SCCA CAM-C Thread

[Norm Peterson]

How so? I get 0.21 deg per side, which is also a lot, but not crazy.

Same here, assuming 26" OD tires. About double what I'd start out with unless there was a lot of toe out in bump going on.


Norm

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[shogun32]

Same here, assuming 26" OD tires

wait, the only datapoints we have are at the wheel lip. 2.5mm at edge of unknown size tire is not a data point.

If my formula or inputs are wrong, please enlighten.

 

[Norm Peterson]

Sin^-1(2.5/19*25.4/2) no?

No. For starters, he said 2.5mm per side, not total.

You need to work with the diameter to where the measurements were taken, which in the case of toe conventionally registers to the tire tread at 26" - 27" or so. That's unless the 2.5mm was referenced to the wheels, in which case it'd probably be closer to a 20" diameter due to the height of the wheel flanges not being included in the wheel diameter.

arctan( [ 2.5/25.4 ] / diameter ) or arcsin( [ 2.5/25.4 ] / diameter )


Norm

 

[Brian@BMVK]

Sin^-1(2.5/19*25.4/2) no?

Not /2,

26.3, not 19. Guess it depends on where it's being measured from.

 

[Brian@BMVK]

Assuming wheel lip diameter is 20", that gets us 0.28 deg per side. That's still about double what is typically run/recommended.

 

[Norm Peterson]

wait, the only datapoints we have are at the wheel lip. 2.5mm at edge of unknown size tire is not a data point.

We don't know where the measurements were taken, but toe readings in inches or mm are referenced to the tire tread, which isn't going to vary enough from about 27" to matter enough for talking purposes here.

If the measurements were made to the wheels, we'd need to know that.


Norm

 

[shogun32]

26.3, not 19. Guess it depends on where it's being measured from.

the pivot point is the center of the wheel - that's where the hub is. If you kick in the front (leading edge) of the wheel by X, you also kick out the trailing edge by same.

Huh, so the reference point is the rear-most point of the wheel or tire? The measurement device is bolted to the wheel and is centered on the hub. So if the read-out is 2.5 from the rear-most point then the distance as referenced from the centerline of the wheel would be 1/2 of indicated? That's a screwy way to do things...

 

[Brian@BMVK]

the pivot point is the center of the wheel - that's where the hub is. If you kick in the front (leading edge) of the wheel by X, you also kick out the trailing edge by same.

Huh, so the reference point is the rear-most point of the wheel or tire? The measurement device is bolted to the wheel and is centered on the hub. So if the read-out is 2.5 from the rear-most point then the distance as referenced from the centerline of the wheel would be 1/2 of indicated? That's a screwy way to do things...

Toe is measured relative to the chassis.

 

[TeeLew]

Everytime I try to get some work done!!!

The measurement is at the rim. For argument, let's call it 485mm. That works out to ~0.3 deg. per side. No doubt this high for a street setting, but I consider it a fairly unremarkable starting point for track duty.

 

[Brian@BMVK]

Everytime I try to get some work done!!!

The measurement is at the rim. For argument, let's call it 485mm. That works out to ~0.3 deg. per side. No doubt this high for a street setting, but I consider it a fairly unremarkable starting point for track duty.

It's higher than I see anyone running on this platform by about 2x, even in competition use. Food for thought.

 

[TeeLew]

Toe in on the rear causes some under-rotation at the limit, which is often nice when you're putting power down on corner exit, but IDK what it will do if you have a lot of toe in during the mid-corner phase.

Toe out on the front causes under-rotation as well (after initial turnin), which people tend to forget. The classic advice is front toe out to increase turn in, but its the opposite after the initial turnin. I guess it might vary by the corner radius, that would be some fun math to work out.

I've found rear toe-in to have it's largest effect during braking when applying throttle. It will increase understeer steady-state, but I've found that effect to be less pronounced. I don't have an issue turning the car on throttle. That is actually quite good and I think overall powerdown traction is reasonable.

Your point on front toe is a good one and not often appreciated. Many settings which increase response tend to reduce overall capacity steady-state. I've found front toe-out to be one of those which often acts counterintuitively.

 

[TeeLew]

It's higher than I see anyone running on this platform by about 2x, even in competition use. Food for thought.

I'll have to give it a test and report back next time out. This could certainly be something which contributes to my trail brake complaint.

 

[Norm Peterson]

Toe out on the front causes under-rotation as well (after initial turnin), which people tend to forget. The classic advice is front toe out to increase turn in, but its the opposite after the initial turnin. I guess it might vary by the corner radius, that would be some fun math to work out.

That'd be your Ackermann geometry getting involved. Toe out is neither constant nor necessarily linear with how far the front tires are steered.

Norm

 

[Brian@BMVK]

That'd be your Ackermann geometry getting involved. Toe out is neither constant nor necessarily linear with how far the front tires are steered.

Norm

Yep. The effect of toe out on balance with significant wheel angle is dependent on the Ackerman amount built into the steering system.

 

[NightmareMoon]

Ok I need tire advice. I've only really run on the RE71R with this car.

The RT660 seems unobtanium until April now? (per Tire Rack guy)

So between the SC3, a set of closeout RE71Rs, and the Rivals in 305/30R19, which would you pick?

I'm also considering taking a 2nd mortgage (j/k) and getting a set of 18s to run the A052, which I think we all know is the fastest setup.

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