Just dropped 40 lbs from the rear of the m3

[ChewToy]

Hey guys. I just dropped 40 lbs from my m3 and would like to hear some thoughts on what I can expect. Do you think it'll make a NOTICEABLE difference while I'm autoxing? If so in what way? Acceleration? Grip? Overall handling? More Understeer? More Oversteer? Please EXPLAIN why you think the way you do. I've been racing with this custom bass box for the past 2 years, and now that I'm a little more competitive I figured I might as well prep the car the best I can... and reducing weight I felt could only help.

I've been wishing for a little more oversteer and I'm thinking that dropping the 40 lbs that was sitting right over the rear shocks might help in this dept.. maybe tooo much. What do you think? I'll find out at the next race Sunday.

Anyway I was just looking for some thoughts from the fellow enthusiasts on the pros and cons of what I just did.

Thanks for reading.
///Matt

Click Here to see pictures of the bass box... which incidentally maybe for sale soon

[SI///M3]

I had a sub+box+amp in my trunk for about 3 weeks until the extra weight just got on my nerves to much and I had to remove it. When I'm autocrossing I've tried both with and without a spare tire and the car definatly feels more responsive without it. It feels a tiny bit more oversteer prone, but nothing super dramatic. If your class allows it take the spare out too, weight is the enemy.

[badmonkey]

FWIW, I recently sold my Vaders, installed Sparco shells, the car weighed 3040lbs with no changes (other than aftermarket exhaust & testpipe). Removed back seat brought it to 3000lbs, dropped the spare tire it went to 2960lbs. Weight the car before any mods, it was 3175lbs as advertised. I figure it can get down to 2900lbs with a little more work. Totally changes the way the car feels.

[megatron]

BadMonkey, 2900 should be doable. I got down to 2880 in SCCA StreetMod trim by doing the following:

No spare tire, No Back Seat, Sparco Pro 2000s on Speedware mounts, 11lb Hawker Battery (got for $25 ), VIS Carbon Hood, Dinan Exhaust, Hoosiers on Kosei K1s. This was with the fuel light just about to come on. 2700's should be achievable with fenders and Al doors.

I figure for street use, with the fenders and Al doors, and with composite back seats, composite sunroof panel and LTWs or some other lightweight wheel will net a car somewhere in the high 2700s to low 2800s. At that weight, we only need about 245fwhp +~5-10% (not as meaty a torque curve) to have the power/weight of an e46.

Edit: Also took out rear carpeted panel above spare (about 10lbs)

[megatron]

ChewToy, as in the answer to this question you asked in the other thread, you'll likely pick up a bit of mid-corner push but the car may turn in better.

Reason being you move weight out of the back, the CG moves forward and the front wheels now have a shorter moment arm to rotate the car about the CG. The front tires apply force along wheel axle axis, towards center of turn. Torque at CG trying to spin the car is (in ideally smooth roads, assuming identical wheel rates front/rear) this force times the moment arm distance (distance from front axle axis to CG). The car is kept from spinning by the rear tires applying an opposing torque, but since the front moment arm is now shorter, and the rear moment arm longer, the front torque is less, the rear torque is more, the car will want to rotate less.

This is at midcorner - ie: "balanced throttle". With less weight in the back, at initial turn in your weight dist is skewed more towards the front, so the car may actually turn in better.

40lbs may not be enough to really notice (like 1.5% of the car weight). But pull out the spare tire, the carpetted trunk panel above it, and put in an LTW battery, and it will start to be noticeable to the point where you may need to adjust F/R bar settings, damping or spring rates to get it back to where you like it.

[Erik@EDGE]

careful removing too much without having the ability to weight jack to get the car closer to balanced. Using the Dynabatt with nothing else in the trunk, the passenger rear corner is WAY light. Took me about 10 times on and off the scales to get it set right. I honestly think that if your suspension isn't height adjustable you may hurt the overall balance with too light of a battery. My $.02

[badmonkey]

Originally posted by megatron


40lbs may not be enough to really notice (like 1.5% of the car weight). But pull out the spare tire, the carpetted trunk panel above it, and put in an LTW battery, and it will start to be noticeable to the point where you may need to adjust F/R bar settings, damping or spring rates to get it back to where you like it.

Which is why you remove the AC

[megatron]

if you do decide to do an ltw battery, don't buy the dynabatt. it's good, but it's overpriced. it's a rebadged hawker g16ep. you can get a g13ep (same stats and dimensions but 13AH instead of 16AH) for 25$ from a surplus source i ran into when doing robot stuff. send me pm if you want info on this. i'm running that 25$ battery in my car and my friend in his se-r and it works great!

[Cannon]

Or buy a LTW..

[ChewToy]

I had a great day autoxing yesterday, and even though I finished 2nd in SM I was very pleased with the way the car was handling. I don't know if it was the course, my driving or the 40 lbs I just dropped in the back of the car, but it felt really good all day. I'm still curious how much of a difference dropping 40 lbs from the rear helped. It seemed it helped the rear end rotate around a little easier but maybe I'm just imagining things. For those wondering how I lost 40 lbs I had a bass box that was in the car for the past 4-5 years. It was a complete PITA to install so I was reluctant to remove it. Well this past Saturday I said the hell with it and ripped the b@st@rd out.

On another note, I've been thinking about switching my 3.15 differential for a 3.38 but after yesterday I've got mixed feelings. I was hitting the rev limiter (which is 7200 rpm) at the finish lights. If I had a 3.38 I would have shifted to 3rd, maybe on more than one occassion. It's a trade off I'm still thinking about. Talking to Bob Tunnell, he thinks the 3.38 is the way to go even with the extra shifts. Any thoughts on this?

[ceegeezM3]

I've been running the 3.38 for a couple years, at Bob's advise too, and I agree with him. There are usually enough slow corners where having the 3.38 to 'launch' you out of the corner with will more than make up for the occasional 2-3 shift.

[Erik@EDGE]

I went 3.38 last year. I would never go back!

[qwickm3]

Speaking of weight I finally had a chance to weigh my car since the race conversion. With 3/4 tank of gas the car weighed 2786lbs. This is about what I expected. For my class the car has to weigh 3048 with me in it. I weigh 225 so it looks like I'll have to add some ballast.