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Chassis Rake and it's effect on balance / handling

 
Old 10-11-2007, 10:38 AM
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AudiOn19s
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Default Chassis Rake and it's effect on balance / handling

In a recent thread about ride heights I posed a question that wasn't really answered so I thought that I'd throw it out here.

I'd simply like to know how increasing or decreasing chassis rake can effect the car's balance and handling.

For example: Stock GT3 height specs are as follows:


Lets look at the MkI specs for example. Based on the factory recommended heights you could have anywhere from a 13mm rake to a 33mm rake and still technically be within specs.

Obviously I could take the car somewhere and let them corner balance it and still be mindless to this topic...but I'd really like to better understand this and how changes in rake will change the attitude of the car.

Andy
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Old 10-11-2007, 01:29 PM
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As with most setup items, there are multiple things going on at the same time.

First, a spec, like those above, doesn't tell you much. It is just a measurement point that does not really indicate rake.

1) When you change rake, you are moving a VERY SMALL amount of weight. That has almost no effect on handling.

2) When you change ride height (rake) you are moving the roll center and CG. If you started at the ideal point for front and rear grip, changing the roll center reduces grip on that end. So, raising the rear will add oversteer in almost all cars. Obviously, changing ride height effects things like bump steer as well, so you can sometimes get unexpected results.

3) In a aero car, especially one with a flat bottom, you are changing total downforce and center of pressure. A flat bottom creates a venturi effect with rake.

For a given car, you should set it to factory (or other) recommended heights. Then play around from there, if you are able. See what happens when you change one end by 1/4 or 1/2 inch. Then try to fine tune.
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Old 10-11-2007, 02:09 PM
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Originally Posted by SundayDriver View Post
2) When you change ride height (rake) you are moving the roll center and CG. If you started at the ideal point for front and rear grip, changing the roll center reduces grip on that end. So, raising the rear will add oversteer in almost all cars. Obviously, changing ride height effects things like bump steer as well, so you can sometimes get unexpected results.

See what happens when you change one end by 1/4 or 1/2 inch. Then try to fine tune.
Mark,
This is exactly the information that I was hoping to get.

Thanks.
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Old 10-11-2007, 04:31 PM
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In a 944 spec car (or any 944 type) rake does play a large roll in balance. I am not sure why, but level front to rear at the rockers seem to provide the most neutral balance. Higher in front that rear will generate understeer and higher in the rear generates oversteer. I have noted change in balance with as little as 1/4" ride height change at one end of the car.
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Old 10-12-2007, 08:49 AM
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Guys,
Thanks again for the help. The main reason I asked was becuase in my previous M3 you had to do everything *** backwards to make the car turn like it should...and in looking at some of the cup car pictures they run some of their cars really low in the rear...and while I realize that's a bit of an apples to oranges comparisons I just had to make sure.

I currently only have a 10mm rake in my C2...I'm going to up that to around 15mm and see how the car reacts.

Andy
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Old 02-06-2016, 10:09 AM
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Sunday Driver - understand - but how do you determine what ideal ride height is. Seems this is a wide variable between cars and car set up.
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Old 01-18-2018, 01:18 PM
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This seems to be a tightly guarded secret. I've been searching for a starting point for my boxster track car and not having much luck. New dampers and springs going on now so it will be a clean slate. Previous owner had it set level and pretty much slammed (98 mm front and 99 mm rear +,- 2mm) to the chassis points in the shop manual.
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Old 01-18-2018, 01:36 PM
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Originally Posted by Gus View Post
Sunday Driver - understand - but how do you determine what ideal ride height is. Seems this is a wide variable between cars and car set up.
Look for the best balance and most go too low. Never seen negative rake (front higher than the back) on a well handling car, only to fix a major problem before you can fix it correctly.
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Old 01-18-2018, 04:42 PM
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Originally Posted by Deadeye View Post
This seems to be a tightly guarded secret.
Sometimes it is a secret because people really don't know or understand it.

I think drivers all want to think the winner knows some secrets. Reality is winners are really better drivers. A good car seems to add to their confidence.

You could volunteer in "tech". Then you get to measure and scrutinize your competition.
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Old 01-18-2018, 04:48 PM
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I dropped the front of my GT4 10mm (Bottom of factory tolerance) in an attempt to reduce understeer. I think it worked.
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Old 01-18-2018, 05:04 PM
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SundatDriver? M758? Blast from the past. Where are they now?
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Old 01-18-2018, 05:12 PM
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Originally Posted by ProCoach
Look for the best balance and most go too low. Never seen negative rake (front higher than the back) on a well handling car, only to fix a major problem before you can fix it correctly.
Almost every track BMW I have come across uses a 0.5 inch negative rake. Front 0.5 inch higher than front. Maybe we have it all wrong.
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Old 01-18-2018, 05:54 PM
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Originally Posted by Thundermoose View Post
Almost every track BMW I have come across uses a 0.5 inch negative rake. Front 0.5 inch higher than front. Maybe we have it all wrong.
Hmm... Learn something new every day. That doesn't sound right to me, but if it works...

The E36 LTW that I campaigned, as well as all the historic stuff, GT4 ClubSports, Lambo SuperTrofeo, Ferrari Challenge, ALL the purpose-built formula and sports racers all have or had positive rake.

Some people may measure body height, rather than chassis height? Just don't know how to answer that.
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Old 01-18-2018, 06:46 PM
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On my 996 which had USA M030 suspension on it (before I replaced with coilovers) it almost looks looked like it had negative rake with the front higher than the rear or at least nearly even from the factory.

Last edited by Nickshu; 01-19-2018 at 06:35 PM.
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Old 01-18-2018, 07:18 PM
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Originally Posted by ProCoach
Hmm... Learn something new every day. That doesn't sound right to me, but if it works...

The E36 LTW that I campaigned, as well as all the historic stuff, GT4 ClubSports, Lambo SuperTrofeo, Ferrari Challenge, ALL the purpose-built formula and sports racers all have or had positive rake.

Some people may measure body height, rather than chassis height? Just don't know how to answer that.
I'm gonna throw your comment on the M3 forum and see what other folks think. Maybe with the way we measure rake (top of fender to center of wheel) that 0.5 difference actually provides a positive body rake.
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