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Suspension modeling

xXDavidCXx

Autocross Champion
Location
AZ
Car(s)
2017 GTI SE DSG
Based on my experience in the last two years autocrossing with different setups, and much discussion and disagreement on these forums I've decided to make a concerted effort to model the Mk7 GTI suspension so that we can have concrete evidence for the choices we make.

Caveat: Only a few of these measurements are correct so If anyone has bits of the front suspension lying around, please speak up so we can make this model more accurate.

One of the questions I want to solve is how much body roll results in zero camber on the outside wheel. Any more body roll will result in less contact patch and thus less grip. This might feel like good grip on turn-in, then as the body reaches maximum roll, less grip or understeer.

I've definitely experienced this, but was never sure exactly why.

One reason is not enough tire pressure to support the weight transfer to the front tires. I used to race between 30-35 psi hot, but later in the year, started at 36 and at some event was running 38-40 hot and had much better grip, but still understeered more than I'd like.

The other reason could be that the body roll was putting the outside tire into positive camber. If this is indeed the case, and I'm not willing to compromise ride comfort with more front and rear spring, then a stiffer front bar is required.

Here is where the idea that a larger front swaybar can be beneficial for increased grip, by keeping the roll under -3.3 degrees in this example.

Tire contact patch is depicted by the blue and black bars under each tire.


EDIT: Moved to a pay program for more functionality.
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Here is the free racing aspirations suspension to play with. Most measurements are accurate-ish.

Using my winter 225/45/17 wheels and tires.

https://www.racingaspirations.com/mods/zbvcdta5/

1607480566517.png


x degrees of body roll with x static negative camber when the tire will have zero camber. This is the amount of body roll we need to stop by or before to maximize traction.
1.2 = -1
1.8 = -1.5
2.4 = -2.0
3.0 = -2.5
3.5 = -3.0
 

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Phur

Autocross Champion
Location
IN
Car(s)
2013 VW GTI Autobahn
An anti-lift kit will add Caster to help the contact patch when cornering aggressively. I know that I have a MK6 and it’s not the same car, but I got the BFI ones and it helped considerably.
 

w124_karl

Ready to race!
Location
NC
Following.

I’ve run mine in SCCA GS trim several times now, with the usual big rear bar, a 25mm Eibach for me. I’ve wondered out loud elsewhere if a front bar would be better, for keeping the front of the car flatter. It’s kind of a painful experiment to perform though with having to drop the subframe just to try it. GS only allows changing one bar.

If I ever do run the car seriously again, I’ll probably give it a shot. This years plan was to codrive an SSC FR-S, so that’s mostly what I’ve one.
 

q74

Go Kart Newbie
Car(s)
R
I have some of the measurements written down (subframe pickup, up-top shock, lca), as I've played with racingaspirations before. I'll see if I can find them again. The biggest trouble I had and never solved when trying to play with it was all the upright measurements.

I have a pair of unused CSS knuckles I can measure, but I'm not sure... well how to measure them.
 

odessa.filez

Autocross Newbie
Location
Roswell, GA
Car(s)
2016 GSW 1.8tsi auto
thanks for posting. Been a while, butI took a half hearted crack at this but stumbled with specifics on certain measurements. Racingaspirations help was not robust then. Might take another crack at it in the spring.

until then, I have some hand-waving about control arm angle, load on my springs and camber for my car.
 

jmblur

Autocross Champion
Location
Massachusetts
Car(s)
2017 Golf R
I have a set of CSS knuckles and LCAs I can measure more accurately than you'd ever want (50 micron accuracy laser scan). Let me know if that'd be helpful. It wouldn't have exact ball joint locations as I don't have spares there.

One very important note here - as with any modeling, this is truly a "GIGO" situation. Garbage in, garbage out. No conclusions should be drawn until you can confirm proper inputs.
 

bfury5

Autocross Champion
Location
CT
Ha, I was actually going to start 3D modeling the front suspension this week for this exact reason. I was curious to see roll vs bump suspension characteristics
 

xXDavidCXx

Autocross Champion
Location
AZ
Car(s)
2017 GTI SE DSG
An anti-lift kit will add Caster to help the contact patch when cornering aggressively. I know that I have a MK6 and it’s not the same car, but I got the BFI ones and it helped considerably.
What is an anti-lift kit?

bonus points if it’s SCCA STH legal.
 

Raguvian

Autocross Champion
Location
Bay Area, CA
Car(s)
2019 GSW 4MO 6MT
I'm very curious to see if having a stiffer front spring rate vs the rear is beneficial at all, or if something like a Whiteline roll center correction kit and softer front springs vs rear would be the better way to go.

Stock springs from the factory are softer in front than rear (regardless of model), so I'm curious why all the aftermarket suspension companies go the other way around (stiffer front than rear).
 

xXDavidCXx

Autocross Champion
Location
AZ
Car(s)
2017 GTI SE DSG
I'm very curious to see if having a stiffer front spring rate vs the rear is beneficial at all, or if something like a Whiteline roll center correction kit and softer front springs vs rear would be the better way to go.

Stock springs from the factory are softer in front than rear (regardless of model), so I'm curious why all the aftermarket suspension companies go the other way around (stiffer front than rear).
I would not use stiffer front springs than rear.

I run 425/750, it’s decently comfortable.

stiffer rear allows the rear to catch up over bumps, and helps unload the rear for more rear rotation.

I suspect the aftermarket goes stiffer front for liability.
 

Raguvian

Autocross Champion
Location
Bay Area, CA
Car(s)
2019 GSW 4MO 6MT
I would not use stiffer front springs than rear.

I run 425/750, it’s decently comfortable.

stiffer rear allows the rear to catch up over bumps, and helps unload the rear for more rear rotation.

I suspect the aftermarket goes stiffer front for liability.

Basically every single off the shelf kit goes with a much stiffer front than rear. I can't speak to companies like Ohlins but most of the other ones are much stiffer front, and then the band aid solution is to put a big ass RSB to get the rear to rotate more.
 

xXDavidCXx

Autocross Champion
Location
AZ
Car(s)
2017 GTI SE DSG

tpellegr

Go Kart Champion
Location
Boston, MA
Car(s)
2016 GTI S 6MT
I'm very curious to see if having a stiffer front spring rate vs the rear is beneficial at all, or if something like a Whiteline roll center correction kit and softer front springs vs rear would be the better way to go.

Stock springs from the factory are softer in front than rear (regardless of model), so I'm curious why all the aftermarket suspension companies go the other way around (stiffer front than rear).
What are the stock spring rates?
 
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