Weight Distribution

[ironworks]

I have learned two things about weigh placement in my short dirt racing career, one is get the biggest fuel cell you can so you can easily add and remove weight when you want it. I only use 5 gallons of race fuel in a 25 lap main event, but I have a 22 gallon fuel cell. The second is how important the weight placement is behind the rear axle center line as it creates "swing weight" the weight right above the rear axle is for traction and the fuel cell and rear weight creates swing weight. Depending on driving style you want some or none. Like was said before the when the car spins and has lots of tail weight or swing weight it begins to act like a rain drop and the weight leads the way for the rest of the car.

Rodger

[ccracin]

Originally Posted by ironworks

one is get the biggest fuel cell you can so you can easily add and remove weight when you want it.

the weight right above the rear axle is for traction

Rodger,

I would use a bit of caution in relating weight placement and distribution on a dirt circle track car and anything else. It has been my experience that they are unique animals and most concepts that work there don't transfer well to even asphalt circle track cars let alone cars that turn right and left. You will never deal with a hard surface race track changing like a dirt track will with in a given race let alone day. (I'm sure you have that well figured out by now ) That is typically why weight placement and transfer change so much in a dirt car and will not in others.

Around here the only guys using that size cell or larger are guys that run alcohol simply because you burn so much more of it during a race. Most try to run as small a cell as they can get away with so that you minimize fuel movement even with foam in the cell. We ran a 12 gallon cell with 6 gallons in it for a 25 lap feature on a .5 mile track as an example. That was the smallest cell we could get off the shelf at the time. Any weight additions are usually done with lead as it easier to know exactly how much you have and it can be more precisely placed. Any weight we added was first placed to optimize our front to rear and left to right bias. Then if we were still under weight for the class we added it as low as we could in the car as close to the CG in the horizontal plane as we could. The car would then be setup slightly tight to start so that it would move to neutral or free as the fuel burned off.

I can't see the benefit (Other than in a dirt car) of adding weight above the rear end line. In most hard surface situations you try to minimize the body roll while creating side bite. (managing weight transfer) This is done by shortening the moment arm length from the CG to roll axis of the car. By adding weight higher than the roll axis, you are effectively creating more roll moment you then have to counter act with stiffer springs or sway bars. (lengthening this arm on Dirt is good but that's another book)

My main point here is that be careful trying to apply what works on a dirt circle track to what most folk ere are trying to accomplish as the 2 in my humble opinion are VERY different. I really wish I could have spent some more time on dirt it was a blast. I only got to play a bit. I really ad a tough time racing my asphalt cars and helping friends with dirt cars because they were so different.

[Sieg]

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[ironworks]

Originally Posted by ccracin

Rodger,

I would use a bit of caution in relating weight placement and distribution on a dirt circle track car and anything else. It has been my experience that they are unique animals and most concepts that work there don't transfer well to even asphalt circle track cars let alone cars that turn right and left. You will never deal with a hard surface race track changing like a dirt track will with in a given race let alone day. (I'm sure you have that well figured out by now ) That is typically why weight placement and transfer change so much in a dirt car and will not in others.

Around here the only guys using that size cell or larger are guys that run alcohol simply because you burn so much more of it during a race. Most try to run as small a cell as they can get away with so that you minimize fuel movement even with foam in the cell. We ran a 12 gallon cell with 6 gallons in it for a 25 lap feature on a .5 mile track as an example. That was the smallest cell we could get off the shelf at the time. Any weight additions are usually done with lead as it easier to know exactly how much you have and it can be more precisely placed. Any weight we added was first placed to optimize our front to rear and left to right bias. Then if we were still under weight for the class we added it as low as we could in the car as close to the CG in the horizontal plane as we could. The car would then be setup slightly tight to start so that it would move to neutral or free as the fuel burned off.

I can't see the benefit (Other than in a dirt car) of adding weight above the rear end line. In most hard surface situations you try to minimize the body roll while creating side bite. (managing weight transfer) This is done by shortening the moment arm length from the CG to roll axis of the car. By adding weight higher than the roll axis, you are effectively creating more roll moment you then have to counter act with stiffer springs or sway bars. (lengthening this arm on Dirt is good but that's another book)

My main point here is that be careful trying to apply what works on a dirt circle track to what most folk ere are trying to accomplish as the 2 in my humble opinion are VERY different. I really wish I could have spent some more time on dirt it was a blast. I only got to play a bit. I really ad a tough time racing my asphalt cars and helping friends with dirt cars because they were so different.

I agree Chad, I'm just saying in a car your going to drive on the street you might want more then a 5 gallon tank. And you can add weight or drive range with a bigger tank. granted a 22 gallon tank with 15 gallons of fuel might not be the best idea for a big sweeper autocross, but it might be for a tight track if your set up is tight and your driving style is with the gas pedal, not the steering wheel. Plus if you had 2 11 gallon tanks with proper baffling designed for your applications you would not have much slosh. Plus a full tank of fuel wont slosh.

