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How Not To Modify Your 205's Suspension...


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189 replies to this topic

#181
Batfink

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Its not just about limiting body roll, but about balancing the front and rear of the car to get the car turning in nicely.

#182
engine killer

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Camber Compensation not an issue really, as your travel is pretty limited, or should be. Also getting enough Camber isn't an issue either.

I feel the only issues that needs sorting is getting the heat spread across the rear tyres and too much load transfer under braking to start off with.


My car is now back on the road again with Roll Centre and Bump Steer correction kits on, with a new camber caster adjuster.

The Roll Centre Correction kit works fine and it gives a tad more -ve camber too. I have to agree that the camber compensation shouldn't be any serious issue since a lowered car may only have about 2-3 inches (or may be less) compression travel. But putting the theory in practice is more fun than experiencing the outcome sometimes :D
Attached File  roll center kit 3.8.JPG   58.2KB   9 downloads

Probably the extra caster given really gives a good sensation when cornering. About 16mm extra caster has now been dialled in with about -1.2 degree of camber.

Unfortunately the weather on this side of planet is "rainy" most of the time, so I couldn't give it a bash.

#183
Cameron

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I know how the axles are built up.
What I meant; it is stated in this topic that it's better to drive without front ARB, but stuff in a thicker rear ARB?
The RC is lower at the front, so a ARB would have most effect there to avoid body roll.

I can't see the reasoning why it's better to have stiffer springs and remove the front ARB.
I'm willing to be believe that it works better in practice, but like to understand the reasoning/theory behind it.


I drove without a front ARB for a track day then promptly put it back in. :lol:

The thinking behind removing the front ARB is that they work by reducing load on the inside wheel and adding it to the outside - i.e. creating more load transfer. Increased load transfer is never a good thing as it reduces grip at that axle, so the thinking is that removing the ARB improves front end grip. However it's important not to overlook the effect this has on body attitude (roll) and the knock-on effect that has on front camber while cornering. In my experience it was more detrimental to grip and (much more) to handling to have front camber out of control than it was to increase load transfer on the front axle.

You're right about the low front roll centre meaning an ARB is more suitable on that axle, since you have a larger roll moment (torque) the more you increase the distance between the CoG and the RC, and therefore a greater roll angle. Maybe for people who have the front end only slightly lowered removing the front ARB is more beneficial than it was for me.. and this was the point I tried to make when I said other people's setups and conclusions drawn from them can't be compared to mine.

Furthermore, having a low front roll centre is good for 2 reasons as I explained earlier:

1 - Lower roll centre means less load transfer
2 - Lower roll centre means less tyre shear force at contact patch, which means improved grip and tyre life

So because of the above reasons (particularly reason 1) you can add a front ARB in to control the body without it being detrimental to front end grip, since the load transfer effects of ARB and low RC act to cancel each other out while allowing better camber control. In my opinion this is the setup that offers the greatest improvement, since you lower the CoG at the same time - lowering CoG is probably the biggest improvement to grip that you can make!

Edited by Cameron, 02 March 2012 - 11:32 AM.


#184
kyepan

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In my opinion this is the setup that offers the greatest improvement, since you lower the CoG at the same time - lowering CoG is probably the biggest improvement to grip that you can make!


I hear you on this, but i still think limiting the role couple via stiff ARB only masks the increase in roll couple length.
Plus the bump steer issues because of the different tie rod / wishbone arcs become noticeable.
Plus the increased destabilisation over rough surfaces where the left wheel and right wheel are doing different stuff.

Granted on a track this is not an issue (except when mounting kerbs), but for road / rally the car becomes a wildly unstable beast. Coupled with some torque steer it's a monstrous combination, or was on my car.

Compare my pug when lowered to my mazda mx-5, and you see how poorly McPherson struts perform when they are run out of the geometry range. The mazda is still incredibly stable.

If lowering, fixing the bump steer and wishbone angles ( by that I mean static camber) is the most important thing to combat in my opinion, especially in real world situations.

J

#185
Rippthrough

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See, I run without a front arb, however, I compensate by running 3 degrees of -ve camber and 7* of castor, which gives an appreciable amount of camber compensatation vs roll, even with my soft springs.
But that gives me a car with much improved traction in wet/low grip conditions, and the ability to hammer over both rough roads and big kerbs without loosing appriecial amounts of lateral grip, and a car that is very much steerable on the throttle.


I've said it before, and I'll say it again, these are road cars, for 99% of people, it's about making the car fun to drive on road and track, they are not racing cars, doing everything you can to raise absolute grip is worthless. Massive amounts of grip don't make you grin, they just push the limits far past where you can access them without loosing your licence.

Edited by Rippthrough, 02 March 2012 - 01:31 PM.


#186
engine killer

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If lowering, fixing the bump steer and wishbone angles ( by that I mean static camber) is the most important thing to combat in my opinion, especially in real world situations.


Fixing the bump steer is much easier, get a pair of male M14x1.5 rod end joint and drill out the hole to 14mm. get some nuts and washers ready to try will do the trick.

Correcting the wishbone angles can be a nightmare if starting from the wrong angle.

#187
wicked

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Tnx Cameron,

I'm about to refit my subframe and have a 205 gti and 309 gti front ARB on the shelf.
This topic made me worry, but I'll pick the 309 one first...
(My car: 309 gti 8v rear beam, 24mm ARB to be fitted, plate diff, Weitec coilovers on the front, 2l turbo lumb)

#188
Cameron

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I hear you on this, but i still think limiting the role couple via stiff ARB only masks the increase in roll couple length.


That's the whole idea! I'm not saying fit a whacking great front ARB btw, but it certainly makes keeping the 205 one on there look like a good idea.

Sorting bump steer is an absolute definite as well.

Tnx Cameron,

I'm about to refit my subframe and have a 205 gti and 309 gti front ARB on the shelf.
This topic made me worry, but I'll pick the 309 one first...
(My car: 309 gti 8v rear beam, 24mm ARB to be fitted, plate diff, Weitec coilovers on the front, 2l turbo lumb)


At least you have both options, so if you find the 309 one gives too much understeer then you can always swap it out. What front springs and torsion bars are you using?

#189
wicked

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Standard 309 GTI 8v torsion bars, 20mm iirc.
On the front I've Weitec Hicon GT coilovers, with the original springs. Donno the spring rate.

I intend to lower it 40-50mm on the front, little less on the rear.

#190
Cameron

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I'd probably stick with a 205 ARB up front then.