Mustang Body Roll

Shocks, Springs, Brakes, Frame, Body Work, etc

Moderator: Team

j-c-c
Guru
Guru
Posts: 6545
Joined: Sat Sep 03, 2016 9:03 pm
Location:

Re: Mustang Body Roll

Post by j-c-c »

lefty o wrote: Wed Mar 03, 2021 6:55 pm yes, because like the old saying goes. you can lead a horse to water but you cant make them drink. you already think you know it all, so there is no point in further discussion. :roll:
If "I indeed know it all", why did I bother to ask the previous question, which your answer kinda avoids, you think?

You know what I really think, you are stumped on this topic and can't explain your position, and you can't admit it, but I might be wrong, and I am not afraid to admit it.
David Redszus
Guru
Guru
Posts: 9633
Joined: Tue Nov 27, 2007 9:27 am
Location: Chicago
Contact:

Re: Mustang Body Roll

Post by David Redszus »

One of the first steps in chassis tuning is the determination of roll couple distribution.

RCD will determine dynamic vehicle weight transfer, in all directions.
While total weight transfer is determined by CGH, G force and wheelbase/track width,
the distribution of the weight is a function of RCD.

The factors that determine RCD are corner wheel rates (not merely spring rates) and sway
bar stiffness (front and rear).

Suspension analysis programs exist that will calculate weight transfer among other things.

Let us suppose that 600lbs is transferred to the rear under acceleration. Some of the
transferred load will go to each rear corner. The load distribution to each corner spring
will determine the compression displacement.
User avatar
frnkeore
Expert
Expert
Posts: 825
Joined: Wed Dec 25, 2019 3:06 am
Location: Oregon

Re: Mustang Body Roll

Post by frnkeore »

With all four wheels on the ground, roll couple transfers weight to the end that has the lowest roll center (usually the front). I suppose you might gain traction, off the line, with a lower roll center, in the rear. But, if the tires have enough traction, to raise the front, at launch, with the wheels up, your dealing with torque reaction and I can only see a stiffer spring being able to "control" that.
lefty o
Guru
Guru
Posts: 3445
Joined: Tue Oct 15, 2013 8:50 am
Location:

Re: Mustang Body Roll

Post by lefty o »

j-c-c wrote: Thu Mar 04, 2021 10:07 am
lefty o wrote: Wed Mar 03, 2021 6:55 pm yes, because like the old saying goes. you can lead a horse to water but you cant make them drink. you already think you know it all, so there is no point in further discussion. :roll:
If "I indeed know it all", why did I bother to ask the previous question, which your answer kinda avoids, you think?

You know what I really think, you are stumped on this topic and can't explain your position, and you can't admit it, but I might be wrong, and I am not afraid to admit it.
you are wrong, it is good if you can admit it.
David Redszus
Guru
Guru
Posts: 9633
Joined: Tue Nov 27, 2007 9:27 am
Location: Chicago
Contact:

Re: Mustang Body Roll

Post by David Redszus »

frnkeore wrote: Thu Mar 04, 2021 1:55 pm With all four wheels on the ground, roll couple transfers weight to the end that has the lowest roll center (usually the front). I suppose you might gain traction, off the line, with a lower roll center, in the rear. But, if the tires have enough traction, to raise the front, at launch, with the wheels up, your dealing with torque reaction and I can only see a stiffer spring being able to "control" that.
Weight transfer is not determined by roll center location; it is primarily a function of center of gravity height and wheel base length.

The formula for longitudinal weight transfer is:

Transferred weight = (CGH * vehicle weight * G force) / wheelbase

i.e. 20 in * 3000lb * .6g /100in = 360 lbs transferred from the front to the rear. Now how much goes to each
wheel is determined by the wheel rates. The stiffer spring will resist deflection; the softer spring will collapse more.
The tire with increased vertical loading will transfer more force to the ground and increase traction.
Ideally, one would want the weight transfer plus static corner weights to be the same for each tire.
User avatar
frnkeore
Expert
Expert
Posts: 825
Joined: Wed Dec 25, 2019 3:06 am
Location: Oregon

Re: Mustang Body Roll

Post by frnkeore »

Most people do know that weight transfer, is a product of CG position x acceleration. It became obvious, back in the days (1960's) of the original "funny Cars".

What I was talking about is the leverage of the suspension of one end over the other (Front vs Rear). The chassis moves about the roll center. If the roll center is at ground level, at one end and is at 12" at the other end, can you tell us how roll couple works?
j-c-c
Guru
Guru
Posts: 6545
Joined: Sat Sep 03, 2016 9:03 pm
Location:

Re: Mustang Body Roll

Post by j-c-c »

lefty o wrote: Thu Mar 04, 2021 6:40 pm
j-c-c wrote: Thu Mar 04, 2021 10:07 am
lefty o wrote: Wed Mar 03, 2021 6:55 pm yes, because like the old saying goes. you can lead a horse to water but you cant make them drink. you already think you know it all, so there is no point in further discussion. :roll:
If "I indeed know it all", why did I bother to ask the previous question, which your answer kinda avoids, you think?