I also agree traction is not as much a concern in asphalt as dirt, where forward bite is hard to come by. But in high horse power cars that might have say leaf springs you might need to add weight since you cannot control the instant center like a 4 bar that will help with traction and forward bite. But the things a drift car does to set up their car is similar to a dirt car, so if you want the car to be tighter and not super loose your going to do the opposite of what a drift car would. Drift guys want the swing weight and they are also going to use the biggest rear sway bar possible to be able to do that. The main difference is the fact that on dirt your driving off the right rear tire when track conditions are tacky and off the left rear when the conditions are going slick. No where have I ever seen a car set up to turn left with the softer spring on the right then the left but in dirt racing.
As much as the racing on dirt is like a women's emotions, you have no idea what your going to get from one day to the next, vehicle dynamics are going to be similar, you just have to use those dynamics for your driving condition and driving style.

[ccracin]

Originally Posted by ironworks

Plus if you had 2 11 gallon tanks with proper baffling designed for your applications you would not have much slosh. Plus a full tank of fuel wont slosh.

I actually thought about doing this on the truck and mount them on both sides of the drive shaft in front of the rear. Then I decided to just try and finish the truck!

But the things a drift car does to set up their car is similar to a dirt car
Never put those 2 together, but yep I can now see the tie in. Cool, might have to go drift something!

No where have I ever seen a car set up to turn left with the softer spring on the right then the left but in dirt racing.

We tried this on asphalt as part of the soft spring big bar fad in the late 90's. It worked, but corner entry was limited. If you didn't get it right, oh boy!

As much as the racing on dirt is like a women's emotions, you have no idea what your going to get from one day to the next, vehicle dynamics are going to be similar, you just have to use those dynamics for your driving condition and driving style.
I think this says alot. Getting back to the OP's question, Weight placement and distribution can be very different for different cars, uses, driving styles, etc. But a good general rule would be when in doubt Low and near the CG with compromises moving out from there.

Gotta agree with all that you said above! I made some comments in red too! I knew where you were coming from, I guess I just didn't want the person that has never experienced this stuff to go to his dirt track buddy and think all the stuff he does will work on the street, autoX etc. Make Sense?
Oh, and post some more racing videos, I need my fix. Racing is all but done back here!

[ironworks]

Originally Posted by ccracin

Gotta agree with all that you said above! I made some comments in red too! I knew where you were coming from, I guess I just didn't want the person that has never experienced this stuff to go to his dirt track buddy and think all the stuff he does will work on the street, autoX etc. Make Sense?
Oh, and post some more racing videos, I need my fix. Racing is all but done back here!

I need to get a camera. I have spent all my spare time getting the blue bomber ( my race car ) ready for the next 3 big races. More horsepower and some other set up ideas.

[byndbad914]

cool, looks like a solid meeting of the minds here! what? clear direction from an internet thread? never!

Originally Posted by ccracin

Originally Posted by ironworks View Post
Plus if you had 2 11 gallon tanks with proper baffling designed for your applications you would not have much slosh. Plus a full tank of fuel wont slosh.
I actually thought about doing this on the truck and mount them on both sides of the drive shaft in front of the rear. Then I decided to just try and finish the truck!
So that is exactly what I had said to begin with, two smaller cells, in front of the rear axle, but as support to may PITA comment, Chad noted it was easier to just finish the truck

But the things a drift car does to set up their car is similar to a dirt car
Never put those 2 together, but yep I can now see the tie in. Cool, might have to go drift something!
Previous posts talking about a bunch of weight behind the axle = drift car. 911s like to drift out for example having the engine hanging off behind the rear axles.

As much as the racing on dirt is like a women's emotions, you have no idea what your going to get from one day to the next, vehicle dynamics are going to be similar, you just have to use those dynamics for your driving condition and driving style.
I think this says alot. Getting back to the OP's question, Weight placement and distribution can be very different for different cars, uses, driving styles, etc. But a good general rule would be when in doubt Low and near the CG with compromises moving out from there.
So were it me, starting from scratch with a pickup, I would not compromise and get two tanks in front of the wheels, then compromise and put the mufflers behind the axle, so forth. And compromise is the best point to make - if we look at each concept as an individual component, then you can always determine the best spot for them - low and centered. Problem is everything can't occupy the same space just have to pick your poison and determine what weight is the most important, control that as best as possible, and then the ancillaries go where they can fit. Or you spend $800 on a Lithium Ion battery, then you don't care where that 8lbs goes and remove an item from the list of must have locations

[badmatt]

Unfortunately I can not utilize the dual fuel cell idea, Although I really like the idea. I can run a larger central center mount tank. dead nuts center around 1" behind the rear cab wall and fairly close to centerline of the truck.

Also I have a 5 gallon dry sump tank that will be bed mounted (front of the bed thinking passenger side)

The mufflers are going to be mounted close to the rear of the truck cab.

NOW... for the battery(s) those I can put Anywhere behind the rear axle, and I can mount them low IE: top of the battery level with the top of the frame. and either in board or outboard of the frame rails. (frame height from the ground is around 3.5" under the cab, 7"s at the very rear of the truck)

This is by far one of the better discussions on this issue I have seen on the net, Please keep this coming so other people who are looking for idea's and help can come in!

The swing weight is really important to me , because honestly.... I don't want this to be a typical truck that is WAY to tail happy.

Thanks
Matt