84-87 vs 88 front suspension (Page 8/10)
zkhennings APR 03, 07:47 PM
Do you know what rate front spring you are running? Because base model springs with 1 coil removed are still too soft.

And yea I did say the rolling resistance is basically negligible, I wonder how much force is applied at the contact patch during hard acceleration when you take into account the change in inertia of the (front) wheel. Still probably negligible in regards to suspension design but interesting to think about the full fbd.
ericjon262 APR 03, 09:32 PM

quote
Originally posted by zkhennings:

Do you know what rate front spring you are running? Because base model springs with 1 coil removed are still too soft.

And yea I did say the rolling resistance is basically negligible, I wonder how much force is applied at the contact patch during hard acceleration when you take into account the change in inertia of the (front) wheel. Still probably negligible in regards to suspension design but interesting to think about the full fbd.



all I know about my front springs, is that they're the springs that were installed in the car when I got it, and presumably stock, 1985 spec springs with no coils removed, Right now, I'm not sold that I would want to go a whole lot stiffer than what I have with the anti dive kit installed.

------------------
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cognita semper

Will APR 04, 10:04 AM

quote
Originally posted by zkhennings:

Has the anti-dive had an effect on the nose of the car raising under acceleration? I imagine that it has stayed relatively the same?

Is the only way to really control front end lift under acceleration with stiffer shorter springs that do not compress as much as therefore do not extend as far under accelerating?




quote
Originally posted by pmbrunelle:
Part of the motorboat feeling though comes from the pro-squat in the rear of stock 84-87 Fieros, so you may want to work on an anti-squat mod in the rear. It's on my agenda, but not for a while...



Correct, most of the nose lift under acceleration comes from the rear.
The rear end is the front end of a X-body or A-body FWD car. The forward pivot of the control arm is lower than the rear pivot in this design. At the front of a car, this results in anti-dive. At the rear, it results in pro-squat.

The control arm pivots need to be moved to level or the cradle rotated until the control arm pivots are level in order to reduce/eliminate rear pro-squat.


quote
Originally posted by ericjon262:
it stays flat as Patrick said, the fronts are just rolling at that point unless you're inputting force some other way, braking, driving the front wheels, ect. the rear squatting could give the impression of front end lift though.

in this clip, I hit it from about 15-80, you can see there's pretty much no rise from the nose of the car.

https://youtu.be/60SUceya6Y4




The '88 cradle doesn't have nearly as much bad behavior as the early car's cradle. Also, you can raise the forward pivot of the trailing arm to give you more anti-squat if you want it.
zkhennings APR 06, 10:18 AM
Cool, so the rear also contributes to the front dive as well then in the pre-88, under braking the rear must lift and transfer some weight to the front suspension. I was not aware that the rear end was pro-squat, the rear control arm looks like the pivots are relatively level but I have never measured it. It is very apparent up at the front control arm that the front pivot is higher than the rear.

I have to buy some DOM tubing and rod ends and start fabricating. I need spherical joints to mess around with misalignment. I was thinking 1.25" x 0.125" DOM tubing for lower control arms and 1" x 0.125" DOM for the uppers based off reading on motorsports forums. I can do the FEA for forces during cornering accelerating and braking, but I am not sure how to set it up for hitting pot holes and that sort of thing.
pmbrunelle APR 06, 12:29 PM

quote
Originally posted by zkhennings:
Cool, so the rear also contributes to the front dive as well then in the pre-88, under braking the rear must lift and transfer some weight to the front suspension.



Lifting the rear, or changes in the pitch angle of the car, do not materially change the steady-state weight transfer to the front tires.

The weight transfer will happen, and just depends on wheelbase and CG height, and acceleration.

You cannot easily change the weight transfer (not without shifting mass around the car), but you can change how the suspension extends or compresses at each end.

Starting at 1:50, check what happens during braking when you go too far with anti-squat:
https://www.youtube.com/watch?v=UBSd2QvY7wQ


quote
Originally posted by zkhennings:
I need spherical joints to mess around with misalignment.



You know about Will's spherical bearing kit for the front lower A-arms?


quote
Originally posted by zkhennings:
I can do the FEA for forces during cornering accelerating and braking, but I am not sure how to set it up for hitting pot holes and that sort of thing.



Here's a discussion on that:
https://www.eng-tips.com/viewthread.cfm?qid=300205

I don't know how things are at other shops, but in my version of the truth, the OEM will instrument a test vehicle, and then datalog it while doing different types of driving. You could have datalogs for highway, city, etc, and each datalog would represent a certain number of driving hours in the vehicle's life.

We then study the datalogs, and then make a simplified/condensed test which attempts to simulate the datalog. Before prototypes and testing, while still in the paper/theory stage, the simplified test could translate into a series of stresses for different numbers of cycles, which could be compared to the SN curve of the material and Miner's rule.

So for example, you could say in the life of the vehicle, there will be A number of bumps at X intensity, B number of bumps at Y intensity, and C number of bumps at Z intensity.

Almost always the OEMs specify a life cycle for their own vehicle; it is not universal for all vehicles.

