staggered and understeer

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scheherazade
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staggered and understeer

Post by scheherazade » Mon Sep 14, 2009 4:06 pm

So, the tyre bible says that tyre width doesn't affect traction.

Then people swear up and down that a staggered tyre setup causes understeer... supposedly because the wider rear has more traction than the narrower front?

So which is it : wider = more traction, or not?

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complacent
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Re: staggered and understeer

Post by complacent » Mon Sep 14, 2009 4:20 pm

My first shot would be that there are a ton of variables needed to determine that. (swaybars, etc, suspension type (wishbone, mcpherson, etc) )

My second comment (and not particularly helpful at that) is since most of us own Subarus, we're all little AWD weenies with two or three differentials and can't have tires with rolling diameters that differ by anything greater than 2 or 3 64ths of an inch. We also can't force the front to rear torque split greater than about 60-ish percent. Staggered tire sizes aren't particularly helpful for us, generally speaking. I'm willing to bet that unless someone here has a good history with RWD vehicles, it's going to be tough to get a decent answer out of one of us. Sorry. :-(

My honest guess would be that all things being equal, understeer would be increased by a greater rear tire size if a car had pretty neutral handling before tire changes. I've always viewed mega-hp RWD cars and staggered tires a necessary evil in order to provide traction to the drive wheels. I've honestly not pondered the potential handling characteristics beyond that as the only RWD car I owned for any length of time was a eunos roadster.
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zaxrex
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Re: staggered and understeer

Post by zaxrex » Tue Sep 15, 2009 7:31 am

scheherazade wrote:tyre width ... So which is it : wider = more traction, or not?
-scheherazade
complacent wrote:... we're all little AWD weenies with two or three differentials and can't have tires with rolling diameters that differ by anything greater than 2 or 3 64ths of an inch... . I'm willing to bet that unless someone here has a good history with RWD vehicles, it's going to be tough to get a decent answer out of one of us. Sorry. :-(

My honest guess would be that all things being equal, understeer would be increased by a greater rear tire size if a car had pretty neutral handling before tire changes.
Well Complacent may have swapped staggered widths for different diameters, and I am a weenie with no real world experience, but I do have an answer...

If you put more traction on the front it will help with understeer problems, but will be more sensitive to throttle lift and trail braking oversteer.

You can get this by having stickier compound tires if front, getting wider front wheels and tires, or staggering tire widths on the same size wheels.

There was a write-up in SCC that had an evo running 8.5 in. rear tires and 9.5 on the front, both on 9.5 wheels. Since primedia went down, so did the SCC website and I can't link to it. The reason for doing this was that the front contact patch stayed flatter, while the stretched rear had acurved profile. The pre-strained sidewall with the smaller contact patch meant that rear broke away first, but at a progressive and predictable way. One of the problems with that was the potential of rolling off your rear beads if your compound is too sticky.

But like the man with the apples said, on a street driven car, many other factors will influence the handling characteristics more noticeably than the rear tire track. Once you have reached the limits of these other suspension bits, then the staggered width would be the next step. Until you get to that point, the extra width would just cost you more money in rubber, add extra unsprung weight, and you would most likely need to get new wider tires all around along with wider front wheels or risk rolling off your rear bead.
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scheherazade
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Re: staggered and understeer

Post by scheherazade » Tue Sep 15, 2009 10:43 am

So, would that mean that for a given tyre width, running a slightly smaller rim would improve stickyness?
You would let the tyre lay-down-flat more, increasing maximum traction?

I had the impression that a 'stretch' makes the tyre more stiff, making a 'sudden change' more likely to cause loss of traction.
Because sudden change would transmit to the traction surface, rather than causing an initial 'flex' in the tyre itself, making the force application more gradual.
But I thought that absolute traction would be the same/similar if driven appropriately.

-scheherazade

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zaxrex
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Re: staggered and understeer

Post by zaxrex » Wed Sep 16, 2009 9:24 am

scheherazade wrote:So, would that mean that for a given tyre width, running a slightly smaller rim would improve stickyness?
You would let the tyre lay-down-flat more, increasing maximum traction?
From what I can remember about the article, it has all about to do with the contact patch. If you have a tire matched "appropriately" to a wheel, you should have the largest contact patch. If you use the same wheel and put a narrower tire on it, the edges are going to be pulled down a bit. This is going to decrease your contact patch, and decrease the available traction compared to the original setup. More traction in front lessens understeer, less traction in rear helps rotate.
scheherazade wrote:I had the impression that a 'stretch' makes the tyre more stiff, making a 'sudden change' more likely to cause loss of traction.
I would agree, and this is what the driver wanted to happen. Get the rear to step out on trail braking and on throttle - both are helping to reduce understeer.
scheherazade wrote:But I thought that absolute traction would be the same/similar if driven appropriately.
In this instance, the car was not super powerful. It may have been spec'ed to compete in events or something, I don't know. He gave up some traction to be faster since he was not overpowering the tires. He made the rear tires narrower to help reduce understeer, rotate the car quicker and give him faster times.

Did he decrease the overall "absolute traction"? Yes. Did he have the driving skill to take advantage of that and go faster? Yes. Would I be faster with the same setup? Hell no.
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