Which is "more" important: Front/rear bias or center of gravity?
 

If have 62# of ballast to play with need to know the best place to put it. The car is rwd.

I have 4 positions in mind.

#1 position places the weight 3" above the present center of gravity and makes 50F/50R weight distribution.

#4 position places the weight 6" below present the center of gravity but makes 51F/49R weight distribution.

Positions #2 ans #3 fall somewhere between these 2.

So what would your choice be, better front/rear weight bias or lowering center of gravity?

How far back is the weight in order to get it to 50/50? That is something else to consider, is it back by the rear bumper, or right behind the rear seats? Also, rather than focus on front/rear weight distribution, especially when dealing with 60ish pounds, have you thought about focusing on balancing your corner weights and getting a near even cross weight for the car?

Hopefully people don't yell at me for this, but I don't think the majority of us (well, maybe all but three or four people around here) would be able to tell where 60lbs of weight went. I know I probably wouldn't be able to.

Matt

Are those ratios posted with the driver?

Is a passenger part of the equation?

Have you checked/factored the weights with the fuel tank full and empty?

These numbers include a 225# meat sack in the front seat and a full load of fuel.

#1 position is 36" behind the rear axle line.

#4 position is 20" in front of the rear axle line.

To focus on the question, assume corner weights are good.

Thanks

I think number 4, as low as possible. You can tune a suspension around a F/R bias, but CG height is much more of an inherent characteristic to a good handling car.

But if the car is for autocross, maybe think about NOT going outboard of the axles with the ballast...autox cars need to rotate a lot more quickly than road racers.

Hi Rusty,

Assuming your goal is handling performance ... you do NOT want to put any additional weight ... especially 62# ... 36" behind the rear axle line.

Packaging stuff/weight in a car is one the challenges we all face in building a car for performance. For road courses, autocross & spirited curvy road driving ... ideally, all 4 corners would weigh the same statically, and ...

* The majority of weight is inside the wheelbase.
* The majority of the weight is as close to the center (both ways) of car as possible.
* The CG is at a height optimum for the speeds the car will spend the majority of its time running in (low, but how low is the question).
* Build the car as light as is practical & safe. Lighter cars accelerate, decelerate & turn better ... period. So don't add a bunch of weight to balance the car if you can avoid it.

Add the 62# 6" below present the center of gravity sounds very good.

And if you achieve a streetable g-machine with 51F/49R, that will be better than 90-95% of the cars built. So, good job !

Economy scales can be picked up for around a $1000 for a set of four. Sometimes you find them a bit less, better ones are a bit more. You also can substitute grain scales, shipping scales, even large truck scales, but you give up a bit of accuracy.

To balance things, you can look at alternate materials, subtracting items or adding items. Moving things around will only gain you so much before you have to get into some nitty gritty details of alteration.

Thats probably a true statement. Some guys are really in tune with their car, some of us are not. 62# is about the difference between a full fuel tank and an empty fuel tank. If you can feel the introduction of a fuel load push as you burn up gas, you will notice a change in ballast weight, and yes, the tires will notice it long before most of us do. However, unless your running some long running events that will really put some heat into the tires, and/or your at the edge of control to begin with, the tires may not broadcast what the change does to them in a typical autocross length event and they certainly won't show it in commuter driving.



I agree with all of this. I'd also add that you can control the front/rear weight bias via roll couple percentages much easier than you can alter COG heights and location. If you have the chance to keep the COG low, do it. That extra 1-2% of front bias can be controlled with shock tuning. Even if the actual numbers turn out to be 5% or greater, springs can be changed to alter the neuteral line of the car.

Maybe it is just my drag race background wanting to get more weight on the rear.

I figured better rear bias would help out of the hole, accelerating out of a curve plus help more equally load the brakes/tires.

Yes, when I put weight in the back, after the axle line, the car would react slower but hook better. That 62# is worth 83# on the rear axle and -20# off the front .I created as spread sheet to calculate this for me.

In drag racing we could just adjust when we left the line to compensate for how the car reacted, slowness?.

Can't do that when driving a road course, so it looks like CG is getting the nod.

Thanks

Rusty,
In my drag racing days, we would do the same. If we needed to add ballast to make weight, it "usually" was mounted to rear frame rails, behind the rear axle.

But in any racing where you're "turning" (oval, auto-x or road course ... even spirited driving on curvy roads) ... you want as much weight concentrated in the center of the car ... versus out past either end of the axles.

I am very visual, so for me to digest things, I always need to see examples. Sometimes exaggerated examples make it the clearest for me. I can always imagine the concept as less, once I "see it".

Imagine ... or simply go do this ... having 50# of weight you're holding. Hold it out all the way with straight arms ... then "quickly" rotate it all the way left ... and then "quickly" back all the way to the right. Now tuck in close to your body & do the same.

When the weight is "out there" it is harder to get it moving ... and once it's moving, it is harder to get it to stop. Changing directions is hardest. With the weight tucked in close, it is "easier" to get it to do these things ... especially change directions.

When you think about running on an short oval, auto-x or road course track you are constantly asking the car to change directions & go ... quickly.

A car with the weight concentrated inside the wheelbase allows a car of the same weight as another car (let's say with more of its weight past the axles) to turn quicker ... and when traction is exceeded ... not snap the end around as hard & quick.

Sorry if this post rambled on & covered stuff you guys already knew.

Signing off,

Captain Obvious