Ballast Positioning Effect

[dradernh]

I would stick the ballast near/under rear diff...

FWIW, the rear subframe will hold ~50 lbs. of lead.

[ETM]

I deal with 3 cars that have fuel cells where the spare wheel tub was once located. All three are faster around the track with 3/4 of a tank of gas than they are with 1/4 of a tank. They just make more grip exiting the turns allowing them to get on the gas harder and sooner.

I think polar inertia can be a bigger factor in quick transitions (autocross slaloms) than it is on your typical road course. In a practical sense it is not the defining issue to going fast on a race track but is instead one factor in a long list of tradeoffs. In my above examples the increased polar inertia is not noticeable while the additional traction exiting turns is obvious to both the driver and the stopwatch.

Of couse YMMV. Luckily experimenting with ballast placement in a test session is one of the cheapest and easiest tests you can do to determine what your car and driving style prefer.

[JS154]

I tried a number of different ballast locations.

I do not like the way the car behaves when there is more weight behind the axle. Grnated, I'm in a E30 with a stock fuel tank, but even driving on the track with a trailer hitch and full fuel tank attached, the car was noticably more tail heavy (especially once the tail started to get happy either by choice or by chance). ALso consider which tracks you;re running at. At Lime Rock I want a car that is flickable. At Watkins Glen I want a car with more rear grip so I can get on the gas sooner.


I've never been too too concerned about cross weights, close enough and it;s good enough for me. No track has an equal number of right and left turns anyways, so I try to get cross weights close enough, set the car up for the most important turns and let the front-rear balance fall where it does.


Ballast is easily moveable, so try bolting 50# on the floor, do some laps, and then move it to the spare tirewell and do some laps. The stopwatch will tell you what to do.

[TXBDan]

What's the current state of thought here? My car is currently pretty well balanced, good cross weights, 51f/49r, handles fine. But i need to add around 150lbs to it. I plan to use a mix of gas in the stock gas tank, lead bricks behind the passenger seat on the floor and 45lb plates in the spare tire well.

I've had more than a few fast E36 drivers tell me to put it all in the spare tire well. I've also heard PTG used to run a 49f/51r weight distribution (of course that doesn't mean weight it was in the spare tire well, but still). Then of course, we all "know" we want the weight between the axles.

How would you guys divvy it up?

[Def]

I'd put it all in the back. Maybe also put some weight on the rear subframe to get the CG down a tad more.

My S13 has a rear mounted gas tank stock, and it seemed to be more balanced in all phases of cornering with a full tank vs. going down about 8-9 gallons. That's a lot of weight (~50-60 lbs) on a sub 2500 lb car, but more weight on the rears seemed to help it anytime it was turning. Car at ~1/2 tank of gas was around 52.5/47.5, so it probably was around 52/48 with a full tank.

[Spec3HOR]

Wow this is old!

Tested both ways, I VASTLY prefer weight between the axles. Weight behind the axles created more "swing" (once the car started sideways it wanted to stay sideways more instead of coming back). Purely IMO and in my own testing.

[TXBDan]

Thanks for the input. Yeh, i'm having a hard time with intentionally defying physics. Maybe behind the passenger seat is best. Back, but in front of the rear axle.

[19Spec3]

Wow this is old!

Almost as out-of-date as your bimmerforums profile info and signature photo!