Sedan with coupe rims,
What kind of tire sizes should reduce some of the understeer in the sedan?
I know the staggered sizes might make the car understeer further so I want to know what kind of sizes should I put on?<P ID="edit"><FONT class="small"><EM>Edited by Renesis on 08/11/04 03:57 AM.</EM></FONT></P>

 

Are you describing power on or power off understeer. Going in or coming out of corner. Which do you want to fix.
Understeer is primarily set with the overstiff front sway bar. Reducing the tire traction by shifting weight.

The roll couple ratio [springs and bar] front and rear vs static and loaded weight. On most front engine cars the front tires are woefullly inadequate as to load index reserve so they slip faster than the rear tires which get less weight transfer, now when you accelerate the engine force decrease the rears availble traction for turning.[sliping around a curve with power on].

 

I acutally want to reduce both but I think the sedan probably needs to reduce power off understeer primarily. I don't have the rims yet but I am guessing that the sedan will still understeer or maybe understeer more than with OEM wheels.

 

This is a helpful resource: http://rogerkrausracing.com/overundr.html

2004 | 6MTs | Diamond Graphite/Graphite
Upstate NY

 

I am thinking about a few options that I have.
225/45 R18 all around
225/45 R18 front 245/45 R18 rear (OEM)
or
245/45 R18 front 225/45 R18 rear (but this is a drifting set up and risky for daily driving)

Any other size combinations possible for OEM coupe rims that has a good selection of tires?

 

Understand that the terms: over, under , neutral are really telling you the ratios of the front to rear tire slip angles.

Under steer assuming the spring ratios are correct for weight bias, is the result of front tires being overloaded.
Tires are sized to just meet or barely exceed the static plus braking loads on the front..........not an ounce of safety reserve.

You either reduce the weight on the front or find stronger [load index] tires that have more peak to static load reserve. Going up 100 pounds [peak load rating] per tire will usually reduce the slip anles at the same load by roughly half the percent or better. [[ All this assumes the same oem tire just a bigger stronger size version]].
A 95 load index [1521 pound tire] replaced by a 98 [1640 pound tire] is 7.8% stronger for the same slip angle so expect a 4% improvement [reduction] in slip angles with the stronger tire.

Study tires and you might find you really want a stronger tire on the front axle......more air volume stronger sidewall stiffness.

The opposite of the bigger wider rear to protect novice drivers in wet acceleration.

Ideally a G would understeer braking going into a curve , be neutral at the apex [brakes off] and over steer with power on coming out. Obviously you can't use power on coming out in wet [or you would spin!] so in wet the best you can accomplish is understeer to neutral [but neutral in the apex may be oversteer depending on speed and water levels].

 

it should be minor and you should be able to reduce it by adjusting tire pressures. since the rims are already staggered, why ruin the look by running same size tires.

dropping pressure in front and raising pressure in the rear can reduce understeer. sway bars can help as can an alignment. put a little more negative camber up front (0.5-1 degree additional static negative camber). If you can't fix it with the alignment or tire pressures, then invest in sway bars. if they don't work then look at same size tires. remember with driver only, you may see less understeer on left turns than right turns due to the drivers weight. 0.5 psi or so difference in the front tires should correct that. it did for me.

Better Life thru Chemistry
Black on Black 03.5 Sedan 6MT - Goodyear F1 GS-D3's 225/55/17, Custom Intake Tube, '03 "Z" Suspension, Magnaflow resonator, 6 Wire 4 Guage Grounding Kit

 

I think messing around with the tire pressure is an easy way to reduce under/over but then in the case of the G, it is a little bit different. It is a kind of PUSH uner because of the weight transfer to the front wheels, if I reduce the tire pressure in front, i think the front tires will have to support too much weight.

 

Please, do NOT under-inflate the front tires. All you will do is ruin your tires. The added rolling resistance will cause the tires to run hotter and not only decrease tire life, but also increase the likely-hood of having a blow-out at speed. The easiest way to reduce understeer is to run the same size tires on the front and rear. i.e. 245/45/R18 all around. An added benefit to doing this is that you'll be able to do a true tire rotation and get even more life out of your tires.

 

Thats what I thought. I was thinking about getting 225/45 R18 all around for my sedan so that I can reduce understeer.

 

I went with 245x45x18 Goodyear Eagle F1 all the way around, and the 'excessive' amount of understeer is a thing of the past. Even my wife commented on it this weekend on a drive up Hwy 1 (West Coast). Currently running 35psi in all tires, but am still experimenting. Any other suggestions on air pressure?

 

Again the front tires [size load index] are inadequate [to maintain the lowest slip angles]for aggressive driving.

Measure the static weight on them and then calculate the transfer weight at what ever G you want. You will see what limits the lateral acceleration.
Only during straight ahead braking can you get close to 0.9 G for a few seconds before the tires overheat.

Front tires should always be stronger [or at least equal] than rear tires.......brainwashed by looking at rear engined race cars........the end with the most weight must have strongest tires if neutrality is desired.

 

powerover (drift) into an apex.
bestway to counter understeering.

 

I use Stillen adjustable sway bars on my coupe and they greatly reduce my understeer. You can adjust them to create more so I suggest you do some reading I don't know how to exactly to set them up on sedan but mine are the softest in front and on the hardest setting in the back.

 

I would try 37psi front 33psi rear and see how it runs.

but getting swaybars would be much easier and get better results.