Probably for a low-budget DIY project, you'll use empirical vertical/longitudinal/lateral fudge factors. Look around, people use different fudge factors.

[This message has been edited by pmbrunelle (edited 04-06-2022).]

zkhennings APR 06, 02:37 PM
Hmmm I understand that there is weight transfer even if the suspension had 0 movement from the moments created at the contact patches, but for example when you set up cross weights on a racecar by adjusting ride height at each corner it only takes a minor change in ride height in the rear to put more weight on the front wheels as the CG is physically moving forwards unless CG is at the height of the wheel center (unlikely).

I am a$$uming that since the rear will physically raise more during braking that that would additionally load the front wheels creating even further dive. And same thing with squatting further under acceleration, the front will become even lighter than it normally would be as the CG moves further to the rear of the car further exaggerating front end lift. The higher the overall CG for the car the more pronounced this would be.

Is this not the same pro-squat phenomenon used in drag cars to get more weight over the rear wheels during a launch?

In Fiero terms, for the pre-88 Fiero, if you left the front suspension alone but put an 88 rear cradle in it, do you think you would measure the same vertical displacement in the front end under acceleration and braking as with the stock rear? I am suggesting you will measure smaller displacements purely from a physics point of view, but I am not an automotive engineer like yourself so I may be missing some variables. I also have not done the math and could imagine it being imperceptible to the driver.

And edit to say yes about Will's spherical bearings but I would like to make my own control arms to gain further adjustability/run a coilover up front. If I had an 88 where I wouldn't be messing with suspension geometry much I think it is an ideal solution. I guess there is always the possibility of modifying the stock lower control arm to accept a coilover, and I do like that the spherical bearings are protected with seals.

[This message has been edited by zkhennings (edited 04-06-2022).]

pmbrunelle APR 06, 09:08 PM

quote
Originally posted by zkhennings:
Hmmm I understand that there is weight transfer even if the suspension had 0 movement from the moments created at the contact patches, but for example when you set up cross weights on a racecar by adjusting ride height at each corner it only takes a minor change in ride height in the rear to put more weight on the front wheels as the CG is physically moving forwards unless CG is at the height of the wheel center (unlikely).

I am a$$uming that since the rear will physically raise more during braking that that would additionally load the front wheels creating even further dive. And same thing with squatting further under acceleration, the front will become even lighter than it normally would be as the CG moves further to the rear of the car further exaggerating front end lift. The higher the overall CG for the car the more pronounced this would be.



Yes, technically the CG moves around and weight transfer is affected. But for the kinds of analyses you might do on your Fiero, is the effect really significant enough to matter? You can estimate how much the CG moves, and see how much that affects your calculations, then judge if you want to consider CG movement or not.

It is like with finite element analysis; you can study with "small displacements", or with "large displacements" in which the system of equations is constantly being modified to reflect the deformation of the structure.

Besides modeling errors/inaccuracies, a most common source of calculation error comes from the human making calculation mistakes. Keeping things simple (but not too simple) reduces the chance of the human screwing things up.


quote
Originally posted by zkhennings:
Is this not the same pro-squat phenomenon used in drag cars to get more weight over the rear wheels during a launch?



I don't hang around dragstrips enough to see what drag cars do.


quote
Originally posted by zkhennings:
In Fiero terms, for the pre-88 Fiero, if you left the front suspension alone but put an 88 rear cradle in it, do you think you would measure the same vertical displacement in the front end under acceleration and braking as with the stock rear?



Assuming the same wheel rates, yes. Where "the same" displacement means close enough that you can't practically tell the difference.


quote
Originally posted by zkhennings:
I am suggesting you will measure smaller displacements purely from a physics point of view



Right, of course.


quote
Originally posted by zkhennings:
but I am not an automotive engineer like yourself so I may be missing some variables.



"The car" is a broad topic... even though I work in the vehicle world (never touched a passenger car though), I'm still missing a bunch of know-how.
Will APR 07, 08:09 PM

quote
Originally posted by zkhennings:

Hmmm I understand that there is weight transfer even if the suspension had 0 movement from the moments created at the contact patches, but for example when you set up cross weights on a racecar by adjusting ride height at each corner it only takes a minor change in ride height in the rear to put more weight on the front wheels as the CG is physically moving forwards unless CG is at the height of the wheel center (unlikely).



Nope, not correct.
Cross weight changes because the springs are being compressed differentially. If you add weight to the left rear, the right front gains weight as well, but the left front and right rear lose weight. The front/rear totals and left/right totals remain the same.
Horizontal movement of the CG is the sine of the roll or pitch angle times the height of the CG. If the car rolls or pitches 3 degrees, and the Fiero CG is 19" high, then the horizontal CG movement would be about 1 inch. This does not materially affect weight distribution, especially not during corner-weighting.

