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Driver and fuel weight considerations for suspension settings

Motorsport Wheel Alignment Fundamentals

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Discussion and questions related to the course Motorsport Wheel Alignment Fundamentals

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After finishing the Alignment class modules I did not hear any mention of weight and it’s relationship to suspension settings and alignment. I am under the impression that it’s best practice to have the driver’s weight in the seat and the average fuel level in the car before performing any ride height adjustments, wheel alignments, and especially corner balancing. I found it odd that it wasn’t mentioned.

on vehicles with known fuel pick up issues that are not fitted with a surge tank we tend to align the cars with the fuel at or above 3/4 a tank, certainly not less. On cars with mulitple drivers we average their weights and add that to the driver’s seat via a combination of sand bags (wrapped in shrink wrap and in trash bags) and standard iron weight lifting plates.

You're completely correct, everything should be setup as its raced, with the actual driver in the car, the average amount of fuel in the car etc. If you can't manage those it's fine to approximate by putting in weight at approximately the correct places. It's also helpful to take multiple measurements if it's available. If you have a 160lb driver and a 250lb driver you can see what the difference is, same for full and empty fuel tanks. This matters far more to corner balancing than it does an alignment.

Weight itself isn't overly mentioned in the Alignment modules. Which somewhat makes sense. Alignment is typically a static value set in a garage, weight, and the change of it moves the various measurements through their dynamic range to some degree. The scale of how much things change are pretty far apart. Is ~30 lbs of fuel (6.3:1 for gas I believe) going to change things much on a car that weighs 3000 lbs? Even if it sits on one corner it's still less than 1/10th of the static weight on that tire. Suspensions setup to deal with loads in the hundreds or thousands of lbs. The driver and fuel differences might change things by .1 degree of camber, but nothing massive.

However, its pretty easy to increase your accuracy by throwing in some weight to approximate them, so why wouldn't you? Ultimately, there's 1 correct point where you setup the alignment. If anything changes you're no longer perfect, and all you can really do is decide at what point you want to be perfect. For a multi driver car, I'd aim to setup the alignment at some driver/fuel weight everyone see's. The heaviest driver might have perfect alignment once the fuel is low, and the lightest when it's full, or you're going to end up changing it per driver so they're all at the exact middle range. The further you deviate from wherever perfect is the worse things get, so a medium weight driver would benefit the most.

Agreed, I would try and optimise it for the way the vehicle is used. But as part of that I'd've also have tried to balance the mass distribution and corner weights as best I could.

As an aside, been bemused in the past by some VERY overweight people who have gone to a lot of time and expense for fractional gains, when they could lose 100+ pounds and make a much more significant contribution to the overall performance and save a small fortune on food at the same time. One chap with a motorcycle would have been a prime example.

While we setup the car with a driver weight, we also note the values without the driver. Now I can setup a 1300 lb 2-seat sports racer, knowing that I just need to leave the left front heavy by 12 lbs (relative to the right front). On single seaters, we don't even bother. We do all setups with no driver -- all ride heights, etc are measured without the driver or fuel in. All changes made when the driver is installed (ie, in the pit lane), are just done by turns of a pushrod adjuster, or toe link.