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Old 04-30-2004, 03:47 AM   #1
rex n effect
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Default Does toe out reduce grip? Please explain its purpose.

I know a lot of autocrossers like to have some toe out in the rear to aid in "rotation". Does that mean it reduces grip in the rear? Why would you want to reduce overall grip just to loosen up the rear end? Would more swaybar or a little less negative camber be a more tire-friendly option for the street? Thanks for the input.
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Old 04-30-2004, 05:04 AM   #2
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From what I understand, rear toe-out will make the car pivot more. In a sense making it more unstable. Toe-in makes it more stable.

At the moment I have toe-in the the back and toe-out in front. The toe-out in front improved the turn-in phase at the detriment of straight line and braking stability. To what extend? I don't know, but I will try toe-in in the front on my next alignment to see for myself.

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Old 04-30-2004, 08:29 AM   #3
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fwiw, toe-out on your front tires is a "stable" condition, while toe-in is an "un-stable" condition. i wouldn't try toe-in for the front unless you enjoy going slow. don't confuse more responsive with unstable. unstable means the car did something without you wanting it to. more responsive means the car did what you wanted it to do very quickly. drive a car with 1/4" rear toe-out on snow tires at 85mph if you want to know unstable. :shudder: i did once. won't do it again.

front toe-out, as i've posted before, can be thought of as dynamic ackerman. more toe-out and the car behaves like it has more positive ackerman, less toe-out and the car behaves like it has less positive ackerman/more negative ackerman. there is no "ideal" ackerman percentage, so you're looking at a best compromise situation. theoretically, a course with a lot of higher speed, large radius corneres would require a lower ackerman effect while a slower, tighter corner would require more ackerman effect. you would run less toe-out in the first case, a little more toe-out in the second. the difference is small though and you're better off just driving through it.

rear toe-out _is_ an unstable condition. it also has the effect of altering the vector angle that the tire will operate at it's max grip relative to the car. for example, we take a well setup car that is cornering at 1g. the inside rear tire is nearly completely unloaded, so we can ignore it's effect on handling dynamics. the outside rear is operating at a 12* slip angle. (the path the tire is following is turned to the outside of the turn 12*'s from the path the chassis is taking) if we add a little toe out, say -.05*, the tire has to still reach 12* of slip angle before it generates it's maximum force. to get to the point where the tire is operating at that slip angle the rear of the car has to "step out" wider then the front of the car. now the car is turned more for the same amount of steering input. best unwind the wheel a bit or you'll be waffling those inside cones!

nate

*edit* cliff notes/short answer. no, toe-out does not reduce grip.
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Old 04-30-2004, 11:10 AM   #4
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I've never thought of it to that detail before. I knew from experience it felt like adding rear wheel steering when I increased toe out so I guess that makes sense now.

My alignment guy races spec miata's and he was telling me to increase the rear sway bar instead of adding toe out but I've always felt I have enough or maybe even too much stiffness in the rear sway bar (not enough rear suspension indpendence). I increased the rear spring rate to 8kg/cm (same as the front) and I couldn't tell much difference at all. I increased the rear toe out to 1/4" and the car handles so much better. More throttle off oversteer and the rear end really feels like it has rear wheel steering. After I increased the rear toe out I was running too tight at the exit of the turns because i wasn't used to the car's front end tucking in instead of pushing. Its a significant difference.

I don't think rear toe out would be good for high speed driving but its great for autocross. Its an easy way to fine tune the handling of the car after you get everything else set like you want it.

To answer the original question... my times are faster since I've gone from 1/8" rear toe out to 1/4" rear toe out. The car is a little twitchier but its much more neutral (less understeer to no understeer) in most situations.
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Old 04-30-2004, 11:47 AM   #5
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rear toe out can be a way to get the car to rotate better, but it wold be preferable to increase rear roll stiffness.

usually guys who opt for toe out are in stock classes and don't have the option of increasing rear roll stiffness.



Quote:
Originally posted by Silver Bullet 02
I've never thought of it to that detail before. I knew from experience it felt like adding rear wheel steering when I increased toe out so I guess that makes sense now.

My alignment guy races spec miata's and he was telling me to increase the rear sway bar instead of adding toe out but I've always felt I have enough or maybe even too much stiffness in the rear sway bar (not enough rear suspension indpendence). I increased the rear spring rate to 8kg/cm (same as the front) and I couldn't tell much difference at all. I increased the rear toe out to 1/4" and the car handles so much better. More throttle off oversteer and the rear end really feels like it has rear wheel steering. After I increased the rear toe out I was running too tight at the exit of the turns because i wasn't used to the car's front end tucking in instead of pushing. Its a significant difference.

I don't think rear toe out would be good for high speed driving but its great for autocross. Its an easy way to fine tune the handling of the car after you get everything else set like you want it.

To answer the original question... my times are faster since I've gone from 1/8" rear toe out to 1/4" rear toe out. The car is a little twitchier but its much more neutral (less understeer to no understeer) in most situations.
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Old 04-30-2004, 11:48 AM   #6
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Quote:
Originally posted by jmott
rear toe out can be a way to get the car to rotate better, but it wold be preferable to increase rear roll stiffness.

usually guys who opt for toe out are in stock classes and don't have the option of increasing rear roll stiffness.
Please explain...

