Battery Location For Autocross

[FURIOUSS]

Hi, I'm brand new to Autocrossing my '85 XR. My question is about the best place to put the battery.

I moved my Battery to the rear of the car near the gas filler. Then I saw a picture of the Mustang GT-R with the battery placed where the stock XR muffler would be.

My muffler isn't there anymore, so I could easily make a box to partially drop it into that space. (Partially because otherwise it would scrape the ground.) It's much lower, but more forward. I dont want to give up rear traction for a lower center of gravity. Or do I?

Thanks, I'm FURIOUSS

[turboxr]

Over the right rear tire.

[John Brennan]

I'd put it in a box on the floor behind the passenger's seat, or in place of the rear seat right behind that. That would have it low, inboard of the axles, and in a safe place. Do you have a fuel cell?

[anglin]

Hi, I'm brand new to Autocrossing my '85 XR.

Nice. Quick question. Are you brand new to autocrossing in general or are you brand new to autocrossing XRs? There is a difference and I want to know what sort of experience you have, so I can adjust my conversation accordingly.

Let me preface the entire technical content of the post by saying the following. Your goal, when designing a race car, should be focused on getting the car to change directions as fast as possible. To do this you need to consider two important things about the physical characteristics of the car:

- First is weight reduction; the heavier the car is, the harder it is to get the car to change directions.
- Second is reduction in polar moment of inertia (rotational inertia), aka getting all of the mass of the car as close to the center of gravity as possible. The ice skater analogy is most frequently used here. They start a spin with there arms extended and draw them in as the spin, increasing the rate with which they are spinning. They didn't do anything but change their polar moment of inertia (rotational inertia).

As an aside; the frequency of hand inputs in autocross has been compared to the frequency of hand inputs in F1. According to the people who have made this statement, no other form of sub-F1 motorsport has more work on the steering wheel than autocross.

My question is about the best place to put the battery. I moved my Battery to the rear of the car near the gas filler. Then I saw a picture of the Mustang GT-R with the battery placed where the stock XR muffler would be.

My muffler isn't there anymore, so I could easily make a box to partially drop it into that space. (Partially because otherwise it would scrape the ground.) It's much lower, but more forward. I dont want to give up rear traction for a lower center of gravity. Or do I?

You'll notice that I didn't mention anything about center of gravity height and how it relates to cornering speed. I just mentioned polar moment of inertia (rotational inertia) and how it relates to getting the car to change direction. (Lowering the center of gravity does improve cornering speed.) When you are putting the battery way in the back of the car, you are increasing your polar moment of inertia (rotational inertia). In fact, you have probably moved the battery farther away from the CG than it was when it was in the engine bay (but it is probably a little bit lower). The CG on a stock XR is near your right elbow when you are sitting in the car.

My goal, when I started my battery relocation project, was three-fold:

- install a lighter battery
- move the battery to a position closer to the center of gravity (there by reducing the polar moment of inertia [rotational inertia])
- move the battery to a position lower than the center of gravity (thereby lowering the net center of gravity)

Here is a picture of the battery I chose and the location I decided to put it in:

http://www.merkurtech.com/merkurtech/sh ... ery005.jpg

Here is some comparison information:

stock battery:
weight: 31 lbs
estimated horizontal distance from CG: 38" in front of CG
estimated vertical distance from CG: 6" above CG

racing battery:
weight: 14 lbs
estimated horizontal distance from CG: 14" behind CG
estimated vertical distance from CG: 8" below CG

The net changes are pretty large. The battery is 17 lbs lighter. The location is 24" closer to the CG, horizontally. The battery mass moved down 14". You'll also notice that I left it on the right-hand side of the transmission tunnel. This is also intentional since left/right balance is important too.

One thing that I can't gloss over with the application of this battery to my racer is this. My racing class rules allow easy weight reduction from the back of the car (in the form of legal rear seat removal). Reducing weight from the area in front of the CG isn't so easy. This has basically allowed me to remove 31 lbs from in front of my CG and put 14 lbs behind my CG. This matters because with suspension geometry that is essentially stock, you want to keep the weight distribution close to stock.

Basically, the stock location of the stock battery puts a 98 ft lb moment on the CG in the forward direction (counterclockwise when viewed from the driver side of the car, makes the front heavier) and my relocated racing battery puts a 16 lb ft moment on the CG in the rearward direction (clockwise when viewed from the driver side of the car, making the rear heavier). This is a total change of 114 ft lb.

Back to the horizontal distance an polar moment of inertia. Your car will change direction faster

Why do you want the car to change direction faster? Just think of a slalom. The faster you can get the car to change direction, the faster you can go through the slalom. Ideally, we'd move all of the heavy parts of the car to the location of the center of gravity. The car would change direction the fastest in this manner. This is why all open wheel cars (excluding compromised designs like Formula Vee) are mid engined. They can change direction faster this way.

