Technical Need Pre- war race car handling help! Understeers badly

Not likely. Frame is very stiff due to boxing and additional cross members. Check out the front and rear pics I posted while jacked at L/R.

 

Greetings to all involved in this conversation! As has been mentioned by several, roll center height also plays a large roll in determining which end of the car has better lateral grip. I am wondering if the rear radius rods not pivoting on the axle and thus turning the rear axle into an " antiroll bar" ( also sometimes called a sway bar) but the inherent bind may also cause their attachment point to be the roll center height. The front roll center height is largely determined by the panhard bar attachment point heights ( split the difference if one end is higher than the other ) but the quarter elliptical spring attachment also adds some lateral location. If the rear of the chassis/body were allowed to roll over the rear axle by eliminating the bind and the car still wanted to understeer attaching a panhard bar level and above the center of the rear axle should help....the higher the rear roll center the less the car should " push" ( most American oval track guys term for understeer!)

 

Greetings dirty old HAMBer!....you know after thinking on this a bit more and looking at the photo of the racecar in question I'm wondering if the fact that the car appears to be pivoting around the contact patch of the tire on the ground if that has caused that to be the effective roll center?...in 40 years of wrenching on dirt modified oval track cars I have learned that the laws of physics and geometry WILL give an answer to every question if in fact we ask the right question!

 

FWIW. one guy with an opinion the rear panhard bar is the absolute brains of the suspension. You can change EVERYTHING else and the car may respond somewhat but if that rear bar is in the wrong place you will not fix loose or tight.

 

Try jacking your car under the L/R frame instead of the rear axle ,and have a good look.......

If you want to be high tech ,then do this with corner scales [the car could be "wedging" and diagonally overloading the front]

Your car is doing the opposite of what it should be doing. With rear roll stiffness the way you have it, it should have bad oversteer .[but it doesn't]

You have "Drag Racing" rear suspension in a "Roundy Pounder" Car.

 

Those rear wishbones must pivot inline with the universal behind the gearbox

 

Although the rear axle is a very stiff torsion bar, the entire assembly including the split wishbones is not very stiff, due to the length of the wishbones. This is for two reasons: the angle of twist in the axle for any angle of roll will be very small, and the long wishbones are rather flexible as cantilever beams. I'd expect far more bending strain in the shallow forward parts of the wishbones than torsional strain in the axle, and a very low spring rate for the assembly overall.

As I've said before, the failure mode I see here is not wrapping the car around a tree due to sudden vicious oversteer, but one of the axle-to-wishbone fixing holes wearing oval and the attachment becoming sloppy.

 

Honestly, looks more like the front end is flexing vs. the rear. Is there a way to secure the front end (ratchet straps over the frame to the axle), then jack the rear end? Can you get a smaller bit of wood as to get the jacking point as far out board as possible? You may only be getting an inch to a few inches of travel before the same side front starts to unload (as a SWAG if the frame is as stiff as you say and if all split rear bones mounted to the frame is evil crowd are correct.)

The stock model A rear spring is, as Enbloc pointed out, way over kill for the job (figure it was originally set for 4 adults, a full load of stuff and a body) 2.5k Lbs vs. what ever the current weight of your car. If you want to pull a leaf, take out the second to the largest first, then take out the second to the smallest if you want to go even softer. By removing the second to the largest, you will reduce the resistance on the initial loading of the spring.

Sorry, completely forgot about Enbloc and his shenanigans across the pond. Have fun and keep us posted on what you learn!

 

What viscosity are you using?

 

The problem is the OP's car is "Fighting" the laws of physics, It should be harnessing the laws of physics instead.

With a high CGH where the driver is sitting combined with a tall engine, there will be a lot lateral weight transfer. With that ladder bar set up it should "unload" the inside rear [causing oversteer on the outside rear]
But it doesn't do that.........which is the Mystery.

I am willing to guess the the "roll centre" at the front is very high.
And combine that with no bodyroll on the rear and the outside front is subjected to increased lateral loads without increasing the vertical loads on the outside front.[Increased load with out the increased grip]

If the RC was lowered in the front [which also lowers the roll stiffness] it would suddenly start oversteering and the OP would then need to address the rear ladder bar issue.

 

What I'm saying is that the rear wishbones aren't really adding a lot of roll stiffness to the rear. I agree that it's never a good idea to have mystery elasticity in a suspension system due to bending or twisting things which aren't supposed to bend or twist, and that includes the frame rails.