You know what I really think, you are stumped on this topic and can't explain your position, and you can't admit it, but I might be wrong, and I am not afraid to admit it.
you are wrong, it is good if you can admit it.
Thanks for that sharing that personal uninformative blurb.

I will add in the same vain, Einstein once noted, "a person knows a subject well when they can explain it successfully to their grandmother", it would be hard for me to say Einstein is wrong in this matter, but not for you, it appears.

Back on topic, I am not sure how the typical calculated roll center plays a role here, as all the "roll" here is the torgue induced, which has a much different force path then COG.
David Redszus
Guru
Guru
Posts: 9633
Joined: Tue Nov 27, 2007 9:27 am
Location: Chicago
Contact:

Re: Mustang Body Roll

Post by David Redszus »

What I was talking about is the leverage of the suspension of one end over the other (Front vs Rear). The
chassis moves about the roll center. If the roll center is at ground level, at one end and is at 12" at the other end,
can you tell us how roll couple works?
The topic of vehicle dynamics was studied at GM during the 1920's by Maurice Olley and later by Bill Milliken who authored an excellent textbook on the subject.
And who had forgotten much more than we have ever learned.

The front and rear suspensions each have their roll centers. Typically, the front roll center is below ground and
the rear about axle height. A longitudinal centerline is drawn connecting the roll centers forming a roll axis.
The distance from the sprung weight gravity center height (at the longitudinal center of gravity) from the roll axis, determines a moment arm. This moment distance times lateral G force will determine the lateral body weight
transfer acting on the springs.

The resistance of the springs (and sway bar) will determine the amount of body roll. Roll couple distribution is determined by combining the front axle spring rates (including bar) with the rear axle spring rates (including
bar) and dividing the front rate by the total rate. i.e. 1400 front, 600 rear = 2000 total. 1400 / 2000 = .70.
Since 70% of the roll resisting spring force is at the front, we say the roll couple distribution is 70%.

But weight transfer is another matter and does not involve roll centers.
Using a vehicle with front corner weights of 900lbs and rear corner weight of 600lbs, a 100in wheelbase, a 20in CGH,
and an accelerating force of 0.60g, the front corners weights will be 720lbs and the rear corner weights will be 780lb.
Which means that a total of 1560lbs of force can be transferred to the road surface. Try for more and the tires will spin.

That is for a vehicle with laterally balanced weight distribution. But in the real world, weights often are not balanced side to side. If LF=1100, RF =750, LR=650, and RR= 500, then with 0.60g, our corner weights become: LF=920, RF=570, LR=830, and RR=680. The left rear tire has gained traction but the right rear tire has lost grip and will spin earlier.

Using data collection, we have often observed a drag vehicle assume an angled attitude as it goes down the track.
Crabbing costs power, affects aero (drag and downforce), and can make the vehicle harder to drive. Be careful when
you lift off the throttle.
User avatar
frnkeore
Expert
Expert
Posts: 825
Joined: Wed Dec 25, 2019 3:06 am
Location: Oregon

Re: Mustang Body Roll

Post by frnkeore »

I can see that you understand roll couple. Now tell us what happens when the engine torque, exerts chassis roll, please.

Normally, a vehicle has it's roll center lower, in the front, that is to transfer weight, to the front and that is accomplished by chassis roll. I'm suggesting (for a drag race only chassis) lowering the rear RC, to transfer some weight to the rear, under chassis roll.
David Redszus
Guru
Guru
Posts: 9633
Joined: Tue Nov 27, 2007 9:27 am
Location: Chicago
Contact:

Re: Mustang Body Roll

Post by David Redszus »

frnkeore wrote: Sat Mar 06, 2021 1:59 am I can see that you understand roll couple. Now tell us what happens when the engine torque, exerts chassis roll, please.

Normally, a vehicle has it's roll center lower, in the front, that is to transfer weight, to the front and that is accomplished by chassis roll. I'm suggesting (for a drag race only chassis) lowering the rear RC, to transfer some weight to the rear, under chassis roll.
The front and rear roll centers determine the slope of the roll axis. Assuming that the center of gravity height remains unchanged, lowering the rear roll center will move lateral roll forces toward the rear. But this only applies to the weight transfer of the unsprung weight. Actual weight transfer includes total vehicle weight and does not involve roll.

The observed body roll is a function of lateral G force and CGH distance above the roll axis.

Body roll as a function of engine torque, results from a torque reaction. "For every action there is an equal and opposite reaction," said Lord Newton.

