fatbillybob

I hope ProRaceEngineer chimes in on this.
First there are heavy things that can be moved around. Some say move the battery to the RR of the car. Yes that puts weight where you have none but isn't it better to put the battery in the passenger footwell to lower the weight about 1 ft at the lowest point in the car and place the weight by the CG, vs. having 50lbs way out on a moment arm?

If you further needed ballast to make weight for a race class lets say you need another 100lbs. On a C5 the RR is about 45lbs light left to right. What do you do? Do you put 45lbs in the RR and then 55lbs by the CG? Or do you put 100lbs at the CG and leave the RR with less weight untouched but corner balancing the car for 50/50 crossweight?

My pea brain says putting weight at the CG with a 50/50 cross is the way to go vs. "moving some weight where you need it" and then taking the balance of the ballast and putting it at CG.

Comments?

Solofast

It depends a lot on your setup and where your particular car needs the most help. I know that sounds a bit evasive, but there can be reasons why you might want the weight that might not fit with the 50% cross weight logic.

For instance, on a C5Z the left side of the car is heavier than the right, so you want to offset the weight as far to the right as you can, in particular if you are racing on most road courses since they all run clockwise. Guys running Spitfires and Spridgets in the lower production classes commonly used to convert their cars to right hand drive, to put the drivers weight on the inside and that helped most corners.

You need to look at where your car needs less weight and where it does you the most good. For instance, if you have a setup with smaller tires in the front, you might want to unload the left front (put all the weight in the RR), to save the left front from getting punished and be better late in a race.

Just getting to 50% cross weight doesn't effect the front to rear balance. You might want less weight on the front tires and more on the rears, to help you put down more power coming off of corners. In that case, you might put the ballast beyond what it takes to get a a 50% cross weight, in the back of the car.

While the 50% cross weight makes your car handle the same in both left and right hand turns, it often isn't the fastest way around a lot of tracks, since some tracks reward the ability to power off of corners, and others reward the ability to maintain speed. Serious racers have setup sheets for each track, and it isn't necessarily 50% cross weight for any particular track. You have to test and figure out what you want the car to do at that particular track and what works best for your setup.

I doubt that you would want to put the weight at the CG, I'd be putting it as far aft as is reasonable, just to help the car put power down on corner exit, and offset to the right, to help cure the natural heaviness in the left side of the car.

Hope that helps, you need to think about where the weight will do you the most good, and then try and test.

Edit..

Perhaps I can simplify it a bit more...

Let's think globally for a moment. You are thinking corner weights, but corner weights can be adjusted, the real thing to think about is the CG of the vehicle. Moving weight effects the CG, and not just the corner weights.

Right now the CG is just forward of the middle of the wheelbase, and to the left of centerline.

The question is, where would you want the CG? The short answer is you want it as far to the right as you can get it, and as far aft as you can get it to help put power down coming off of corners. Moving 45 pounds around is a lot of weight when it comes to moving the CG of the car, so you have a big gift that you can take advantage of here. The only downside comes from polar moment if you put the weight outside of the wheelbase (like the rr corner of the car) and that can make turn in a bit slower, but that's something you are going to have to decide for yourself with testing.

davidfarmer

If you put the battery down in the storage compartment in the rear, it is still very low.

This is my mentality, having raced against front engine, mid-engine, and rear engine car. I personally like a STABLE car, and so I have no problem with putting swing-weight behind the axle. This increases the moment of inertia (relative to central mounting), which will cause the car to rotate more slowly, but also increase stability. Mid-engine cars turn on a dime, but they also "spin out" a lot. I don't think there is a right/wrong, just preference.

In a Vette (front engine, rear drive) I personally like to put all the weight as far rearward, as far to the right, and as low as possible. I've hung lead below the rear bumper before, just invisible in the body work. You might have to turn-in sooner/quicker, but you'll have a car that is stable and "grippy" coming out of the corners

Again, my choice, based on my driving style......

yooper

Regarding the C5Z, does tire width and/or the front to rear stagger of concern in this discussion?

fatbillybob

The entire game of racing is managing tires so yes . But we are limited by race group rules so in this case no. So for this discussion tires are not considered.