A go-kart with the same wheelbase and CG location as the Fiero would have the same degree of weight transfer under acceleration/braking as a Fiero, if the minimal movement of the CG is neglected in the math.


quote
Originally posted by zkhennings:
I am a$$uming that since the rear will physically raise more during braking that that would additionally load the front wheels creating even further dive. And same thing with squatting further under acceleration, the front will become even lighter than it normally would be as the CG moves further to the rear of the car further exaggerating front end lift. The higher the overall CG for the car the more pronounced this would be.



The horizontal CG movement is not a significant factor in weight transfer. Pro-dive and pro-squat do not materially affect weight transfer... just suspension and body motion.


quote
Originally posted by zkhennings:
Is this not the same pro-squat phenomenon used in drag cars to get more weight over the rear wheels during a launch?



Drag race cars use ANTI-squat. A well tuned drag race suspension will leave the line with the fronts just barely off the ground, but won't go higher than that. The geometry of the suspension changes the direction in which the tractive force of the tires is applied to the body.


quote
Originally posted by zkhennings:
In Fiero terms, for the pre-88 Fiero, if you left the front suspension alone but put an 88 rear cradle in it, do you think you would measure the same vertical displacement in the front end under acceleration and braking as with the stock rear? I am suggesting you will measure smaller displacements purely from a physics point of view, but I am not an automotive engineer like yourself so I may be missing some variables. I also have not done the math and could imagine it being imperceptible to the driver.



An early car with an '88 cradle will show similar front end lift to an '88 under acceleration. The early car might actually have a little less, since the early front springs are slightly stiffer than the '88 front springs.
Under braking, the same car may show the same front end drop in terms of distance, but less rear end rise due to the lesser anti-squat geometry in the rear. This would translate to a lesser pitch change from braking.

Squat and dive geometry uses tractive forces on the tire to apply extension/compression loads to the suspension... Since these loads are in addition to the spring, but can be positive or negative, then they will change the pitch attitude of the body relative to the steady-state (constant acceleration) cases without the squat or dive geometric effects.

[This message has been edited by Will (edited 04-07-2022).]

zkhennings APR 08, 02:18 PM
Alright I trust you guys but I could swear that I have seen drag cars with lower front pivots in the rear suspension linkage, and I could have sworn my friend who races spec Miata raises the rear for extra twisty tracks to help the rear end be looser, and he lowers it for racing in the rain to get more rear end traction. He did confirm that it affects the handling of the car drastically but also confirmed the front to rear weight distribution only minorly changes. Very anecdotal information, I know.

I guess I am still unsure how the front end on a pre-88 with a 88 rear end will have less front end lift under acceleration than a stock pre-88 from this conversation though, I can see it having less overall angular pitch because the rear won't drop, but why would the front raise any less if the weight transfer is unaffected by the lack of rear squat? And I am defining "front" as the point directly above the front suspension, if you are referring to the extreme forward point at the end of the front bumper will raise less, that makes sense due to the overall angular pitch of the car being reduced.

But if the actual front spring extends less under accel with an 88 rear end then I need answers lol.

This has been an enlightening conversation! I want to do out some of the math to see how minor the the weight transfer actually is. Would be interesting to work backwards and adjust suspension and measure the pitch of the car static while wheels are on scales, see where CG ends up via scale weights, and using the angular change of the car apply some trig to figure out the CG height.
Will APR 08, 09:34 PM

quote
Originally posted by zkhennings:

Alright I trust you guys but I could swear that I have seen drag cars with lower front pivots in the rear suspension linkage



Pic?
Also, was it a successful drag racer?


quote
Originally posted by zkhennings:
, and I could have sworn my friend who races spec Miata raises the rear for extra twisty tracks to help the rear end be looser, and he lowers it for racing in the rain to get more rear end traction. He did confirm that it affects the handling of the car drastically but also confirmed the front to rear weight distribution only minorly changes. Very anecdotal information, I know.



While that does move the CG vertically a smidge, that has a much larger effect on the height of the rear roll center. Raising the rear roll center is one effort that has been demonstrated to significantly improve a Fiero's handling, moving the car from limit oversteer toward neutral, because it carries its weight higher in the rear. FieroGuru has a bracket kit that does this for the '88 cars. My fabbed knuckle design for '84-'87 cars will lower the outer ball joint for a similar effect.
There are also "roll center kits" for front wheel drive cars, which mostly consist of rather scary looking ball joint extensions.


quote
Originally posted by zkhennings:
I guess I am still unsure how the front end on a pre-88 with a 88 rear end will have less front end lift under acceleration than a stock pre-88 from this conversation though,



IIRC, the '84-'87 cars have slightly stiffer springs, which would result in less suspension deflection for the same load changes.


quote
Originally posted by zkhennings:
This has been an enlightening conversation! I want to do out some of the math to see how minor the the weight transfer actually is. Would be interesting to work backwards and adjust suspension and measure the pitch of the car static while wheels are on scales, see where CG ends up via scale weights, and using the angular change of the car apply some trig to figure out the CG height.



The magazine articles I've read from back in the day quote the CG height at 19". 1" lowering makes that 18" of course, but engine swaps might change it that much as well. Because of the narrow bank angle, the V6/60 carries its cylinder heads high up, which may give it a higher CG than one might expect.