Design Height of Axle

[URracing]

When designing your suspension geometry, what height is usually used for the centre of the axle?

We plan to run 20.5 x 7.0 x 13 Goodyears for both wet and dry tires. I am assuming 20.5" is the TOTAL overall diameter of the tire (including tread) and everything, once inflated to maximum recommended pressure, with no load. Now technically your centre of the axle would be at 10.25" but theoretically, when the tire is loaded with the weight of the vehicle, the axle would drop closer to the ground. According to our CAD, we cannot exactly determine the total vehicle weight until it is actually built but we are able to make a fairly close estimate.

I measured the front wheel on one of our old cars (weighs approx 830 lbs) where they used 20.5" tires and with the weight of the car, the axle was sitting at approximately 9.75" off the ground. This means the centre of the axle dropped about 0.5" when it was loaded.

Is this the height I use when doing my static suspension geometry to find my role centres and all that other wonderful stuff?

TIA,

Brett

[URracing]

When designing your suspension geometry, what height is usually used for the centre of the axle?

We plan to run 20.5 x 7.0 x 13 Goodyears for both wet and dry tires. I am assuming 20.5" is the TOTAL overall diameter of the tire (including tread) and everything, once inflated to maximum recommended pressure, with no load. Now technically your centre of the axle would be at 10.25" but theoretically, when the tire is loaded with the weight of the vehicle, the axle would drop closer to the ground. According to our CAD, we cannot exactly determine the total vehicle weight until it is actually built but we are able to make a fairly close estimate.

I measured the front wheel on one of our old cars (weighs approx 830 lbs) where they used 20.5" tires and with the weight of the car, the axle was sitting at approximately 9.75" off the ground. This means the centre of the axle dropped about 0.5" when it was loaded.

Is this the height I use when doing my static suspension geometry to find my role centres and all that other wonderful stuff?

TIA,

Brett

[nickerss]

Brett,

I may be misinterpreting what you're saying, but the height of your axle has nothing to do with your suspension geometry (instant centers, roll centers, etc). Your inboard chassis a-arm pickups and your ball joint locations on your upright determine all of that.

[Brian Evans]

He must have a solid axle with no locating members (panhard rod, etc...). In that case, the height of the axle is the height of the RC. Be fun to watch, if nothing else...

[Marshall Grice]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> Your inboard chassis a-arm pickups and your ball joint locations on your upright determine all of that.
</div></BLOCKQUOTE>

The ball joint locations are directly affected by change in axle height.

The short answer to the OP's question is yes, use the most accurate dimension you can come up with for doing suspension calcs. Keep in mind that the calcs end up being nothing more then ball park figures to start from. So do your best to get good numbers then build/test it.

[Marlin]

He is asking for loaded wheel centerline. Which obviously depends on the load, the tire, and pressure.....

[URracing]

Sure. Let's go with that. By spindle centre I meant wheel centerline which to me is the same as the centre of the axle/hub/spindle/wheel.

Before the discussion gets too far on the wrong topic, yes, we are using unequal length wishbone suspension, and yes, the reason I ask is because the a-arm connection to the upright is determined where the "centerline" of the wheel will be.

Obviously this point will change kinematically as the tire heat and rolling radius will change from static, but I was just curious to see what a good starting point may be. You can simply tell that a good starting point is necessary since designing the spindle to be at exactly half the diameter of the spec, this will drop the ride height a half inch from designed height as soon as the car is dropped to the ground.

[Pete Marsh]

Brett,
We use the same tyre and your estimate for loaded hieght seems reasonable to me.(is your car around 450 - 500 lbs?) As long as your aware the tyre has a spring rate that you still don't know taint your results acordingly....

Pete.