acceleration and weight

[iknowwhatidontknow]

I am trying to determine the difference if any in the performance of my RX8, weight= #3100 vs a base model at #3050 for a 60 second autocross. I can't see much of a difference. I understand the numerous variables, ie COG, tire grip, lateral motion, braking, etc. but should the difference in time be noticeable like some of my racer pals are telling me?

 

[MintJulep]

50/3050 = 1.6%

If you make the crude assumption that in an autocross you are accelerating 1/2 the time, that's half a second in a 60 second run.

 

[GregLocock]

But if your cg was lower than theirs then you might see a smaller differential.

Cheers

Greg Locock

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

 

[safetydan]

Based on what conditions you are giving, weight difference of 50#, in autocross racing, you probably will not notice a difference. Theoretically, there is a difference of 1.6% if power of the cars is the same, same driver, same conditions, but given the scope of your racing, you probably won't notice a difference. If you were professional drivers with a lot of experience, then you would notice a difference, but it would be slight.

 

[NormPeterson]

If you could keep the car at maximum acceleration in the optimum direction for the whole run, you might pick up 0.7 second. That's based on grip being approximately an 0.7 power of vertical tire loading (IIRC, that correlation comes from one of Greg's posts a while back).

But since transitions take time, and because most of us can't stay on the friction ellipse for that long or trace precisely the same line from one run to the next, you probably won't see anywhere near that much. If your run times are scattered over a two second range without generally showing improvement, you wouldn't notice it. And if your car preparation isn't otherwise apporaching the allowable limits for your class/category I wouldn't worry about it yet.

That said, it definitely is a step in the right direction, as 0.100 seconds will show up in the results even at local or regional events (I should know - the last time out I was 0.06x second behind the next higher position).


Norm

 

[NormPeterson]

I owe you a correction, as I dropped a SQRT function from all of that. So in 60 seconds you could theoretically expect something more like 0.34 seconds on a 60 second run.

Sorry.

Norm

 

[evelrod]

Weeeell---If I dropped 50 lbs from my race car, I can guarooontee a major improvement in lap times!

(This assumes, of course, the 50 comes from around my middle)!!!:-(

Rod

 

[patprimmer]

Good point Rod

I know a boat racer here who was drilling the centre out of bolts to save about a gram per bolt. He was 30 Kg overweight himself. A diet and exercise would have had a much greater improvement in performance and health.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[evelrod]

A friend and super fast Mini Cooper racer from Canada and I are the same age (67) and same weight (220 lb.)...the standard joke is that if either of us really wanted to go quicker we would simply use a hand pump to inflate our tires!!!

Gotta run, Pat....my ice cream is melting!

Rod

 

[MacGyverS2000]

I simply changed out my heavy stock exhaust for a stainless steel/titanium one, replaced the cat with a straight-through pipe, removed the spare tire, junked the heavy stock struts, etc. If I really wanted to get nasty, I'd rip out the carpet, extra seat, A/C, replace the convertible top and motors with a hardtop, etc... but I like having a fast grocery getter and eating ice cream


Dan - Owner

http://www.Hi-TecDesigns.com