Suspension Travel in Sedan Based Race Cars

[Yawpower]

How much suspension travel is "typical" in low, or no down force sedan based race cars? (ALMS GT, FIA GT, V8 Supercars, BTCC, etc.)

Is it typically split evenly between compression and droop? Meaning, if a car has 2" of bump travel, would the static deflection be in the same range, giving the car a total of 4" of travel?

Or is it typical for the travel to be greater in compression than in droop, due to the low static deflection that accompanies high spring rates?

[Yawpower]

I had no idea this post would generate so much excitement.

[DaveW]

I had no idea this post would generate so much excitement.

Probably tells you that there is no simple answer to your question, or you have asked the wrong question.

The short answer is that the c.g. should be run as close to the ground as possible, but that requires many caveats involving (for example) driver preferences, engineering preferences, vehicle layout, power train, suspension design (& friction), springs, dampers, "installation" stiffness, tyres, aero, the track, race regulations etc.

Arguably, a multi-post rig test can help to understand (at least) the influence of vehicle layout, suspension design, friction, springs, dampers, "installation" stiffness, & tyres.

[Luke]

How much suspension travel is "typical" in low, or no down force sedan based race cars? (ALMS GT, FIA GT, V8 Supercars, BTCC, etc.)

Is it typically split evenly between compression and droop? Meaning, if a car has 2" of bump travel, would the static deflection be in the same range, giving the car a total of 4" of travel?

Or is it typical for the travel to be greater in compression than in droop, due to the low static deflection that accompanies high spring rates?

I believe in the categories you mentioned there is a big variation in suspension travel used. ie FIA GT versus v8supercars are hugely different. mainly because the fia gt cars are homologated with set spring rates, and these spring rates are set based on european track which are generally much smoother than those experience in australia.

even series that have open spring rates like v8supercars you will find a big difference track to track of travel used due to the requirements of each track. ie from 100mm total travel at bumpy tracks to 50mm travel at smooth tracks. more bump than droop, as generally circuit cars aim to stay on the ground, and they have anti roll bars that limit the droop on the inside wheel anyway so having more droop from the suspension isnt necessary

[Yawpower]

I believe in the categories you mentioned there is a big variation in suspension travel used. ie FIA GT versus v8supercars are hugely different. mainly because the fia gt cars are homologated with set spring rates, and these spring rates are set based on european track which are generally much smoother than those experience in australia.

even series that have open spring rates like v8supercars you will find a big difference track to track of travel used due to the requirements of each track. ie from 100mm total travel at bumpy tracks to 50mm travel at smooth tracks. more bump than droop, as generally circuit cars aim to stay on the ground, and they have anti roll bars that limit the droop on the inside wheel anyway so having more droop from the suspension isnt necessary

Thanks Luke.

The cars I have work with in the US have 50mm to 65mm bump travel, with droop travel effectively determined by the spring rates. With a quarter car based ride frequency of 3Hz, which might be typical at the front, that gives approximately 30m of droop travel.

I have recently wondered if the best use of available suspension range over rough surfaces would come from using equivalent amounts of bump and droop travel.

This would require a decrease in spring rate, and a corresponding increase in compression damping to keep the suspension working within the previously stated 50 to 65mm bump travel.

Of course, nothing is quite this simple, but it is this idea that prompted my question.

[Jersey Tom]

I have recently wondered if the best use of available suspension range over rough surfaces would come from using equivalent amounts of bump and droop travel.

There's a difference between having available travel, and using it. There's always the mechanical grip vs. platform control trade off (for more than just aero). But so long as you're not hitting a travel limit (in compression or extension), then perhaps it's best not to think about too much.

[Yawpower]

There's a difference between having available travel, and using it. There's always the mechanical grip vs. platform control trade off (for more than just aero). But so long as you're not hitting a travel limit (in compression or extension), then perhaps it's best not to think about too much.

I would say that if you have available bump travel that you are not using, the ride height should be lowered.

[Lycoming]

Why? it's not like you're wasting anything. Ride height should be dictated by factors other than a desire to use every last mm of bump travel you have.

[Yawpower]

Why?

CG Height.

[Greg Locock]

"I have recently wondered if the best use of available suspension range over rough surfaces would come from using equivalent amounts of bump and droop travel."

In production cars the answer is... it depends. Obviously we don't worry about aero. Generally the problem is getting the power down on rough roads, especially coming out of corners, so the tendency is to use soft springs on the driven axle, and perhaps surprisingly, more rebound than jounce travel. I assume we are talking RWD, in which case you then restrict the front suspension rebound to encourage limit understeer.

[Yawpower]

"Generally the problem is getting the power down on rough roads, especially coming out of corners, so the tendency is to use soft springs on the driven axle, and perhaps surprisingly, more rebound than jounce travel. I assume we are talking RWD, in which case you then restrict the front suspension rebound to encourage limit understeer.

Yes Greg, I am referring to RWD. Do you use more rebound than jounce travel because it helps to achieve the stated goal of putting power down on corner exit, or is that where it ends up after other priorities are attended to?

Thanks.

[Greg Locock]

That I don't know. I'd add our more powerful variants tend to have lower ride heights, so by default they have even more rebound travel than standard. But with higher rate springs the probably hit fully drooped more quickly. Sorry, that's all i know.

[DaveW]

I believe in the categories you mentioned there is a big variation in suspension travel used. ie FIA GT versus v8supercars are hugely different. mainly because the fia gt cars are homologated with set spring rates, and these spring rates are set based on european track which are generally much smoother than those experience in australia.

A suspect that the main reason for the difference is tyres. Last time I looked, GT3 ran on tyres that were almost 50 percent stiffer than V8 tyres, which implies that GT's will support higher spring rates.

GT's are homologated, but do have some scope (I think). They often run "helper" springs, mainly to ensure that the wheels move clear of the bodywork for wheel changes.

One issue with the V8's is the back axle (150 kg, I recall). Curiously that limits the springs (too soft looses control over the axle).

[PhillipM]

and perhaps surprisingly, more rebound than jounce travel.

Most Safari and Dakar racers are the same, provided you have the luxury of enough travel to go with the required ride height for clearance.

[DaveW]

I'd add our more powerful variants tend to have lower ride heights, so by default they have even more rebound travel than standard. But with higher rate springs the probably hit fully drooped more quickly.

It will (hit fully drooped more quickly). The normal solution is to cause the damper to extend fully by inserting a "helper spring", a low rate spring that is designed to collapse fully, see here, for example.

The two springs can be active together, of course, but life will be much simpler if the helper is designed to be fully compressed in normal operation.

p.s. The "headroom" (available compression stroke) required will depend on spring rates and damper settings (and on other things, normally taken care of by bump rubbers). Road vehicle dampers are normally rebound biased. This helps "ride", but requires more headroom (higher c.g. height). Conversely, race vehicle dampers are normally compression biased. This hurts "ride", but keeps the wheels on the ground better, and requires less headroom. Regulated ride heights can (and often do) affect the distribution of damping.