Wider tyres at front than rear - anyone tried this?

[sambb]

Any news? How'd it go??

[kaanage]

LOL. I don't think I'll get out again until early next year.

I've also picked up a good set of 2nd hand Hankook Z221's which I will put on to my "cruise" rims so I can do some comparative testing. Which means I'll have to play around with the roll bar settings since the Hankooks are all the same size

[sambb]

I did a bit of a test on my car similar to what you were thinking about. I'd been running dunlop sport maxx RT's all round with a whiteline RARB. In this guise the car felt perfect. If I lifted off mid corner to tuck the nose in, the weight would transfer but I guess the tyre balance meant that nothing extreme ever happened with the back end. Afterwards though I put a set of advan AD08R's on the front only to see if I could sharpen up the steering. I loved it initially because now the front was just bolted down and the steering became so accurate especially at low/mid speed tight corners when understeer is normally at its worst. But I did start to notice that when the car was on fast 3rd gear corners (understeer not an issue any more) that if I lifted slightly to tuck the nose in that the tail would get very light and unstable. I'm not talking about deliberate stamping on the brakes followed by a lift to hang the tail out type fun - this was pant dirtying terror. The weight shift + bar was overpowering the dunlop rears.
After you posted I disconnected a droplink on one side of my RARB and swung the droplink out of the way - no problems there....the bar doesn't seem to hit anything or bang even on some pretty rough roads so should be fine on a track that way. Now I'm back to completely stock suspension but with street semis on the front. In relative terms thats kind of the same as you - less relative tyre grip on the rear, just achieved in the opposite fashion. I'll be removing the rear bar altogether for a while now I think. Yes the front isn't as bolted down, yes it understeers a little more on tighter corners ( but still way better than stock due to the sticky fronts), but atleast on faster corners I can lift slightly or make steering inputs without the car getting scary light in the rear. I totally see your point and think your dead right about not wanting narrower rears with your rear ARB still in place.
The Shine racing handling stuff you put up ages ago says that you don't want to rear bar a car with a high rear roll centre and I tend to agree now. Once the rear is lowered though ( CG and roll centre lower) then maybe i'll try out the bar again but only if I have the same tyres all round. In my experience then, having relatively less tyre grip at the rear has a far bigger impact on handling than I thought it would and works pretty well and is a hell of a lot safer on a stock car instead of running a rear bar.

[kaanage]

That's a terrific write up of your observations, Sam.

The only thing I'd add is that the understeer with the rear ARB disconnected is due the increase in rear grip, rather than loss at the front since we can feel a change in the balance far more easily than change in absolute grip.

Will you get semi-slicks for the rears, eventually?

[sambb]

Yep i'll get matching semi's for the rears too. I had full tread on my rear dunlops still, so figured I'd get my moneys worth out of them 1st.

What I don't understand is that if the inherent high rear roll center produced by twist beam rear ends means that you should counter that by getting the rear low and keeping it relatively soft and un-barred, why do so many spring kits that come out produce wedge lowering with the arse up relative to a lowered front - infact the direct opposite to what the Shine racing guys and many other engineery types advocate?

[seangti]

Interesting that your results were so pronounced, I wouldn't have expected it to be so obvious. I'm still yet to decide if I'll be running semi slicks or AD08R's for next year time attack, there's advantages to either approach, though I don't classify an AD08R as a semi.

My red car is quite a bit lower in the front than the rear, with a 65/35 front bias. That was professionally set-up, my silver polo is 60/40 and sits even.

For set-up, I read quite a few auto-cross sites about FWD set-up. Have also spoken with a few respected specialists about FWD handling and all have said that to make a FWD handle, it'll have a very firm rear and drive bumps like a raging pig on the street. Do you have a link to the Shine Racing content you're referring to? My search didn't provide information, just a shop that has since closed down.

