Wider tyres at front than rear - anyone tried this?

[kaanage]

With the death of one of my 205/50R15 A048 semi-slicks yesterday (stupid rumble strips at Winton ) I am now wondering if I can make use of the old stock 195/55R15 A048's that I have remaning from when I upgraded.

I was thinking that if I were to run the 195's at the rear, I could remove the rear anti-roll bar and maybe stiffen the front to keep the car balanced.
Of course, I would have to revert the settings when I switch back to daily use to prevent terminal understeer :p

Thoughts and experiences?

[seangti]

I say go for it. Doubt you'd find much if any difference. The rear only follows the front.

I think Alan (aka Grandturismo) runs staggered front and rear on his mkVI golf. But he runs the wider front from improved traction, rather than any other reason. Perhaps check out his thread on golf.net. He's done it in the past 2-3months. I can't access golf.net at work, otherwise would assist with a link.

[kaanage]

Thanks, I'll check Golf.net

I need the car balanced - mine is very adjustable on the accelerator (and brakes) and I use this a LOT on corner entry and for mid-corner line adjustment.

[brad]

i'd do it. If the balance isn't there then put the narrow ones on the front & see how that feels. 10mm isn't a game changer IMO

[HaydEn]

99% of fwd race cars will use skinny tyre on the back. You'll be fine

[sambb]

I was going to do that for the street but went for a rear anti roll bar instead. I still might take the rear anti-roll bar off and do it. I reckon it'd be a good way to get some nice and progressive lateral slip into the rear without changing the cars overall balance - something that's hard to do with a rear bar.

[kaanage]

But that's exactly what you would be doing - changing the balance (front to rear slip angles).

I can't run the 195's on the front as they're the old batch that fall apart if I push them too hard - on the back, they should be OK.

I'm going to try it since the only cost will be the strip and refit charge + a potential wasted track day (I'll be damned sure to work up the pace gradually so I don't back it into a wall)

[sambb]

Yes you are right, its just that one way alters balance with outright mechanical grip of the tyre being a bigger factor and the other way it is more through roll stiffness.
My guess is that narrower rears/no rear ARB would break away at the rear more progressively than wider rubber/with ARB. It would probably go well in the wet for that reason and also since it would 'lean' on the tyres more at lower speeds.
I'd have thought that the presence or lack thereof of a rear bar would have a bigger influence on the cars balance than slightly narrower rears of the same compound. Increasing the roll stiffness of the front as well might take things too far the other way. Changing one variable at a time might give you a better picture. For example you could turn up to the track with narrower rears + rear ARB. Who knows it might be unreal in the dry. Then if its horrible that way or if its wet, it'd be nothing to disconnect one side of the bar and do a back to back comparo. That'd give you a good idea where you're heading with things before the front was fiddled with.
good luck. I'll be interested to hear how it goes.

[kaanage]

I guess the small variation in width may be more subtle than the antiroll bar variations allowed by the mounting holes that are commonly provided. I'm wondering if intermediate settings can be achieved by using different mount holes on the left and right. Now if we had sliders like a proper race car...

I need to make sure there aren't any clearance issues with using only one rear ARB mount unless someone else has already tried this.

The reverse stagger would actually make the car more responsive for motokhanas (where I really struggled in the tight turns last Sunday) plus I should put the rear ARB to full stiffness and disconnect the front to maximise the roll stiffness (and hence grip) differential. But with track sessions as well on that day, I wasn't about to fiddle with the suspension to that degree.

I think Alan (aka Grandturismo) runs staggered front and rear on his mkVI golf. But he runs the wider front from improved traction, rather than any other reason. Perhaps check out his thread on golf.net.

Reading through his thread (all 140+ pages of it), I think the stagger he is using is in offset, not width.

[sambb]

I was speaking to an engineer/mechanic about softening the rear ARB on my car further (my whiteline adjustable is already on full soft). He was explaining how rear ARB's physically mounted to twist beam rear ends (by way of shackles/bushes) actually work. He was saying that they do not bend along the lengths of bar that lead out to each damper mount at all really.. These arms just act like levers to do the real work which is twisting the section of bar that lies along the rear axle. That's why moving to the furthest hole out or running longer drop links softens the bar because the added length makes these lever/arms more efficient at twisting the bar in between.
So he said to stiffen the bar for a given hole setting you could space out your shackle mounts to the maximum width, mount a third shackle/bush onto the axle between the other two and /or use alloy bush material. To soften it you could narrow your bush mounts which effectively increases the length of the 'levers' and allows them to act in a more concentrated way on a shorter section in between the two bushes. Or you could fit longer (or adjustable) drop links.
Maybe if your car with narrower rears and rear bar is too taily you could try some of those tricks to find a compromise without having to take it off all together. Adjustable drop links (rose joints with threaded high tensile rod in between?) would be good for track work.
I have no idea how you would apply any of this to UR style bars since they don't mount to the axle itself and so aren't forced to actually twist. When you think about it, they are virtually a fixed length that bridges straight across from one hub to the other. Normally if one wheel was to hit a bump mid corner then the distance from one hub to another should be able to increase ie go from being a certain length on a flat 90 degree plane to forming a hypotenuse. But with one of these fixed length bars, wouldn't the rising wheel actually drag the other wheel across laterally toward it causing toe changes, because the bar cannot extend? I can't see how they wouldn't cause bump steer. I'm all ears on that one because if its overall a softer bar than what I have I'd like to try one.

[kaanage]

Yep, anti-roll bars work as a torsion bar hence sliders are used for infinite adjustment in race cars to vary the lever length. That's why I was wondering if anyone had different mount points on the 2 sides to provide an intermediate stiffness via the difference in leverage.

The UR suspended style ARB still works as a torsion bar - it simply relies on the torsion beam axle (which is a really big anti-roll bar) providing all the location.

If you're finding the Whiteline rear ARB too stiff at it's softest, I'd suggest either increasing the front roll stiffness, softer rear springs or else ditching the rear ARB. I didn't run a rear ARB until I also had a stiffer front ARB installed as I wanted to maintain the balance that my car had with just the stock front ARB

[sambb]

So do you mean for example use the soft hole on one side and the medium hole on the other in the hope of getting an overall/common setting that's an average between the two?

[kaanage]

So do you mean for example use the soft hole on one side and the medium hole on the other in the hope of getting an overall/common setting that's an average between the two?

Yep. The leverage difference would make one side a bit softer than the other but given the small difference in lever length, it shouldn't be too unbalanced (and may be an advantage if all the fast corners are in one direction as it is on some tracks)

[seangti]

Yep. The leverage difference would make one side a bit softer than the other but given the small difference in lever length, it shouldn't be too unbalanced (and may be an advantage if all the fast corners are in one direction as it is on some tracks)

May actually be advantageous depending on the direction of the circuit. Could aid in turn-in and/or exit.

[sambb]

Nascar style eh? You know what you probably won't even feel much of a difference going to the narrows. If you go really deep and turned in under brakes, that would put lateral forces on the rears whilst they are slightly unweighted too which could be interesting. But otherwise it'll probably go really well.