Understeer

[DWBOX2000]

On a 2002 Boxstet S I switch rims and tires from:
7x17 55
8.5x17 50

To

8x18. 57
9x18 43

I went from old pilot sports to new continentals. I was going for a little less performance with a little softer ride.

Anyhow, I know these cars are built with oversteer in their design (not sure why) but I think it has more now. Would this happen from what I did and Is there some way to counter that somehow?

[Shaun @ Tru6]

Bill Verburg who moderates the Technical forum is the best person on this site to answer your question. I would send him a PM.

My unqualified response is front wheels are a tiny bit more outboard and rear wheels a little more inboard which I think would increase oversteer. My sense is to check and take out a little air in the rear tires.

[DWBOX2000]

Thanks Shaun. I’ll give it a try.

[masraum]

You went from 17" wheels to 18" wheels in an effort to reduce performance and increase ride comfort? Isn't that backwards? Usually larger wheels have lower profile tires which means a harsher ride.

Are you running the stock size tires 225 40 18 | 265 35 18?

And the stock size tires for 17" wheels were 205 50 17 | 255 40 17. Is that what you were previously running?

[stealthn]

You sure you don’t mean understeer?

[Superman]

Shaun is right about at least two things. Mr. Verburg is THE MAN. Also you might try reducing tire pressure on the axle you wish to improve traction, or vice versa. Most folks at autocross like to run slightly higher tire pressures, like in the 37-38 psi neighborhood. Less rolling resistance and less squirrely quicker responding handling. Also, in my view, the Continental Extreme Contact Sport tire does not take a back seat to Pilots in terms of performance.

[masraum]

Originally Posted by DWBOX2000

Anyhow, I know these cars are built with oversteer in their design

Originally Posted by stealthn

You sure you don’t mean understeer?

Agreed. I didn't read that right the first time because my brain was expecting it to say "understeer" so that's what I read. My '08 BS can/will behave either way, but tends more towards understeer.

Originally Posted by Superman

Mr. Verburg is THE MAN.

Absolutely!

Also good call on the tire pressures. My car on 19" calls for 32#fr 37#rr. DWB may want to check and tune the pressures.

[DWBOX2000]

Tires are continental extra contact sport plus (so they say in tires). Fronts are 235/40 zr18. Back 265/40 at 18.
The car plows forward on turning. Previous were 205/50 zr 17 and rear 255/40 zr17.
I meant to say ultimate performance wasn’t my only objective. Price/comfort/performance/durability. A compromise I guess.
Anyhow, I reduced the rears from 36 to 32. I’ll test out and see what happens.

[DWBOX2000]

Cayman Rims

[Shaun @ Tru6]

You couldn't ask for better weather for driving around. I took the bike down to Walpole yesterday and then Hanover then back to work. Glorious day! Today looks to be the same.

[DWBOX2000]

Geez, should have called. We could have grabbed coffee. I was just sitting around thinking about understeer.

[masraum]

Originally Posted by DWBOX2000

Geez, should have called. We could have grabbed coffee. I was just sitting around thinking about understeer.

jab the brakes, jerk the wheel, problem solved!

[Shaun @ Tru6]

I know, was gong to call but super busy day.

[DWBOX2000]

For giggles, could you make a boxster be more oversteer bias?

[DWBOX2000]

Anything’s possible, I was thinking the offset more than the rims and tires. I believe the rims and tires are the same size as the 18’s that would have come on an S less the offset being different. Would 5mm spacers in the front help or hurt? Regarding the suspension, my gut says it’s all pretty good as far as health. Previous owner took to porsche every year for its annual check up and they changed anything they could think of. Heck, new rear axles 4K miles ago. Not saying it couldn’t but nothing changed regarding bangs, clunks and grinds. Drives as nice as always. Maybe an alignment would help, with someone in the know.
Thanks.

[DWBOX2000]

I did mean understeer built into design. Brain fart, my bad.

[masraum]

Originally Posted by DWBOX2000

For giggles, could you make a boxster be more oversteer bias?

Sure. I'm no expert, but I think in order of ease/cost
change alignment (more front camber and less rear camber would impact the balance)
change sway bar(s) (smaller/softer in the front and/or larger/stiffer in the rear)
change springs (softer in the front, stiffer in the rear)

Originally Posted by DWBOX2000

Anything’s possible, I was thinking the offset more than the rims and tires. I believe the rims and tires are the same size as the 18’s that would have come on an S less the offset being different. Would 5mm spacers in the front help or hurt? Regarding the suspension, my gut says it’s all pretty good as far as health. Previous owner took to porsche every year for its annual check up and they changed anything they could think of. Heck, new rear axles 4K miles ago. Not saying it couldn’t but nothing changed regarding bangs, clunks and grinds. Drives as nice as always. Maybe an alignment would help, with someone in the know.
Thanks.

That had occurred to me. I believe changing offset can have an effect (increasing or decreasing the front or rear track. I think increasing the track gives that end more grip, and decreasing gives it less grip, but I'm not certain about that.

[cockerpunk]

anything with more than 50% of the weight over the rear was inherently built with understeer, because understeer is faster. or more correctly, when driven well, a car with built in understeer is MUCH faster. the 911 even moreso than a boxster. the entire point of the 911 is quell the understeer on turn in, and ride that engine out of the corner.

you can add more oversteer bias, very easily. rear toe, bigger rear bar, less front bar, change the spring rates, the rake, etc. adding 5mm of track with spacers would be a very minor effect.

but honestly, what when you notice a balance change after swapping tires, its probably that your rear tires are actually working, and they werent before. if anything your wheel change should have slightly added oversteer. not a big change, but slightly. but when you swap from old tires, to new tires, often the balance will shift to more understeer, because the rear tires are actually working well.

[911 Rod]

Change swaybars.

[Superman]

I just noticed something. Your rear tires used to be 24% wider than your fronts. Now they are only 13% wider.

265/235 = 1.127
255/205 = 1.243