Does ride height effect fuel consumption.

[allmotor8v]

Was curious fo the effects of ride height on an auto in relation to fuel consumption. I am trying to resolve an argument and would like some solid info to back it up. I am thinking that, on an average car, a lower ride height would result in a better mpg rating. Resoning: more air going over the car as opposed to going under it.

Opinions?

 

[btrueblood]

I'd argue the opposite; lower downforce on the car (higher ride height) reduces both aero "induced" drag and wheel/bearing losses. The most aerodynamic cars try to reduce ground effects as much as possible (have very aero-looking undersides)... but these cars also tend to get a little light at high speeds.

 

[evelrod]

I can't address fuel mileage per se.(never worried much about it)--- However I can say that in several tests done on the back straight at the old Riverside International Raceway in the early 80's we determined that by lowering the front of the car initially and ultimately the entire chassis resulted in higher indicated speeds. Simply lowering the front of the chassis, in this case a very non aero Lotus Cortina, resulted in speed increases but tended to upset the overall handling in the high speed turn 9 that followed the straight. It took a lot more work to balance things but, in general, the consensus was , "the lower the better" for both top speed and handling. Less overall drag and lower CG (There is a LOT more to it than that, but for this thread, it should suffice).

Rod

 

[GregLocock]

"would like some solid info to back it up. I am thinking that, on an average car, a lower ride height would result in a better mpg rating. Resoning: more air going over the car as opposed to going under it.

Opinions?"

So, do you want opinions or solid info? Evelrod has given you some solid info. I'd agree that that is the trend, for a normal car. However, there is a trade-off. As you reduce the ride height there is less air that is being forced into the venturi. But, the venturi has a narrower throat, so you accelerate that smaller amount of air by a greater amount, so it has a higher speed, which causes higher drag losses, as it passes amongst the underfloor components.

P530 in Milliken shows a wind tunnel experiment that shows Cd INCREASING as the ride height decreases.

As in most real engineering problems, it is a compromise.





Cheers

Greg Locock

 

[evelrod]

May I add that my tests were done with a front air dam that reached to the track surface. The dry lakes boys rake their cars for higher speeds, you know, nose down, ass up.
Can't say as I am in agreement with that approach. It goes counter to my experience with road race cars.

Rod

Old age is where you still have something on the ball but are just too tired to bounce it.

 

[GregLocock]

Rod, check out that graph, it also shows the effect of angle of attack.

FWIW in the weird and wonderful world of solar car racing we found that you could trade ride height off against angle of attack - my guess is that we were probably tuning for ZERO lift, and hence induced drag. The change in skin friction drag is relatively unimportant in that application.



Cheers

Greg Locock

 

[mloew]

C_d can increase with lower ride heights because in most vehicles with non-optimized (dirty) underbodies as the air tends to “pile up” under the car. However, the frontal area will decrease with the lower ride height (less tire exposed and the effective blockage of the air underneath gets counted here as well). Drag is a function of the C_d times the effective frontal area. Drag is proportionally related to fuel consumption, all other variables being equal. So the answer is (as it typically is with multi-dimensional calculations) “it depends”.

Hope this helps.


Best regards,

Matthew Ian Loew


Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[AdrianSonelly]

There was a study done on a road going passanger car shape. Baseline front ground clearance 300mm. Adding simple flat sheet airdam/spiler showed that decreasing body to ground gap changed the front Cl significantly. It also reduced overall CD up to a point. Spoiler depth of 90mm showed Cd reduction of -.04 while Cl change was huge - -0,21. After this point and up to 170mm spoiler depth Cd got back to baseline value but Cl(f) kept improving slowly and showed -0.25 cnange. It was still improving till 250mm where it leveled out -0.27 cahnge from baseline value. Cd increased by 0.04.
This are hard values BUT for that particular body shape. How ever it does represent general trends very well. One point to remember - when Cl(f) was improving Pitch moment was changing also and Cl(r) was degrading. If one asks me what is more important overall Lift (hopefully negative) or aero balance - I'll vote for balance.

 

[Rob45]

Adrian:
Was your testing done in a windtunnel, or on a road?

Because if your windtunnel doesn't have a moving roadway, you're going to get some misleading results.

 

[AdrianSonelly]

Rob,
This wasn't my study It's just a SAE paper that seemed to answer intitial thread question.
On the rolling road in the WT: It's a matter of cars right height. We don't whant boundary layer to get thick enough (in model area) so it will effectively reduce the body to ground clearance. Moving belt under the car (there are some other ways to deal with BL build up) does help to reduce this problem. It's not that easy thought as there many other problems arasing - like suction of the belt to the model for example in case of high downforce produced by underbody of model.
How ever it is assumed that with normal street car (ride height over 100mm) BL build up is not an issue.
If you study a race car (ride heights 60mm and lower) moving road is eccential.
If you're in between than it's up to you

One thing to remember - by it's nature wind tunnel is not giving you absolutely correct numbers - being within 10% of on track measured data is considered good.

Sorry for my English
Thanks

 

[downforce]

One thing to consider if the DOWNFORCE of the 4 tires. Each tire spinning at 500RPM (approximately 37MPH) can generate as much as 30-60lbs depending on the wind conditions. The most important aspect of aerodynamic data for MPG is definitely the Ooefficient Drag and Frontal Surface Area. The faster you go the more those two factors will limit you.

But to answer your question... Which is better for MPG, I would say lower. Yes it creates more DF, but weigh is not too much of an issue, because you are slipping through air more efficiently. With all cars except high end sports cars and racing cars with complete underbody diffusers, lifting your car up will cause so much turbulence that it will drag your car back like a dog on a leash.

Now for my favorite part.... everything above is to be taken for entertainment purposes and I could be wrong so please do not regard anything I write as a fact. Just trying to help.