View Full Version : coupe questions about weight transfer/distribution

01-22-2003, 09:13 PM
1st question is about battery relocation placement.
Assuming no spare tire, what would be better placement for the battery? a) Above the (what is it, evac?) on the shelf part opposite the side where the jack is kept. or b) 6" lower in the spare tire well? If in the spare tire well, front or back? Left or right?

I originally had it on that shelf as far back as possible. My thinking was to get the weight to the very back in order to increase the amount of weight taken off the front...closer to 50/50 bla bla bla. Also to offset driver weight, getting the fl and rr corners closer to the same load.
BUT, the downside is that the weight is a little bit higher. Also someone mentioned it causing a 'pendulum' effect being so far back. they relocated theirs in the middle of the spare tirewell as far forward as possible in order to get the weight over the wheels.
Now I have no clue where to put mine when I relocate again (buying new hardware to do it, stuff i had was ****ty)
Any thoughts?

2nd question, and somewhat related to above...
When removing/adding weight to one end of the car or another, how does it affect the handling characteristics of the car.
Example: If I remove 30lbs from the rear, adding no more to the front (even though the % of overall weight is now higher at the front) would the car be more prone to understeer or oversteer than before?
Thoughts I had on this are: 1) it will tend to oversteer more because there is less weight keeping the wheels stuck to the ground. 2) Then I though, well that’s dumb. It would cause it to oversteer less (understeer more) because there is now less inertia pushing the tail around and higher % of load on the front tires.
So, I’ve done confused myself. Which is it? Does it depend on the situation?

I actually had one more, but now that i've typed all that, i don't remember what it was.

Any imput is greatly appricated!

01-22-2003, 10:18 PM
I've constantly been asking myself the same battery relocation question. I've decided that I'm either putting it behind the passenger seat (prob. on the floor pan where rear passengers' feet would go... if I had rear seats) or on the "shelf" opposite the jack. In any case it would have to be (in order to comfort me) 1/ behind the driver and 2/ to the right of the driver (offset driver weight).

As for taking weight from rear/front, I'm pretty sure the car would tend to understeer. It's (AFAIK) partially attributable to FF cars being hard to "toss around" without a ridiculously beefy rear sway bar or without putting spare tires/sandbags in the trunk.
This is also why I feel bad for taking out the spare tire, sound deadening (from rear only), rear seats and rear interior panels. I feel like I really have a responsibility to relocate the battery, put in a pair of carbon-kevlar seats and dump the A/C compressor.

Now that you bring up the issue of lighter rear end meaning less grip meaning more oversteer vs. rear being lighter and having less inertia to rotate around... that's a very good question.
I'm 99.9% sure a lighter rear end results in understeer, but now I'm confused as to why, too. Damn you! :p

01-23-2003, 07:49 AM
My unscientific thoughts:

1) weight removal is always beneficial, even if it hurts the car's balance.
2) weight placed at the rear will act to increase the polor moment of the car, making it more prone to oversteer - especially the undesireable "snap spin." Personally, I'd move it toward the cg. Also keep in mind when you are counteracting driver's weight that battery relocation is illegal for any reasonable racing - and is useless for HPDEs (b/c you usually have an instructor w/ you).
3) in your 2d question, you are talking about two different types of oversteer. Reducing the weight over the tires will reduce power on oversteer for the reason you stated. But, increasing (or keeping) weight over the rear tires should enhance the "pedulum" effect/ polar moment, resulting in increased lift throttle oversteer. Generally, I think increasing the polar moment is a greater concern that keeping the power down. ESPECIALLY w/ an NA KA.
4) moving more weight over the front tires is bad in any event. That area already carries a higher amount of weight. Also, the front tires do the majority of the braking and all of the turning. Generally speaking, weight plus use means the tires are more prone to overheating. Overheated tires don't grip - understeer.

Hope I helped - others feel free to show I'm wrong (I didn't think much here)

01-23-2003, 08:38 AM
In my mind, the weight priority goes like this:
1. lose weight
2. get the cg closer to the ground
3. get the cg towards the center of the car

So, having said that, how much extra weight have you added to the car in the form of 0 gauge battery cable, box, brackets, etc.? Something to think about.

