leaf spring - what if

[geometryjeff]

OK guys - correct me if I'm wrong. Rear wheel steer on a leaf car is caused by the angle change of rear end housing when car body rolls in a turn; the leaf on outside (opposite direction of turn) flattens which cause the chord length to change moving that end of axle toward the rear while the opposite spring moves in opposite direction moving that side of axle towards the front - right? Rear wheel steer drives the rear of the car away from the direction of the turn for reduced traction and posible spinout. What if you could counter that movement? Would traction improve? What if the front eyes had a swivel point and the rear of the spring were controlled to allow movement to counter the rear wheel steer? Yes, there are still issues with the rear end having fixed points on the axle, but I"m just thinking about theory. If a servo or something would allow real time movement in a turn, would that create better handling coming off the turn with better traction, or would the opinion be that the car would get really squirrely and have a squishy feel? I don't have enough engineering or seat time for a best guess. There seems to be a lot of autoX'ers at this forum and I'm learning a lot from your posts. Keep up the good work for us rookies.

[cornfedbill]

In the days when leaf springs were the norm, many racers used the roll steer affect of the leaf springs to aid the cornering traction.

Tires only generate cornering force when there is a slip angle, the angle between where the tire is pointed and the actual trajectory of the axle. Modern race tires operate at a much lower slip angle than their tall sidewall predecessors. That means what worked in 1969 may not work the same today.

That being said, some drivers liked to set up their cars with roll understeer so that the rolling of the body would cause the slip angle of the rear tires to increase and provide greater cornering force. Other drivers set up the leaf springs to minimize this affect.

So your theory that countering the cornering forces by allowing the leaf springs to turn the axle into the turn is correct. It does work.

However, like many other suspension set-ups, it really depends on what the driver likes, how he drives and what he is using for tires.

[MonzaRacer]

Go look at quadra steer gm truck rearends. Very unique.

[exwestracer]

You could run slide-a-leafs on the axle and locate the housing with solid links instead of the leaf packs. Used to be quite common on circle track cars. I haven't done the math on the rear toe angle change, but I'd be willing to bet that typically you're getting as much from bushing deflection as from spring length change. Most passenger car leaf packs don't have a lot of arch at ride height....

[astroracer]

I have to agree with Ray. Most "dedicated" track cars running leafs will generally use Del-A-Lums or solid bushings in the front, eliminating any deflection there. And, like Ray stated, most performance springs will not have much of an arch. I really don't think axle deflection in a hard turn roll situation will cause much of a "rear steer" condition. The front of the leaf is going to be fairly stiff as compared to a four link bar. Any deflection will probably be in the cross-car direction or side to side movement of the entire axle on the springs...
Anyone have a Go-Pro they can set up to watch the rear shackles deflect in a hard turn? THAT would be interesting to see as it would show just how much "squash" the spring is seeing.
Mark

[Gitter Dun]

You can throw as much theory into leaf spring geometry as you want but theory is just that......theory. Get the car out on the track, get a feel for what the car is doing, and then start to plug in how and why. Use what you've learned from car feel to plug in tuning devices. There are many variables and tuning devices that will get you the feel you are looking for and real world experience is the best way to learn.

[geometryjeff]

All info is good info - thanks for all the input. It'll be awhile before seat time is reality, but working hard to make it happen. Putting car together now and like most projects, underquoted timing. My goal is to get my car on Mid Ohio course next year (worse case); hopefully this year after several local autocross events.

[Gitter Dun]

Keep us posted. I've spent quite a bit of time running rear leaf set up on the road course and might be able to help. The worst thing about rear leafs is limited tuneability. The addition of a Fays 2 watts link will allow you to adjust rear roll center height. From there your limited to ride height, tires, tire pressures, shocks, and sway bars not to mention you can tune the rear by tuning the front and vise versa.

[High Plains Mopars]

I thought we talked about this in your previous leaf springs post?

Yes, leafs have some amount of steer. Yes you can alter that with arch, splay, eye height, bushing compounds, shackles or sliders. The amount you want or don't want varies from car to track to driver. You can add or eliminate it to assist with your goals. However, it isn't always easy to do with leafs because they are rather large and in a stock installation, do not have a large range of adjustability. If you have the room in the chassis, you can fabricate alternate mounting methods and positions to best achieve your goals, provided you don't have a rule book limitation about changing your locations.

