Static cross-weight

While I'm at it I might as well get into cross-weight. You might be thinking...

Well who cares on a road racer? Besides I can just corner weight the cross load out.

Just you hold on a minute. Two things to think about...

First, what is it and what does it do? Percentage of load taken up on a diagonal. I do it as the percentage of load taken up by the RF and LR, but either works. On an oval setup it's a good trick to get power down better out of a corner. With a cross-weight > 50%, you are "preloading" load transfer to the outside on the front axle, and the trade is you can "preload" load transfer to the inside on the rear axle. Having extra load on the LR tire in that case will let you get on the throttle earlier and more confidence. Obviously the trade off is the car will understeer more since you're overloading the RF more.


On a road course car, cross-weight > 50% will give you asymmetric balance, particularly on-center. Car will understeer left and oversteer right. Not what you want when trying to tune the car. Having looked at some of the telemetry from my guys' FSAE car last year they had that issue, and I'm kinda curious now if they had a cross-weight problem.

When doing corner weights, you can get rid of static cross-load by lengthening or shortening pushrods and such. Now that I think about it though, I suspect you could be fooling yourself by doing this, particularly if you have asymmetric wheel rates. Asymmetric wheel rates are surprisingly easy to get if your manufacturing isn't perfect or if your spring rates aren't all the same. Having measured springs on the FSAE car, there's a damn lot of variation. Four springs all stamped "300 lb/in" could easily range from 290 to 340.

An interesting way to check would be to take corner weights with and without the driver.

The above example are some representative FSAE numbers. It shouldn't be too hard to position the seat on centerline. While I'd expect the F/R load split to change a bit when the driver gets in, the cross sure shouldn't! Cross changing with vertical load, or acceleration, could lead to some weird stuff... if you put a different driver in, as fuel burns off, or even if you just get onto a section of track with some banking.

In the above example, if everything had been manufactured perfectly and air pressures were set, I'd suspect maybe the RF and LR springrates are a bit higher than the LF and RR. A swap of the LR and LF springs might fix it.

Food for thought.

Hopefully I'll get back to more design-related stuff soon. Been busy with other crap, and gotta drive up to Buffalo at 430am tomorrow morning...