01-23-2003, 08:47 AM
i moved my battery to the trunk. i have an STB so i cant put it directly over the rear axle like i wanted to (inbetween the shock towers, improving rear traction and taking weight away from out side of the 4 wheels). so instead, since it isnt a drycell ( yet :) ), i moved it directly behind the rear right shock tower, thinking that im offsetting me, as a driving since i sit on the left, and balanced it out with the battery on the right. when i switch to a dry cell, i plan on moving it into the rear right passanger seat. that should move more weight near the CG. :)

01-23-2003, 09:54 PM
1) weight removal is always beneficial, even if it hurts the car's balance.
Gonna have to disagree with you there. The car will be more predictable and easier to drive if it is properly balanced. This will translate into faster lap times even if the car is heavier. You want to keep any weight that is moved in between the axles. Putting weight all the way at the rear will cause a pendelum effect.

Also keep in mind when you are counteracting driver's weight that battery relocation is illegal for any reasonable racing Actually it's legal to relocate the battery in every SCCA Solo II class except stock.


01-23-2003, 10:26 PM
LanceS13 makes a good point about the weight actually added by relocating the battery.

IMO the more elegant solution is to use a lighter battery. Tractor batteries work... Miata batteries also look like a nice inexpensive solution (versus a special racing battery). Miata batteries are tiny, light, and just as an aside - actually rear-mounted in the Miata.

01-24-2003, 05:59 AM
battery weight = ~35lbs
weight reduced over front wheels ~35lbs
weight increased over rear wheels ~35lbs
total weight difference being ~70lbs, makes a big difference in feel.
weight added from wireing and hardware ~15lbs nearly evenly spread over front/rear, 7.5lbs added to each end, mostly on the passanger side.
If mounted in the spare tirewell, I haven't measured it but it appears to be slightly closer to the middle (f/r) of the car than in it's original mounting point, definitely in the left/right middle of the car and also sits slightly lower.
Also allows for a cool intake ;)

After thinking about it, I think the reason the difference I felt was so huge was because of the pendlum effect...also I thinkit was mounted higher than originally, making the rear come around even more.
So when I remount the battery in the trunk, horrozontally in the spare tirewell as far forward as possible.

"weight removal is always beneficial, even if it hurts the car's balance."
i'd agree with that. Sure you don't want 95% of the weight over the front...but for all practical purposes, i can remove every ounce of weight possible from the rear (already did) and not drop more than 60lbs
The reason for the battery relocation was to reduce the overall weight of the car w/out changing the original ballance...doing this and only adding 15lbs total...I still come out on top.

The handleing makes since to me now too...i think :)
more weight on the rear will keep the tires from spinning but will increase inerta. I don't have to worry about my tires spinning so I should only worry about inerta. Making reduced weight in the rear increase understeer on a stock ka.

I think my car is already in SM because the rules are picky and I didn't feel like modding my car in acordance to them :) if i do auto-x it'll just be for ****s and giggles.

Thanks for the help guys! :cool:

01-24-2003, 06:41 AM
To determine the best place for the battery, I think you need to put the car on scales. Without doing this step your are completely guessing as to where it should go by "feel". I'll have to bring my notes from last season to work with me, I could give you what my weights were, may give you some ideas, although my car is pretty well stripped down compared to a streetcar.

I think the concern of making the car tail happy, shouldn't be too much of a concern. Think about this. If we put you in your car and covered the gas gauge and sent you out for some hot laps, not telling you if the tank was empty or full, do you thin you could tell? Most likely not. That tank being full or empty is a bigger weight loss/gain. A full tank of gas is more than 80 lbs. I doubt that you have noticed that the car becomes "loose" with an empty tank of gas, or vice versa.

Concentrate on getting the car balanced AND reducing weight at the same time. Relocate a smaller light battery, after you have determine where it needs to be with the use of scales. You can only corner jack so much, the only way to really get your coner weights perfect is by moving stuff.

Last comment, if you do relocate the battery inside the cockpit or have it mounted in the trunk with no back seat (there is an opening between you and the trunk) get a sealed battery and mount it in a box. Normal batteries are vented, they are full of acid and bad things can happen if they are overcharged or shorted out.

with love, bubb rubb