This right here is a fundamental misunderstanding of basic physics. The only thing that determines corner weights is the location of the CG and the location of the corners. That's it. It doesn't matter what happens between. Simple example is those C-shaped tables you see. The legs only attach to one side, but you can put a load in the middle, or even on the end of that table, and it'll still stand upright. Because that load is distributed to the feet on the ground. It has to be. Those are the only things that react the load.

What happens between is different. With that table, you've created a hell of a moment arm that it has to resist. That table has to be beefed up in other ways to make the design work. Same with a suspension. If you have a solid axle and springs directly over the top of it, then those springs hold almost exactly the corner weight. But when you add a pivot point with, say a trailing arm, you've changed the physics. Now you have a lever arm. When you put a breaker bar on a stubborn bolt, you don't push the lever as close to the bolt as possible. You push at the far end of the bar. Same reason you'd want the coilovers on the far end of the trailing arm. You get the lightest load on the coilover and the maximum motion for the shock to react to.

So if you have the spring directly over the wheel, you get a 1:1 motion ratio. For every inch the wheel moves, the spring moves 1". That also means for every lb the wheel carries, the spring carries the same lb. But if you put that spring halfway between the pivot and the wheel, you get a 2:1 motion ratio. For every inch the wheel moves, the spring only moves 0.5". That also means for every lb the wheel carries, the spring has to carry two lbs. This gets worse the closer that spring is placed to the pivot, in the same way the torque you can put on a bolt gets worse the closer you put your hand to it.

 

Mounting the shocks, and/or the springs, behind or in front of the axle, changes the basic physics of how they act.
The leverage changes. The Motion Ratio etc. They more farther or less far, on each 1" bump at the axle. And at different speeds.

That means a completely different spring in both load and rate.
It also means different shocks, with different extension and compressed distances. as well as different internal valving.
The shocks will move either faster or slower.

So this would be an extremely difficult A to B comparison test.
But the physics says there should not be any difference overall, IF all that is accounted for.

The one thing that could make a difference is more or less unsprung weight.
Mostly in bump compliance.

 

Rear percentage is affected by the mounting points. the more forward that mounting point is the more rear weight you have. you can draw it up however you want but i have an actual set of scales and guess what it makes a difference on rear percentage. only talking about rear percentage.

 

i made you a pic showing a spring mounted front or behind . all that metal ,body plus fuel cell being further behind mounting point makes a difference.

 

With respect,



The red and blue lines have zero impact on weight distribution.

The only thing that determines weight distribution is where that weight contacts the ground: the tires. That's it. Anything in between will not affect weight distribution at all.

 

Rear Percentage????
What term are we talking about?

% rear weight distribution?

The axle position matters.
And the location of the weights...engine, fuel tank, etc.
But how the spring or the axle connects?
Nope.

High School Physics 101. I taught that. And designed suspensions.

Now will moving the weights around affect both the weight distribution, and the polar inertia moment?
Yep

I have been to a lot of different kinds of racing venues, and a lot of drivers & teams kind of make up their own physics, and terms, because it feels different.
Some of them I was able to help.
The ones that believed the science more than myth..

 

I only wish you could see it for yourself. scales don't lie the red line has more behind it and the blue line has less . i guess every dirt late model and every stock car running today is wrong I'll take my chances with facts like scales and personal experience. i wonder why they set the motor back so far . kind of thought it was to add more weight to the middle of the car but i guess if it was over the front springs it would be the same . but I'm sure it added no more weight to the rear tires right? Only because it doesn't matter if its closer top the rear spring mounts so it couldn't possibly have different percentages.

 

By moving the motor, they moved the CG of the car relative to the tires. By moving the mounting point of a spring, you aren't moving weight anywhere, relative to the tires. Not sure why this is so difficult for you to understand. The only thing that matters is where the weight is relative to the tires.

If you saw a change in the scales, you changed something other than just the location of the spring mounts. You moved weight, you added weight, or you subtracted weight, but SOMETHING else changed other than JUST the spring mounting point relative to the tire when you did it.

 

I feel the same so will just have to disagree. Your just stuck with how it was done in the past and scca club racing where they want those rear springs farther back so the none drivers don’t spin out and our more stable. Today we drive cars that 51-53 rear percentage and steer them more off the rear than the front. Basically with bump stops it’s a 2300 lb go cart. Suspension doesn’t travel because we don’t want it to. I’m curious if you even know what a tie down shock is ? Without google

 

I admire the work, but this race track terminology and myths are killing me!

If the amount of weight changes on the tires, then some weight got moved in the chassis, forward or rearward, with respect to the center of the chassis. The attachment points themselves do not matter at all. (Let's leave unbalanced spring jacking out of this).

If the tire weight changes F/R, weight got physically moved.

I have driven autocross 911s with maybe a ~65% rear weight bias. ASP.
It is an advantage.
It had traction galore, and lifts a tire out of every corner, like a dirt car, and brakes so hard it was like you threw out an anchor.
It had a nearly 50-50 weight distribution when you were are on the brakes hard. (Not 75% front like our cars.)

But that rear hanging engine, all behind the tires, made the polar inertia moment so bad, it made it wag it's tail badly going into corners, it took a looong time for the rear to take a set, vs the instant in the front, and was something I could never get used to. Basically it handled like the devil. Two different cars front vs rear. It forced you to adjust to it's personality. I just didn't like it. Oh BTW, the car won several national championships, so it was set up very well, just not the way I liked it, or the kind of car I was used to.