If I'm not mistaken the top STX guys run a significant amount of rear toe out.
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Old 04-30-2004, 02:24 PM   #7
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Quote:
Originally posted by Silver Bullet 02
Please explain...

If I'm not mistaken the top STX guys run a significant amount of rear toe out.
Because it flat out works better for SoloII. Increasing rear roll stiffness is well and good for making the back end slide around since it also unloads the inside rear wheel more, reducing available traction even more. Adding toe out in the back results in the outside rear wheel helping to "steer" the car around while the inside wheel is planted a little better. More power makes for more steering effect, which points the car faster so you can unwind faster and get forward bite quicker without waiting for grip to return.

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Old 04-30-2004, 04:54 PM   #8
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raising the rear roll stiffness with springs does not reduce grip. it raises it.

it is the change in weight transfer f/r due to the stiffness that changes the balance of the car.

I would argue, that the toe out method of getting oversteer would be less effective than moving weight around, or tire sizes, or roll stiffness.

but I could be wrong =)

suspension isn't simple, and the fact is not even top F1 engineers can tell you what the best solution is, you just have to play around with it.

for instance, top E36 BMW drivers Brian Matteucci and Bob Tunnel use vastly different spring rates in the rear. But both are insanely fast. There are often very different but equally fast suspension setups.



Quote:
Originally posted by Storm
Because it flat out works better for SoloII. Increasing rear roll stiffness is well and good for making the back end slide around since it also unloads the inside rear wheel more, reducing available traction even more. Adding toe out in the back results in the outside rear wheel helping to "steer" the car around while the inside wheel is planted a little better. More power makes for more steering effect, which points the car faster so you can unwind faster and get forward bite quicker without waiting for grip to return.

Jay Storm
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Old 04-30-2004, 05:31 PM   #9
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Quote:
Originally posted by solo-x
for example, we take a well setup car that is cornering at 1g. the inside rear tire is nearly completely unloaded, so we can ignore it's effect on handling dynamics. the outside rear is operating at a 12* slip angle. (the path the tire is following is turned to the outside of the turn 12*'s from the path the chassis is taking) if we add a little toe out, say -.05*, the tire has to still reach 12* of slip angle before it generates it's maximum force. to get to the point where the tire is operating at that slip angle the rear of the car has to "step out" wider then the front of the car. now the car is turned more for the same amount of steering input. best unwind the wheel a bit or you'll be waffling those inside cones!

nate

*edit* cliff notes/short answer. no, toe-out does not reduce grip.
Thank you.
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Old 04-30-2004, 05:44 PM   #10
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The biggest thing I learned after getting into national competition with my G-stock 2.5RS (and running an STS prepped 2.5RS quite a bit) and a BS Porsche Boxster, is that different mods will effect the car at different points of the corner. There are really three stages to a corner, corner entry, mid corner, and corner exit.

Rear toe-out on the Subie will aid with the balance of the car throughout the corner, but has the best effect on helping the car to rotate (or better put, not push on throttle) coming off the corner. It helps to get the car to drive like a rear driver coming out of corners.

More rear bar can be used to help the Subie to get the mid corner balance where yo want it, especially useful on NT courses with lots of sweepers.

Way low or way high rear tire pressure can help with getting a Subie loose on corner entry.

What is interesting (and obvious, but not to me at the time, because my experience level was all FWD and AWD) is that these same mods on the Boxster could have different effects, because we were more traction limited. We had corner entry and mid corner balance just right, but without some toe-in at the rear, it had trouble getting loose on throttle coming off corners. upping the rear pressures a couple pounds helped the car to keep the ass inline better, but then it was too much pressure for getting power down to the ground.

Again, changing toe, or pressures, or roll stiffness can effect the car at different point on a corner, and the layout of the car will be different too. Lots of experience helps to figure it all out.

BTW...i learned all about the rear toe-out trick from Brian Priebe about 4 years ago, who had experimented with it on his STS Subie the year before. Once the secret was out, rear toe-out has become a standard for Subies in autocross.

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Old 04-30-2004, 11:14 PM   #11
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I was replying to the roll resistance increased by bigger bars, as quoted earlier in the thread. Yes, increasing roll resistance with springs increases grip, with it's own drawbacks. Moving weight around and tire sizes was not part of the thread subject, but it has it's own benefits and drawbacks, especially with our AWD not liking different tire diameters.
Quote:
Originally posted by jmott
raising the rear roll stiffness with springs does not reduce grip. it raises it.

it is the change in weight transfer f/r due to the stiffness that changes the balance of the car.

