of the tire with less grip). The 3X the torque does not mean the outside tire is getting 3X the torque of the inside tire, but only 3X the grip level OF the inside tire is sent to the outside tire. Most of the torque still goes through the inside tire, causing some level of inside wheel spin.

In an open diff you're rate of acceleration is pretty much determined by the grip of your inside tire. In a 3.0 TBR Torsen, you can now accelerate at 3X the grip level of the inside tire. In a locker or a salisbury differential that can lock 100%, you're not limited by the grip level of the inside tire at all.

Billy, regarding the bushing choice, you said this..."The rear bushing appears to be identical between the old and new arm. Its very common to change this bushing to a smaller diameter and Delrin or polyurethane bushing to reduce unwanted alignment changes when cornering. This is a big problem on old, worn-out bushings but since our car is a dual purpose street/track car, we didn’t want the increased Noise Vibration & Harshness (NVH) of a stiff bushing and decided to use a new large diameter OEM style bushing because they hold up quite well and don’t affect the geometry much when new."

To get the RC improvement, the 302S control arm (M-3075-RA) w/extended ball joint would do the same but the FR bushing kit (M-5638-C) has cautionary words about increased NVH. Would the stock bushing you used on the project work with the 302S arm (without compromising location and intended geometry improvement)? Thx
Yes
 

Norm Peterson

Corner Barstool Sitter
831
616
Exp. Type
HPDE
Exp. Level
5-10 Years
a few miles east of Philly
Torsens don't lock. In open diffs and torsens, torque takes the path of least resistance. Torsens are LIMITED by their TBR in sending power to the tire with the most grip (determined by the grip of the tire with less grip). The 3X the torque does not mean the outside tire is getting 3X the torque of the inside tire, but only 3X the grip level OF the inside tire is sent to the outside tire. Most of the torque still goes through the inside tire, causing some level of inside wheel spin.

OK . . . I think there's a different disconnect going on here. I'm looking strictly at rear tire forces without regard to how much driveshaft torque is actually being productively used vs how much of it is disappearing into inside wheel spin (going to a limited amount of tire spin-up rather than traction so basically wasted).

I'm thinking if the inside tire can only support 100 lbs of tractive force - presumably while doing some amount of spinning/slipping, and the outside tire is then getting 300 lbs, that there's a net of 200 lbs traction acting outboard of the car CG. That says 'yaw moment' to me. Whether that's noticeable or not, I don't know or really know how to judge relative to a weakish Trac-lok.


In an open diff you're rate of acceleration is pretty much determined by the grip of your inside tire. In a 3.0 TBR Torsen, you can now accelerate at 3X the grip level of the inside tire. In a locker or a salisbury differential that can lock 100%, you're not limited by the grip level of the inside tire at all.

So a Torsen could be looked at as an open diff that's less than fully-open? Where the Trac-lok would be more of a limited locker that can be made to open up?


Norm
 
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Torque seeks the path of least resistance. The unloaded tire is the path of least resistance. It's not as simple, cut and dry, or accurate to say that there is a 200lb difference acting in the outside tire in your example.

It might be better to think of it as the inside tires grip (100lb) determines the amount of accelerative force/torque sent to the outside tire. In your example, the outside tire may have 500lbs of tractive capacity, but the diff only sends 300 to the outside tire, meanwhile the inside tire contributes 100lbs and is also spinning. Technically this inside wheel spin reduces the tractive ability of the tire and thus also reduced the torque sent to the outside tire, so it's a bit more complicated than a single dimension.

You shouldnt look at a torsen like an open diff at all even though it performs like one off throttle.

Traction locks are open diffs with static preload that binds them together. Torque is trying to spin the inside tire but once the difference in grip between an inside tire and outside becomes great enough to overcome the preload, the inside tire will spin and more torque will go through that path and reduce the amount of acceleration force to the outside tire.

the only way to add more lock to a traction lok is to increase the preload. This also prevents the car from turning and you'll need to start to be more drastic with the setup to make it work, like a spool.
 

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