 

Sorry, agree to disagree is for things like colors, not physics that has been well understood for centuries. You're just wrong and that isn't an opinion. Weight distribution is determined by center of mass relative to the tire. That is a cold hard fact based on physics. There's no "how things were done in the past here", its literally how physics works. You want more rear bias? Have at it. But moving the spring won't change anything. Moving the axle forward will. Moving weight rearward will. Removing weight from the front will. Adding weight to the rear will. But how the spring attaches has no impact on weight distribution (beyond the weight of the spring and attachment points) and that's an indisputable fact.

 

3 time nascar modified champion I’m not a beginner. Scales don’t lie and that’s all I did was go from the original mounting points to the coil overs in front of axle. Car has more rear percentage because its carrying more of the car and has more of the car behind them. In fact I’m done explaining common sense. Just saying it doesn’t matter the position of the spring in relation to the frame and in fact it’s what is actually holding the car up doesn’t matter wow. Weight is actually physically moved when you move those mounting points you now are carrying more weight. The farther forward those spring mounting points are it’s just ridiculous to think that that weight doesn’t matter. And scales don’t lie.

 

Deleted.

 

The weight of the coilovers themselves is definately measureable on a scale.
And you did relocate those.
Say 40-50# for two?
Race car scales I have used go down to a pound or so.
And you must have moved them at least a foot or two.
So that is measureable.
You are correct, scales do not lie.

Dynamically, the shocks and the springs would require different rates to feel the same to the rear wheel, and to the driver.
So the driver could easily feel a difference if those units were relocated.
The motion ratios of both the shocks and the springs and their resulting spring rates at the wheel would change.
Without a corresponding spring and valving change the rear tire is going to react differently.

As I said before, you can not just change one thing when doing this.
This is an almost impossible A to B comparison, for a driver to evaluate which one feels better, or is faster.
There are just too many things that change at once.

If the tire moves up & down 1.0", with the coilover behind the axle, then the coilover moves more than the axle, say ~1-1/2"
When in front of the axle it moves less, say ~1/2".
It needs a stronger spring while in front, and tighter valving.
3X as much in my example! (If it went from 1-1/2 to 1/2")

It does not have to double or half either.
Even a small 10% change could be felt by a good driver.

Moving the exact same unit forward, would feel "shockingly" different.

 

I recently installed coilovers on my car. Going from the leaf spring mounted behind the rear axle to coilovers mounted in front of the rear axle increased my rear axle weight by 10lb.

The weight difference of the new components compared to the old was...10lb. The rear axle got 10lb heavier because I added 10lb of weight, not because of the spring mounting location.

 

That makes sense.
So is 10# really worth all this work?
And all this arguing?

Physics does not lie either.
50# at the end of a 110" lever at a 98" wheelbase = 56# on axle
50# at the end of a 87" lever on a 98" wheelbase = 44# on axle
A 12# difference.
Pretty much matches Wheersmyhorsepowers experience.

Anyone surprised?

50# of weight got moved.
The car is not lighter, weight just got moved forward.
And the forward mounted coilover now puts less weight on the rear axle.
Cause part of that weight is now being handled by the front tires.

So the rear weight dropped from 1600 to 1590. So what? That's not much. It a 1/2 of one %.
It's like adding 3HP. It ain't much.

 

My car 2380# 1166# on the front tires 1214 # on the rear tires. Seems like it was 49.8 % rear before with the stock spring. The car has a Howe 1 row radiator ,aluminum heads lightweight crank , 0 accessories besides a 50 amp alternator no headlight assembly’s 5 1/4 clutch. 49.8 makes it 1186# before on rear weight. That’s 28# yea that’s worth it to me.

 

Again I’ll be running bump stops with tie down shocks on all 4 corners. There will be no travel there will be no bump steer the very little movement will be adjusted with the packing.

 

You talked about gutting the doors and the custom control arms in your other threads, but how did you get the weight so low? Smokey Yunick style 7/8s model? I thought 3,000 was impressive.

 

Plenty of people can and have won, races without knowing a damned thing about what is actually happening with the car.

If you are using the frame as your reference, then you also have to include reaction forces at all of the suspension pickup points. Spring holds up the frame, but what is it connected to? A trailing arm. What is holding up the trailing arm? The frame (and the tire). That load matters. It counts when you're looking at how loads go into the frame. So yes, the spring is holding more load, but the trailing arm is countering it by providing a force in the opposite direction. And all of that load is ultimately reacted by what? THE TIRES.

This is literally physics 101. Sum of ALL forces. The total forces on the entire car are gravity, and a normal force from each tire. Sum all of the forces, set equal to zero (no acceleration), and you'll get the load at each tire. That's it. When you switch to the frame, you now have gravity, plus spring mounting points, control arm mounting points, and trailing arm mounting points. When you do the actual math, the forces still balance out regardless of spring mounting point. The center of mass of the frame didn't change. The center of mass of the body didn't change. The center of mass of the powertrain didn't change. And since the ONLY forces countering gravity are the tires, there will be no change in weight distribution OTHER THAN THE WEIGHT OF THE SPRINGS. When considering that, keeping the springs farther rearward helps move even more weight back, while giving you a smaller motion ratio to make shock tuning easier.

As I said, you changed something else other than just spring mounting point. wheresmyhorsepower likely nailed it. You added weight with your new suspension and that "shifted" more weight back. It wasn't the spring location.