I would argue, that the toe out method of getting oversteer would be less effective than moving weight around, or tire sizes, or roll stiffness.

but I could be wrong =)

suspension isn't simple, and the fact is not even top F1 engineers can tell you what the best solution is, you just have to play around with it.

for instance, top E36 BMW drivers Brian Matteucci and Bob Tunnel use vastly different spring rates in the rear. But both are insanely fast. There are often very different but equally fast suspension setups.
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Old 05-02-2004, 03:59 PM   #12
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ok so toe in is / \
and toe out is \ /

right?

so thinking common sense wise, for the car to be set up to rotate the best you need the following:

front : \ /
rear : \ /

am i correct?
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Old 05-02-2004, 07:32 PM   #13
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Quote:
Originally posted by MRF582

front : \ /
rear : \ /
am i correct?
Front / \

Rear \ /

will turn the best because as the weight shifts to the outside you are basically already turning. Inside rear doesn't do much usually.
However front toe in is very unstable and usually results in worse cornering ability when going fast. You can see if you were taking a fast right (below), the left rear would want to step out and the right front isn't helping to turn the car as much.

Front / |

Rear \
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Old 05-03-2004, 09:06 AM   #14
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Quote:
Originally posted by NegativeC
Front / \

Rear \ /

will turn the best because as the weight shifts to the outside you are basically already turning. Inside rear doesn't do much usually.
However front toe in is very unstable and usually results in worse cornering ability when going fast. You can see if you were taking a fast right (below), the left rear would want to step out and the right front isn't helping to turn the car as much.

Front / |

Rear \
For autocross situations... Toe in / \ up front is BAD. Toe out \ / front AND rear is good.

Toe in up front makes steering response very mushy. Toe out makes it feel much quicker.

Toe out in the rear really helps the car follow the corners and rotate better.

Zero toe is best for best tire wear.

Road and rally racing will require different settings than autocross.
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Old 05-03-2004, 09:19 AM   #15
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Quote:
Originally posted by MRF582
ok so toe in is / \
and toe out is \ /

right?

so thinking common sense wise, for the car to be set up to rotate the best you need the following:

front : \ /
rear : \ /

am i correct?
Correct on both accounts.
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Old 05-03-2004, 09:55 AM   #16
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I don't know about you guys, but we are already eating the insides of our front tires with 0 toe. I can't image how fast we would go through tires if we had toe out in the front....

-Tom
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Old 05-03-2004, 10:00 AM   #17
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Quote:
Originally posted by trhoppe
I don't know about you guys, but we are already eating the insides of our front tires with 0 toe. I can't image how fast we would go through tires if we had toe out in the front....

-Tom
Go slower... really.
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Old 05-03-2004, 10:11 AM   #18
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Quote:
Originally posted by KC
Go slower... really.
Unless I'm mistaken, isn't our goal to go fast??

-Tom
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Old 05-03-2004, 10:41 AM   #19
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Quote:
Originally posted by trhoppe
Unless I'm mistaken, isn't our goal to go fast??

-Tom
Ahh... but you must go slow to go fast.
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Old 05-03-2004, 10:59 AM   #20
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So you think we're overdriving and killing the insides??

-Tom
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Old 05-03-2004, 11:13 AM   #21
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Quote:
Originally posted by trhoppe
So you think we're overdriving and killing the insides??

-Tom
The inside fronts definitely spin when they're unloaded in a turn. With the ackerman, and a big sway bar, the camber on the inside tire is probably close to -5 if the outside is fully loaded...

Take a bicycle, lift the rear tire a little bit and just pedal it with your hand... then put the tire down.. lightly. You'll leave rubber if you can keep the rear wheel spinning.

Open diff in the front remember. The tire with the LEAST amount of grip in the front gets ALL the power...

Same thing.

--KC
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Old 05-03-2004, 11:18 AM   #22
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Quote:
Originally posted by KC
The inside fronts definitely spin when they're unloaded in a turn. With the ackerman, and a big sway bar, the camber on the inside tire is probably close to -5 if the outside is fully loaded...
--KC
You sure about that? The outside tire goes more negative as you turn, while the inside tire is supposed to go more positive. If you are look at the front of the car and crank the wheel all the way to the left, your tires look like this / /

-Tom
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Old 05-03-2004, 11:34 AM   #23
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If the Camber is / \ to start with, on the turn the loaded tire goes straight up and down (in a perfect setup car and perfectly lloaded suspension) and the unloaded tire has even more neg camber....

Look at my unloaded tire... $10 says that tire is spinning like crazy.






You can SEE underneath 1/2 of the unloaded front tire in those pics!

Granted this was with the soft DMS suspension, but there's more neg camber on those pics than what was dialed into the car.

Same thing still applies to our current setups... if you're lifting your inside tires... the tires will spin faster then the loaded tires due to the lack of an LSD in the front.

That's all a function of the front swaybar I would bet. (Not positively sure).

Just like putting a rear bar and lifting your REAR unloaded tire...
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Old 05-03-2004, 12:03 PM   #24
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Quote:
Originally posted by trhoppe
If you are look at the front of the car and crank the wheel all the way to the left, your tires look like this / /

-Tom
I think if my tires looked like that on coarse I might end up upside down

But yea, KC is right that your wheels will look more like this around a turn / | (you get the idea) then / \ when straight again (obviously)
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Old 05-03-2004, 12:10 PM   #25
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gah damn

swaybars be damned!

hehe
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