Correlation between lap times and weight loss

Any idea on a general rule of thumb on what the relationship is between lap times and losing weight?

Say for example, I dropped 5.45% of my weight - and went from 2750 to 2600. What percentage drop might I expect in lap times? It can't be a direct correlation, is it closer to half the weight drop percentage would equal the drop in times? For example, .5 x 5.45% = 2.72%, so on a 1:50 lap a 2.72% drop in time would be 1:47. That seems too drastic though. There are a million variables, I'm just looking for a general idea for an IT type car. I think I've heard something like .1sec roughly for every 10lbs, that would be about 1.5 seconds which might be more realistic?

Thanks.

I would love to have some engineers drop an opinion on this.

I think that the POSITION of the weight is much more key. Some of my fastest lap times have come over 40lbs over min with weight better placed (by accident - more gas than needed - tank in nice spot). I even move my ballast between two positions depending on what track I am at and the 'centricity' of the turns.

IT is limited in how you can 'balance' your car but I think that is much more key than overall weight. If I were building a car from scratch that didn't have 50-50 weight dist from the factory, I would really try and build my cage to suit my needs as best as I could.

As far as direct percentages - weight is just like HP, it's doesn't affect lap times in a limear fashion...it helps in a small but incremental way. The goal is to add up as many incremental improvements as you can to get yourself a good platform to actually try and drive.

(On edit: I bet that some SWC teams could pull some of their 'rewards weight' data and give you an answer. I am betting that it doesn't kill qual times that much as it does end-of-race performance as the weight takes it's toll on shock oil and tire temps.)

Newton's 2nd Law applies WRT the acceleration advantage you might gain but then it gets very complicated. The lack of weight has handling and braking advantages. Unless you lose traction, you might be able to figure braking. Handling is complex, with traction, balance of forces, CG, and more all coming into play.

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Originally Posted by Andy Bettencourt

I bet that some SWC teams could pull some of their 'rewards weight' data and give you an answer. I am betting that it doesn't kill qual times that much as it does end-of-race performance as the weight takes it's toll on shock oil and tire temps.

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Not much science on how the weight is picked and results surely vary by setup more than just the weight.

REWARDS is a program which adds weight to drivers who achieve a finish in the top-three, maxing out over the season at five percent of the base minimum weight of the car. REWARDS weight is removed if the car finishes out of the top-four. The purpose of the program is to help provide the closest on-track competition between a diverse variety of cars.

Here's an quick/dirty IT version of something I did years ago.......

http://www.vectorbd.com/am-rennsport/vwsim.xls

fill in the green blocks, it calcs the rest. You can see the effects on HP req'd

It makes sense one of the million variables is also where the weight is removed from, could be all over or more front/rear. The weight I am removing is lead in the passenger floorboard, 150lbs of it. I'm curious as to what impact it will have on lap times. I guess if I had time on a test day, run with and without the ballast but that will throw off the setup, so again more factors. Unfortunately my setups are done by a shop and not by myself at the track (no scales).

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Originally Posted by JimLill

Here's an quick/dirty IT version of something I did years ago

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Hmmm.... may have been 'quick', but it certainly doesn't appear to be very 'dirty'. :) I made adjustments to a couple of formulae to closer fit my car (frontal area & driveline loss), filled in the necessary blanks to include some 75-90 mph accel data I was comfortable with and... Voila! Calculated flyweel HP was very close; I'd say within 3 percent or so.

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Originally Posted by Gary L

Hmmm.... may have been 'quick', but it certainly doesn't appear to be very 'dirty'. :) I made adjustments to a couple of formulae to closer fit my car (frontal area & driveline loss), filled in the necessary blanks to include some 75-90 mph accel data I was comfortable with and... Voila! Calculated flyweel HP was very close; I'd say within 3 percent or so.

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Thanks for that feedback...........

Now knowing someone actually used that.......... I improved it a bit

and here's a better version yet.........

http://www.vectorbd.com/am-rennsport/hpcalc.xls

Enter the data in the green boxes...

I couple of years ago I spent an incredible amount of time comparing the effects of weight versus horsepower on my car at Mosport. I was very lucky to be using the Bosch LapSim software which had Mosport as one of the modeled tracks. I input all the data requested in the software, including the tire grip levels, aero effects, spring rates, horsepower, torque, rear end and gearbox gearing, etc.... and asked it to spit out an expected laptime. I also have a full data logging system in my car, so i was able to quantify actual criteria closer than some people.

The number that came out was 2/10ths faster than my fastest ever race lap. It was stunning, especially when I ran the simulation as a model around the track. The gears selected were the same, the little touch of oversteer in Turn 2 was calculated and the same (in the same place). It was mind blowing. The MPH going into the turns was the same, even the braking points and duration was very very close to what I do. As the class that I was moving into allowed open engine mods and weight reduced by over 500 lbs, I worked those calculations into the simulation.

Therefore, based at Mosport in a Second Gen RX7, each 20 hp increase in engine hp was worth a second (assuming an 18% driveline loss). Each 100 lb (45kg) weight reduction was worth 1 second. Surprisingly, switching from my stock 4:10 diff to a 4:88 diff made no difference as the new requirement to use 5th in 2 places per lap negated the acceleration advantage out of turn 5 and 3. All my messing about with bars and spring rates only changed the laptimes by 2/10ths.

In using the other tracks in the simuation I noticed that the effects weren't the same. The tighter the track the less HP made a difference versus reducing weight and adding gearing.

So I guess my response to the initial question is, "it depends".

Eric Nummelin

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Originally Posted by 23racer

I couple of years ago I spent an incredible amount of time comparing the effects of weight versus horsepower on my car at Mosport. I was very lucky to be using the Bosch LapSim software which had Mosport as one of the modeled tracks. I input all the data requested in the software, including the tire grip levels, aero effects, spring rates, horsepower, torque, rear end and gearbox gearing, etc.... and asked it to spit out an expected laptime. I also have a full data logging system in my car, so i was able to quantify actual criteria closer than some people.

The number that came out was 2/10ths faster than my fastest ever race lap. It was stunning, especially when I ran the simulation as a model around the track. The gears selected were the same, the little touch of oversteer in Turn 2 was calculated and the same (in the same place). It was mind blowing. The MPH going into the turns was the same, even the braking points and duration was very very close to what I do. As the class that I was moving into allowed open engine mods and weight reduced by over 500 lbs, I worked those calculations into the simulation.

Therefore, based at Mosport in a Second Gen RX7, each 20 hp increase in engine hp was worth a second (assuming an 18% driveline loss). Each 100 lb (45kg) weight reduction was worth 1 second. Surprisingly, switching from my stock 4:10 diff to a 4:88 diff made no difference as the new requirement to use 5th in 2 places per lap negated the acceleration advantage out of turn 5 and 3. All my messing about with bars and spring rates only changed the laptimes by 2/10ths.

In using the other tracks in the simuation I noticed that the effects weren't the same. The tighter the track the less HP made a difference versus reducing weight and adding gearing.

So I guess my response to the initial question is, "it depends".

Eric Nummelin

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This is the first time I've ever seen reliable empirical data on this. THANKS!

I'm downloading the free-ware version of Lap-Sim, but having looked at the manual it looks like the engine version will allow the holy grail of prediction a-priori how any motor will react to an IT build. The lap-times is more complicated as it requires lap data to make predictions. Check it out http://www.bosch-motorsport.com/cont.../html/3589.htm

James