[sambb]

Yeah when this thread started I was thinking no way could a tyre grip mismatch be that pronounced. I had a good go around an off ramp today and the car was alot quicker although it may not feel as quick when things are softer. For the info goto the main part of the forum and go into the 'wheels and suspension' section. Search in there for "does your VAG handle? It depends" by kaanage.I think its in the last two articles about roll centres where it goes into the real nitty gritty of how rear ride height is so important for managing an overly high rear roll center. But you're best reading it from the start because its written like a series I think. He is basically talking about Mk4 golfs but that theory all transfers to our car. They talk about autocross setups and "GT" setups ie street car, seperately because the setups are quite different.
The other stuff I'd read that agreed with the Shine posts (atleast as far as setting up a twist beam rear axle for street cars) was in racecar-engineering magazine by their resident suspension contributer Mark Ortiz. Basically in that he was saying that rear lateral weight shifts tend to move through the roll center and so the higher the rear roll center (and ours is unusually high in the our platform apparently) the less stable the car will be eg during a mid corner lift etc. This is because lateral weight shift will be happening up high through the cars body/sprung weight and not shared through the springs. If you try to counter that by slapping on more rear roll stiffness (stiff springs or a rear bar) all that will happen is that the rear tyres have their grip overcome easier by the lateral weight shift which remained a constant so long as the roll center remained a constant. He says you can't start increasing roll stiffness of a twist beam rear end until you've lowered the roll center (and even then you have to keep it as soft as you can) and the way to do that is by lowering the rear (which drops roll center and center of gravity together). The front is an opposite/different story though and the shine articles go into why lowering a macpherson strut front end screws the handling.

[kaanage]

What I don't understand is that if the inherent high rear roll center produced by twist beam rear ends means that you should counter that by getting the rear low and keeping it relatively soft and un-barred, why do so many spring kits that come out produce wedge lowering with the arse up relative to a lowered front - infact the direct opposite to what the Shine racing guys and many other engineery types advocate?

Looks and the reduced understeer giving a snappier response. Most people who buy these kits only run them on the road where you never really approach the limits at high speed corners.

My old Peugeot 306 was set up to respond like this and was a joy to drive on roads but the British magazines found it unstable on racetracks. I suspect I nearly died once when I entered a sweeping downhill horseshoe bend (coming out of the forest towards Apollo Bay from Lavers Hill, if you know the Great Ocean Rd) too fast and basically had to accelerate through the turn to keep the car stable (I could feel the rear getting edgy as I turned in) - I think I was going close to 150km/h by the time the turn straightened out enough to slow it down.

My red car is quite a bit lower in the front than the rear, with a 65/35 front bias. That was professionally set-up, my silver polo is 60/40 and sits even.

For set-up, I read quite a few auto-cross sites about FWD set-up. Have also spoken with a few respected specialists about FWD handling and all have said that to make a FWD handle, it'll have a very firm rear and drive bumps like a raging pig on the street. Do you have a link to the Shine Racing content you're referring to? My search didn't provide information, just a shop that has since closed down.

It depends on the track whether a raked setup is beneficial or not. At tracks like Winton and Wakefield where there are no really high speed bends, the reduced rear grip helps turn in a FWD car so you can get on the power earlier. Even at Sandown (and possibly Mallala) where there is only one high speed turn, what is lost in the fast bend may be made up in the slower turns. It's at tracks with large numbers of high speed sweeping bends where the increased stability of a grippier rear is really beneficial - at Phillip Island, the old Oran Park and Eastern Creek (and Bathurst, I'm sure) it would be noticeable.

I don't know about Qld, Tas or WA tracks as I never made it that far from home.

Autocross is almost all tight, slow bends (in a relative sense) so too much rear grip really hurts you. Motokhana is even worse and when I do my next one, I'll run the fuel savers at the back with the rear ARB on full and the front ARB disconnected.

Basically a car using Mac strut front and torsion beam rear end cannot be capable in both high and low speed corners. Here's the thread where I linked posts from TDiClub (originally from Vortex) - I wish I had as much practical knowledge and experience as those guys.
Does your VAG handle? It depends...

[sambb]

Yeah Shine talk of running the rear low relative to a standard or higher front to increase rear end stability for race and "GT" street cars. But that's not what is needed in an autocross setup where an unstable rear is needed so the car can make quick direction changes. It makes sense then that a raked setup would work in autocross because it keeps the rear roll center high. Its two totally different setups.
I've always been careful with going to stiff in the rear of FWD cars....my 205GTI pug tried to kill me too....probably mostly my fault.....they can bite you irretrievably and so suddenly.

[sambb]

if anyone wants their head to explode, read this:

http://www.theoryinpracticeengineeri...ter/2006_2.pdf

and here's another relating to tailoring F:R roll resistance ratios (whether it be elastically via springs/bars or geometrically via roll centre heights) to F:R weight distribution - its not quite what you'd think! It also touches on the adjustments that need to be made when tyre mismatches exist. Its half way down the paper:

April 2000