Your guess of a high front roll centre makes sense, but I don't see anything in the pics which would bring that about. The quarter-elliptics' shackles probably provide enough lateral location to obviate the need for a Panhard bar, and as a general rule you don't want several lateral locating devices which might try to fight one another. In this case however, both the quarter-elliptics and the front Panhard bar will impose a roll centre somewhere within the six inches or so below the level of the front hubs, so there won't be appreciable bind there unless the front spring rates are a lot lower than they seem to be. But I'll look again at the pics, just to make sure.

A technical nit-pick – though I quite understand what you're saying in the present case: a high roll centre doesn't actually cause roll stiffness. Roll centre height is a factor in geometric lateral load transfer, and roll stiffness is one in elastic lateral load transfer. The sum of these (plus inertial lateral load transfer of the unsprung mass, which is usually not easy to change) will give you total lateral load transfer, so the same total lateral load transfer can be achieved by a range of combinations of roll centre height and roll stiffness.

I'm still stumped as to what is loading the outside front tyre in this case (if indeed it is getting loaded. It could be something quite non-fundamental like a softer tyre compound on the back?)

 

I had another look. I don't see anything in the pics which might place the front roll centre anywhere except about 3-4" below the level of the front hubs.

It's also evident from the jack exercise that there is more roll deflection at the front than at the rear; and I'd say therefore a greater rearward bias in roll stiffness than the probable rearward bias in tyre adhesion. Everything suggests that this thing ought to handle beautifully, with a slight and controllable tendency to oversteer.

I'd like to take a look at the steering geometry, though. I don't see a bump-steer disaster but it might be improved.

 

I may recall this incorrectly, but I thought the roll center of a any solid axle was the center of where the leaf springs bolt to the axle?

Could be a useful site...

https://www.racingaspirations.com/

 

It's halfway between the height of the spring eyes and the height of the other mounting – if the spring plays a lateral-locating role.

 

You are correct about the R/C ,it is reasonably low.
If we look at this photo when the body is tilted to the left [of the photo] it also shifts/arcs to the left . This puts the RC [pivot point] below the springs near the panhard height.


I don't agree with your statement "Everything suggests that this thing ought to handle beautifully" [It should oversteer like crazy]
Parallel ladder bars do not belong on a road racing car , and this car has them both front and rear.

The car should be neutral on corner entry, and slightly understeering on corner exit [to allow throttle oversteer]
Some of the engineering needs to be corrected before the OP goes through the tedious task of dialing it in.
The first thing I would consider is getting rid of that rear ladder bar system by pivoting them at the axle.
A torque arm [eg: Camaro style] or an upper 3rd link would need to be added.

Do this and the rear panhard bar won't bind ,and then the dialing in process can begin

 

Mimilan,
Thanks for your latest comments. I did try jacking on the chassis in the rear and I only have 4.5 degrees of roll before the tire starts to clear the ground. I guess at this point I'm wondering how much would be needed for it to improve handling? I don't have time to relocate suspension components before the event 7/19-21 so will have a go as is. I like the idea of a third link and pivoting the track bars but not sure where to put the torque arm. Any thoughts on the length the arm would need to be? Space is limited. I can move the track bar front locating points towards the center of the chassis but only to about 10" a part. Which option would be preferred torque arm or relocate track arms?

 

I just was under the car this AM. The pivoting of the track bars appears doable with a torque arm; in fact it might be the least invasive means to get the rear to "work" better. The torque arm would need to be below the drive shaft. I can make a substantial bracket to bolt to the banjo bolts and run forward. Would I want to have a spherical bearing (heim joint) in front or poly bushings. I assume the front location should be in line with the front u-joint; correct? My lift is tied up or I would post photos of underside.

 

gas it, kick the rearend around with engine power like a big go-cart.

 

You sound like my husband....... He "overcooks it" getting supplies for the BBQ , but never overcooks it on the BBQ.
Backing a car into the corner [outlaw sprintcar style] is a lot slower than late apexing.

But double apexing or "diamond pattern" can be very fast on short tracks.
For that the car needs to have slight oversteer under braking ,and slight oversteer on acceleration.
It is a good technique for drivers who prefer to Right Foot Brake!
They downshift at the point where the car "takes it's set" [the point in the diamond where the car turns in]

It is a good passing technique under braking, and blocking on corner exit.

 

If you have a torque arm up the centre ,it must be able to float [forward and rearward] or it will bind when the outer links travel in an arc.

