al503

First of all, I'm not trying to flame anyone for their choices in aftermarket parts/equipment.

With that said, I have to say that I'm surprised at some of the said choices (wheel/tire combos and lowering springs seem to be the most common). Many people are putting on wheel and tire combos that are questionable in terms of the final overall diameter.

This might be OK in cars that don't have sophisticated traction, skid, abs, etc., technologies and of course, it may be OK in the 2IS, but what if it's not? What if the % of slip allowed is based on the speed of the car which I'm sure it does? By going with wheels and tire that are over 5% larger (in some cases) how does this affect the computer's decision to intervene:
1. too early
2. not soon enough
3. or not at all?

In regards to the lowering springs: Most of the threads I have read use the amount of drop as the main criteria for choosing a set. While this is a most valid consideration, it should also be one of several others including:
1. the OEM spring rates front and rear
2. the spring rates of the aftermarket springs and how that affects over/understeer in relation to the stock springs.
3. whether the springs are linear v. progressive or a combo and how that affects overall handling/comfort.

Most of the time, finding the right set of springs is a lesson in compromise. One may like the drop of brand x but the bias (front to rear) of the additional stiffness may increase the understeer tendencies of the car, etc., etc.

I am by no means an expert in these matters but it catches my attention when people are using springs manufactured for the GS in the IS with apparently little or no research.

Finally, I would like to re-emphasize that I'm not trying to flame or bash anyone. I guess I'm trying to alert forum members to the fact that aftermarket modification decisions should not be taken lightly as your decision could put you in danger.

Just in case [flame suit is being zipped up]

rys

Of course, I agree. On the Scion tC forums (My daily), they have a list of all of the springs, the drop, and the spring rate.

Spring rates should increase as the vehicle drops to keep the car from slamming the wheels into the wheel wells, or in the most extreme cases, to keep the vehicles chassis from slamming into the ground.

-Brian

al503

Not necessarily. Most of the manufacturers will test their springs so that you don't bottom out. The more important consideration is the bias of stiffness. Do the aftermarket springs transfer stiffness to the front or rear or will they keep the bias the same, thus retaining the cars original under/oversteer tendencies.

Most cars are set up for understeer as that is much easier to control by the average driver in emergency situations. What happens when someone changes that dynamic and gets a nasty dose of oversteer at the worst possible time?

rys

That's why we have VSC and VDIM.

I'd be more concerned with replacing the sway bars and causing a change to the oversteer/understeer.

Without any hard technical data, I'd venture to say that our cars oversteer, but are very close to neutral. The electronic safety features assist in keeping control of the vehicle (but are not a substitute for safe driving practices).

-Brian

al503

Which the engineers at lexus spend countless hours of fine tuning to the stock parts and specifications. My point was whether some of the aftermarket choices would affect VSC's and VDIM's ability to do their respective jobs.

Tires and their size/width affect over/understeer to the greatest degree. Springs and sway bars can definitely affect over/understeer but they are usually used to fine tune it, not to change the overall tendency.

I disagree. Cars understeer from the factory. Again, it's easier for the average driver to control understeer. How many drivers out there (including us) know how to catch the car with opposite lock when it oversteers with any consistency?