I wouldn't bother with cutting a hole and building a bucket for the battery, Mustang GT-R style. The rear passenger footwell is almost exactly as low as you'll be able to get it if you put it under the floor somewhere else.

I'm FURIOUSS

Fine.

Over the right rear tire.

That is a drag racing mod, not an autocrossing mod. I hope my post has defined why the trunk/hatch location is such a bad idea for the location of the battery when building an autocrossing XR.

Summary:

- If you want to move the battery, move it as close to, and below, the center of gravity location
- Putting the battery in the trunk/hatch is almost certainly (depending on the actual location) increasing your polar moment of inertia (rotational inertia) which is taking an autocrosser in the wrong direction
- don't bother with the GT-R style, you can get the same results (if not better) by choosing the location of the battery wisely

Hope this helps.

[MerkXRTurbo]

How did I know that Anglin would have alot to say.

[anglin]

How did I know that Anglin would have alot to say.

Apparently you guys have me figured out. I put HUGE amounts of mental effort into questions just like the extremely-upset-guy asked. My mind revolves around racing (as long as the racing involves turning). Heck, whenever Grayson and I get together there is almost exclusively one subject.

(That subject is racing, in case you didn't figure it out.)

[John Brennan]

Well, none of us has any objections to THAT!

It's nice to hear my simple (and correct!) answer explained so fully and expertly!

[turboxr]

I've been owned by Mr. Anglin

[MerkXRTurbo]

That wasn't an objection... in fact, I speak for all of us when I say that it's great to have a guy of your knowledge level around. I just knew that it was up your alley more than any other question that could be asked.

[dunk]

I added a light weight battery and left it in the stock location. This saved the weight and cost of the longer battery cables. Granted I'm not autocrossing, but I'm not sure sure moving the 14 pounds will make that much difference or is worth the total weight penalty? Keep in mind the merkur's battery location isn't in the worst case scenario position (i.e. front left side).

My racing experience is more or less 0 and I am just trying to get the car as light as possible as cheaply as possible without excessively using a sawzall.

So it all depends on how serious an autocrosser you are etc....

[anglin]

That wasn't an objection... I just knew that it was up your alley more than any other question that could be asked.

I was aware that you weren't objecting. I should have thrown an emoticon or two in there.

I added a light weight battery and left it in the stock location. This saved the weight and cost of the longer battery cables. Granted I'm not autocrossing, but I'm not sure sure moving the 14 pounds will make that much difference or is worth the total weight penalty? Keep in mind the merkur's battery location isn't in the worst case scenario position (i.e. front left side).

You pointed out something that I intentionally avoided. I didn't mention this mostly due to the complexity of the calculations. When calculating the weight/mass of a battery you can effectively reduce it to a single point in space. When trying to figure out the effects of things like the weight of cables on the moments of the vehicle, life gets really ugly really fast. Cables have a mass per unit length and each infintesimally small unit of length is at a different distance from the CG. The integral for calculating moments that results wouldn't be so bad if the cable were a straight line, but it's not. You could probably get a fair approximation by using a straight line.

Yes, my setup has the penalty of the cables and the battery mounting system. These add weight to my 14 lb battery that wouldn't be added if I had just installed a 14 lb battery in the stock location, like Dunk did. However, I was shooting for the benefits that come with relocating the battery:

- lower CG
- lower polar MOI
- battery not in the engine comparment
- slightly more convienent installation/wiring of master cut off switch (though I have yet to determine if there is any gain in wiring convienence, since I'm maintaining KAPWR even when the master cut off is switched off)
- lower weight (it would take a lot of aluminum mounts, bolts, cabling, plugs, connectors, lugs, etc. to eliminate the weight savings from choosing the lighter battery - more than the weight difference of 17 lbs, since I get to remove the battery tray, that steel hold-down bracket, the aluminum heat sheild and the stock cables)

So it all depends on how serious an autocrosser you are etc....

That is the bottom line in every single motorsport-oriented discussion I have ever been a part of. Your level of seriousness directly determines how optimized you are going to make every modification. Since I am working on decreasing the weight of my XR using the Dunk-style, non-sawzall method, I have to make every mod count.

There's another long one. I'd better get back to work.

[Ray]

anglin you rule.

[Varthlokkur]

I think my IQ went up!

[anglin]

Nice. Quick question. Are you brand new to autocrossing in general or are you brand new to autocrossing XRs? There is a difference and I want to know what sort of experience you have, so I can adjust my conversation accordingly.

Where is Mr. Furiouss anyway? I'm still waiting on an answer to my question above. Let's make this guy's car fast!

[MerkXRTurbo]

anglin you rule.

touche