Conventional front engine, rear drive cars with live rear axles are subject to wheel load changes due to drive torque; called engine torque reaction. With a conventional counter-clockwise turning drive shaft (viewed from the rear), the right rear tire spins first. The problem occurs because the driveshaft torque must be reacted by a change in rear axle loads, with the left rear increasing and the right rear decreasing. Solving for the changes in wheel load is compounded by the fact that the driveshaft torque must also be reacted by the chassis through the engine mounts. This requires a moment reacting the driveshaft, be produced at the front of the car due to chassis roll.

The driveshaft torque produces a diagonal jacking effect of significant magnitude. But only on solid rear axle cars and not on independent rear suspensions because the differential is fixed to the chassis and the engine torque is reacted through the chassis, not the rear springs.

To make matters even worse, longitudinal weight transfer on asymmetric weighted vehicles
produces asymmetric weight transfer.
Examples follow:

Symmetrical loading
100" WB, 20" CGH, 0.6G accel
LF.......RF
900.....900lbs
LR......RR
600.....600
Lateral CG = 0", Longitudinal CG = 60" from rear
-----------------------------------------
720......720lbs
780......780
An equal amount of weight has been transferred to both rears due to acceleration.

Asymmetrical loading
LF.......RF
1100....750
LR.......RR
600......550
Lateral CG = 4.23", Longitudinal CG = 61.7" from rear
--------------------------------------------
920.......570lbs
780.......730
Note that less weight has been transferred to the right rear wheel.
Vertical tire loading produces tire grip.

So we see that corner weight distribution, engine torque, and engine mount stiffness will
determine the rear tire weights under longitudinal acceleration.

To determine rear tire performance, we frequently use infra-red tire temperatures
to measure tire slippage; some tire slippage is better than none.
User avatar
frnkeore
Expert
Expert
Posts: 825
Joined: Wed Dec 25, 2019 3:06 am
Location: Oregon

Re: Mustang Body Roll

Post by frnkeore »

The front and rear roll centers determine the slope of the roll axis. Assuming that the center of gravity height remains unchanged, lowering the rear roll center will move lateral roll forces toward the rear. But this only applies to the weight transfer of the unsprung weight.
Yes, this is what I was getting at.

In a automatic trans car, you can see that, you also get increased lift and therefore, a higher CG, from the torque arms (read traction bars) and torque, unloading of the RR tire. This also happens with a clutch but, it's sudden and not so obvious. Depending on the front suspension geometry, you may also get a higher roll center, while on a solid axle, you will get little to no change, at the rear.
David Redszus
Guru
Guru
Posts: 9633
Joined: Tue Nov 27, 2007 9:27 am
Location: Chicago
Contact:

Re: Mustang Body Roll

Post by David Redszus »

OEMs spend a lot of time perfecting the engine mount stiffness; which should be proportional to engine torque.

A solid engine mount is bad news for the chassis.

Note that the problem is torque, not horsepower.
Chris_Hamilton
Pro
Pro
Posts: 316
Joined: Sun May 24, 2015 11:50 pm
Location:

Re: Mustang Body Roll

Post by Chris_Hamilton »

Specifically to the OP"s question. Fox Body Mustangs unibodys were always marginal even with stock power levels back when they were new (or nearly new) I can't tell you how many cracked torque boxes and seat mounts I repaired replaced for guys back in the early 90's. If you are'nt going to put a roll bar in, the best practical things to do are full length welded (meaning welded full length of the sub connectors, both sides) subframe connectors that have at least two perpendicular braces supporting the seat mounts. Solid rear tower tie bar, solid front tower tie bar, a lower front crossmember connector. and stronger torque boxes. Several places sell all of this, just google.

You also mentioned removing the torque boxes on your car, hopefully you mean replacing them with stronger ones. :wink:
High quality metal, body and paint work
http://www.spiuserforum.com/index.php?t ... inia.9030/
Chris_Hamilton
Pro
Pro
Posts: 316
Joined: Sun May 24, 2015 11:50 pm
Location:

Re: Mustang Body Roll

Post by Chris_Hamilton »

I'll add that if you can fabricate a brace (square, rectangular or tubing) that joins the the lower A pillars together at/around the hinges, that is one of the best things you can do to stiffen a Fox body. Use 1/8" or larger plate welded to the ends of the brace and weld it fully to the lower a-pillar. You want a wide footprint when doing this. 4'x4' would be good. You can retain the stock dash by bending the brace to clear the dash. I used to use tubing (usually 1 1/2") and bend it to fit around/under the dash using sand and a torch. This in combination with the above mods will make the chassis much stiffer.
Also be sure to reinforce the control arm mounting points (another weak area) and either upgrade the rear control arms or at the very least box the stock control arms to stiffen them. They will bend easily at very moderate power levels. By now most stock control arms will be bent to one degree or another. Be wary of using urethane bushings as they will often bind due to the four control arm setup on a fox body and the flex of the unibody. Solid bushings are better if you can stand them. .
High quality metal, body and paint work
http://www.spiuserforum.com/index.php?t ... inia.9030/
Post Reply