I was surprised to find that say putting 50lbs in the right rear does not move 50lbs on the scales. There is some complicated formula that can estimate how corner weights change. I'm not smart so I'm trial and erroring moving weight around. When you put in ballast the weights on each corner change unpredictablely. So I'm learning that you have to move weight around to the compromises you want. I want a balanced car because I'm too lazy to set up for a specific track. Maybe ill change my ways. That means I want to move weight around to minimize the rf lf split for best braking and get the rr and lr as close as possible.

So how do you set up for other tracks? Wags?
Going left on oval ballast the left of car?
Lots of right turns more weight lr?

redtopz

I thought T1 (now T2) had rules for how you can run ballast. IIRC, it has to be in the passenger seat area in certain size plates with specific size bolts. Also, I think you have to run a stock size battery in the stock location. Not sure if they changed the rules for 2013?

sperkins

I always thought it was funny that some folks said that the old NASA PTA cars weren't real "race cars" because none of them (with the exception of one) had aero, yet the SCCA makes you run a dash and a stock battery in the stock location.

Solofast

You can and should balance corner weights, but that doesn't mean you shouldn't maximize the effect of where you put the weight. If there are rules that say it has to be in the passenger well, there's still a lot of lattitude as to where that weight can be put, from your toes to the back of the passenger area, big difference. As noted above, as far aft and to the right is the best place for a road course car that is turns right most of the time.

Other tracks... Depends on where the most important corners are, in terms of exit speed effecting lap time. If the corner leading onto the straight is a right it's different than if it's a left. Every track is different. Grattan has a big left hander leading onto the straight, so I could see a different setup for Grattan than for a place like Road Atlanta, were the important turns are right handers (entrance to the straight, exit of the chicane, over the hill, bottom of the hill and the end of the front straight are all rights...

Ovals... NASCAR is specific in that they limit the location of the CG, both side to side and fore and aft, it's left of center, it is specifically limited, and you can't do things like moving the engine to one side (Sorry Smokey). NASCAR also has rules on the fore/aft weight distribution too. If there wasn't an advantage in it, there wouldn't be rules limiting it, so there's for sure reasons to get it right.

If you drive mostly typical road courses, and you don't want to dink with it, you want the weight on the inside of the turn, so more right turns = weight as far to the right side of the car and as far aft as the rules allow.

I used to have a spreadsheet that you could put in the existing corner weights, the wheelbase and the track, and it would crank out the the optimum corner weights. Then I had a place where you could put in the lateral and fore/aft location of any ballast, and it would calculate a new CG and spit out the optimum corner weights for the new CG location. It's pretty easy to do, it's just a sum of the moments kind of calculation. It's pretty suprising how it all moves around when you fiddle with the location of the ballast.

RX-Ben

What happened to the magical spreadsheet?

davidfarmer

I've never had a Vette that WASN'T Nose/right-side heavy (with driver). Also every track I've been to except for Laguna Seca runs clockwise.

Putting these two factors together, right rear is where it stays for me....

As for 50lbs NOT equaling 50lbs...of course not, simple leverage. If you put 50lbs BEHIND the axle, it will actually put more than 50lbs on the rear tires, and lift weight OFF of the front tires. Since the fronts are often the most abused, this is usually a good thing. If you put 50lbs directly OVER the axle, it will indeed show up directly over that axle.

Solofast

I haven't used it in 10 years. I'd have to look to see if it was saved on an old floppy disk, the computer it was on went south. You can recreate it easily, it was just a summation of moments and weights to calculate the CG location and then move the CG with the added ballast and recalculate the new CG and then re-calculate the new ideal cross weights for that CG.

I did find it, I'll have to find a way to post it or pm it.

Solofast

Tried to hot link it, but that didn't work, if somebody has a premium membership and wants to post it let me know.

93Rubie

I just got done reading an old but goodie book, How to Make Your Car Handle by Fred Puhn. Great book, I learned a lot. More than I need to know, or know how to use on my Corvette.

He talked about moving weight around, recalculating CG and Polar Moments of objects in the car, etc...Interesting stuff.

At the very least, I now understand WHY my C4 handles the way it does.