Just curious to see if anyone here has put 245's on both the front and the rear in an attempt to give the car more neutral handling. I suppose you could also use different tire compounds front & rear but that doesn't seem as practical.

I have a DB enthusiast w/ zero mods. Still in storage. Damn those New England winters.

-Murry

 

I haven't done it yet (original tires are still Ok), but I may do it soon. I was thinking about going to a 245/255 setup, but my front tires are wearing so much faster than the rears that I was thinking maybe a 245/245 would be good so I could then rotate front to back. I'll probaly use my worn fronts as an excuse to go to Toyo's all the way around, but I haven't fully decided yet. I may just get two more 245 from Bridgestone to put up front, and then start the rotation pattern for even wear. Eh, we'll see.

I talked with raceboy when we were at Laguna, and he said that his 245/255 S-03 setup was much better than the stock setup, but still not where he wanted it. I think he's finally 100% satisfied with the Nismo S-tune suspension kit, but I have no idea how his standards compare to yours or mine.

My impression is that it definitely couldn't hurt, and at the worst, it would allow you to rotate the tires for better life, permitting you to buy tires less frequently and therefore save up for the Nismo (or similar) suspension upgrade.

-D'oh!

 

Granted I do not have a Z, but going from general knowledge, I suspect increasing thread section by 20mm without increasing wheel width will not noticebly help reduce understeer.

If funds is an issue, which always is obviously, one cheapest way to reduce (not totally neutralize) understeer would be to increase front sway bars strenght and/or diameter. This should cost around $300 or less if you can find one for the Z sold seperate from the rear sway bar, if not both should cost around $500-550. I think this would have a noticeable effect.

Ideally, you would want to upgrade the suspension and the sways bars in a packaged setup, similar to "S-tune." That would probably cost between $1500-2000 however.

But then again, d'oh's idea of being able to rotate tires of same sizes is not a bad one either

 

Increasing the front roll bars stiffness actually makes a car understeer more. Increasing the rear (or decreasing the front) will increase oversteer (or decrease understeer).

Stiffer roll bars transfer more vertical load to the outside tire (decreasing by the same amount from the inside tire) in a turn, causing it to lose traction earlier than if there was an equal load on both tires. It loses traction earlier because lateral force vs. vertical load of a tire has a slope < 1 (ie increasing the normal force doesn't result in a linear increase in the lateral force). Or another explanation is that the slip angle of the rear tire increases faster than that of the front.

 

Julian - you are certainly accurate in your assesment, however, that would be true if you had already reached the point that the car is somewhat neutral in its handling. Not that I have first-hand experience with the Z, but it appears that the Z can use less weight transfer and an stiffened "anti-roll bar" (as it is the proper name) would keep the weight in check during cornering.

All of these have give and takes. One thing you do in the front creates the opposite effect on the rear and vice versa. Thus, you are also correct that increasing rear oversteer would create less understeer on the fronts.

In short, I do think, the Z can use stiffer anti-roll bar at the front to reduce understeer with stock wheel/tire setup.

FT!

 

yup, i think you got it right in saying the z needs less weight transfer. but the anti-roll bar actually acts to transfer weight!

that was news to me and kind of counter intuitive. i just learned all this stuff recently (started going through a better book last night) so i'm trying to digest everything.

i think when there's less roll (when you stiffen the bar), it means less *body* roll. But to resist the body roll there has to be a moment/torque to keep the body from rolling. Where does the torque come from? From the chassis, ie the tires via the springs, which is acting against the ground. anyhow, that's how i picture it in my attempt to understand and pass on the info. A rectangle sitting atop a rectangle with wheels, the two connected by springs.

i've also tried experiment on PS2 GT3 as a poor man's experiment.

 

Have you guys seen this chart from the West Texas Region SCCA?

www.wtrscca.org/tech.htm

 

Cool link!

Thanks.

-D'oh!

 

I will be running my new auto-x tires at talladega grand prix event on the 29th. they are going to be 255/40-17s all around. will let you know how it works out.

 

Julian - you are right actually. I went back to my own notes from several years ago when I had adjustable shocks, springs and anti-roll bars; and you are right.

I will try to post some of those notes later on to this thread to help clarify it. In the mean time the SCCA chart is certainly helpful addition to my notes

Happy motoring all,

FT!

 

There is a lot of information on that site to digest. Reminds me of the vehicle dynamics course I took in college several years ago. I have to dig around to find the course notes.

I have my reservations about mucking with the suspension. I think I'll try something with the tires first. I see the ability to rotate front to back as a real asset.

Thanks for all the input

-Murry

 

I am in agreement that my first change will be a tire change (probably SO3s). There seem to be several choices available for tires which meet DOT and are also have improved traction. I'm not competitive enough to add race-only tires yet, and I don't have a race 'budget' which allows for changing tires every 3-4 months.

My initial 'upgrade' will be a pyrometer so that I can see what changes result from manipulating tire pressure alone. Down the road I can imagine changing anti-roll bars/spring rates, and camber plates. My skill and knowledge base isn't high enough to make sensible changes at this time.

There clearly are at least several others on this forum who seem to have a similar basic plan of making handling improvements one at a time; and becoming a better driver at the same time.

I really appreciate the sharing of facts and experiences.