Brake torque will also cause the front of the torque arm to nose dive [and the rear end to lift] This is the main reason why torque arms are not favoured by race car designers

Note: Sprint cars usually have the callipers mounted to a floating "birdcage" to separate brake torque from axle rotation [so they have a torque tube and outer 4 link locators to the birdcages]

If you have a 3rd link [with pivots at each end] it will not bind. But brake torque will cause compression loads on the upper link, and tension on the lower links.

Accelerating forces at the axle centreline get transferred to the frame via both the upper and lower links. These forces applied to each of these links get proportioned by their distances from the axle centreline [picture a plank between 2 blocks and you standing in the centre , or closer to one end]

On a floating torque arm design all the accelerating forces at the axle centreline get transferred to the frame via the lower links.

Because your outer links pivots will be lower than the axle centreline, the accelerating forces at the axle centreline vs resistance to acceleration at the lower pivots will cause the axle to try and rotate downwards [so the torque arm pushes down at the front]

There is also pinion torque reaction that tries to rotate the axle upwards but these forces are less due to mechanical gearing. [eg: 4:1 results in 4 x the axle torque to pinion torque reaction]

With the outer link pivots lower than the axle centreline, pinion torque reaction will increase the compression loads on the outer links And simultaneously reduce the downward forces at the front of the torque arm.

The only time the torque arm causes lift at the front is when the outer link pivots are directly in front of the axle centreline [or the diff head is floating like on sprint car "open tube" axles]

Note: [If you look under a 3rd/4rd Camaro ,the outer links are slightly lower than the axle centreline so axle thrust counteracts pinion torque reaction]


^^^^ Because your existing front pivot points are lower than the axle centreline, I would suggest keeping these links parallel to the ground [this is from pivot point to pivot point, regardless of the shape of the link in between]

With 2 outer lower links, I would recommend a single upper link with pivots at each end [slightly down at the front, creating an "instant centre"]. The upper bar is usually a lot shorter, which causes the instant centre to shorten when the suspension compresses.[dynamic anti-squat]

Again having the outer link pivots lower than the axle centreline, the added forces of pinion torque reaction will increase the compression loads on the lower/outer links And simultaneously reduce the compression loads on the upper link.

This is the same set-up that Ford used in their Mustang S-197

If the lower/outer links were mounted directly in front of the axle centreline, the upper link will be under tension due to pinion torque reaction

You could use your existing panhard bar and crossleaf spring as-is.

 

When was a young dumb kid, i had a roadster with a 40 rearend open drive and split bones still bolted to the backing plate bolts and mounted to the side rails with tie rod ends. Years later when i was a little older but not much smarter, an old hot rodder said to me,,,," slide that rear end out of the back of the car complete with the bones and then straddle it and pick up the front of them bones and pretend your the car, and push down on one side while trying to pull up on the other." It don't work that way. Trying to twist that solid rearend. Now my roadster rearend has pivot points front and rear on each wishbone and a torque arm. Works as it should now. A friend just cracked and broke both rear split bones on his roadster mounted solid in the rear. Just sayin.

 

How did the race go? Update? Inquiring minds and the need to know...

 

Doug, I'm afraid our subject's gone AWOL! He probably has gotten totalyl confused by all the suggested tricks, "do-dads"and "Band-Aids"! (LOL)

 

@panheadguy, any update after Road America event?

Phil

 

At this point I just hope he is ok.


Sent from my iPhone using H.A.M.B.

 

Lets hope to hear from him soon

It should be discussed his driving technique and when the push starts.

Old Dawg, races vintage all over the place, might be some good info here
https://www.jalopyjournal.com/forum...read-hot-august-nights.1159461/#post-13190186

John

 

I'm not too sure I'm any help. To set up a race car (I think you'd know'as well as anyone); you'd have to have everything in front of you. Starting from the tires up. Example: I'm not sure that Coker makes racing tires! And then it goes from there. When everything is viewed and analyzed, including the driver's style; then one can go about redesigning or applying "BandAids" and other "DoDads"!

 

John,
You know better than anyone technique in driving an old race car is essential
It will never be a Vette' or Porsche so you cannot just grab a a hand-full of wheel and expect it to turn
You are also correct in having a look at the entire package, I too think it should be over steering like crazy from the info we have, but it is not
Track time and analysis is in order, I have an idea what is happening, but I cannot offer anything without having more info from the driver

Best, John

 

Thanks, but I'm still learning! Every time I go over to Buttonwillow Raceway to do driving instruction for the Vintage Auto Racing Association's racing and precision driving school; I think that I come away learning more than the students! Oh well.."It ever was thus"!

 

Learning more just opens broader avenues of inquiry.