[geometryjeff]

Thanks again for constructive input. This is where I'm at today. 68 Camaro going thru another build (had it for 30+ years). It's been a fun ride but mostly straight line. This rebuild is going a different direction. Body seams and frame rails have been MIG welded solid; I've added formed 1/8 plate at subframe mounting point with locator holes to accept Al body bushings for aftermarket subframe (lots of tuning adjustment). I had DSE mini tubs installed with lowered Hotchkis springs moved inboard at back (1" per side) to accept the rear end cut for Z06 wheels/P-zeros. The frame connectors are custom bolt on so that bottom of car can go back to stock if future needs are different. I'm working on the attachment points for the front spring eyes. I've got 3/16 plates formed and ready to weld on to both sides of frame rails at front so that modified spring cups (with Cal trac plates) can bolt to connectors and body points (some adjustment capability). Ive got Del-alum style bushings front and back of leafs (question in my mind is whether there's room for improvement). Things on the 'todo' list are to weld up shock mounts (inboard) and modify lower plates for CAl-trac and shocks; add mounting for quick change anti-roll bars (adjustability); and install cage. Now for advice. I've been around cars since a teen - mostly straight line stuff, but I have 1MM+ miles of seat time (city/highway all kinds of weather) and understand handling physics and momentum. Because of the mods already in place on car, stock autox classes are out, so I'm building a fun car that may or may not be competative on a track and that's ok. But I'd like to get a baseline on a course and improve from there. This is for fun and cruising. In your opinions, has anything that I've done so far precluded me from getting onto a track or course somewhere and having fun?

[Rod]

I've got 3/16 plates formed and ready to weld on to both sides of frame rails at front so that modified spring cups (with Cal trac plates) can bolt to connectors and body points (some adjustment capability). Ive got Del-alum style bushings front and back of leafs

these are the only things that throw up a RED flag for me

1) cal tracs produce bind on leaf springs, not good for cornering
2) Del-alum bushings front and rear, Even Global West the makers of the Del-alum bushing dosnt use the Del-alum bushing front and rear it creates bind also, there own Cat 5 leaf kits have Del-alum shakle and sperical bearing front eyelet

[geometryjeff]

Thanks for concern and info. I've incorporated a spherical race within the sandwich between cal-trac plates which have running clearance with the through shaft. Hope that description explains it, but suffice it to say - got it covered. I only hope I've got enough tire clearance inside and out. Also, thanks for your web site. Good info!!

[Rod]

- got it covered.

ok!

[exwestracer]

Rod,
If we are talking about a "true" Calvert Cal-tracs setup 70781, the roller only leans on the spring during axle wrap, not under simple spring compression. Obviously this depends on the amount of basic arch in the pack versus the relative angle of the lower link. There isn't any reason that the Cal-tracs should bind the spring up in cornering, and it could be said that calming the spring down under acceleration OFF the corner might make for a smoother exit. Your mileage may vary....

[Rod]

umm I see that Ray, I seems that you could be right, but my question is that are they mainly for the calvert leafs like they show? I do see that the calvert leaf isnt a laminated leaf spring(multi) theirs is more of a simple suported leaf without any reinforcment of the front eyelet, were its a little easier to read the elasticity of the material and control the rebound of that single leaf with the trac bar, on a laminated leaf I guess you have to weigh the options of use and style of leaf from semi-elliptic to full taper or parabolic spring and also the location of the spring clamps that are used to help with stress distribution from impacts and there impact on leaf wrap, I will keep looking at the options for cal-tracs but the varibles of use on a leaf other than a controled cal-tracs single leaf, to me is a lot of more hassle and stress to add a second binding device on a multi leaf that is already designed to equalize impacts, then again I could be WAY off, Im no expert just a regular old racer LOL thanks for making me use my noggin Ray, now my head hurts

[Skip Fix]

"Even Global West the makers of the Del-alum bushing dosnt use the Del-alum bushing front and rear it creates bind also"

They do have a full Del a Lum kit for upper and lower shackles and front spring eye also.

Caltracs with their front plates and the center sleeve have a moderate amount of side clearance-ie the front of the spring can move around-at least their split leaf springs and bushings and plates I have on my Camaro. The spring perch bolt tightens completely on the sleeve before most of the slack is taken out on the sides of the brackets, on that front upper bolt in the picture. There is also a little slack in the brackets holding the split leaves together aas you can see in that picture. Not sure how solid it would be for precision cornering.

If spring wrap up is a concern I think making a spring like the Guldstrands where the second leaf wraps around the spring eye or like the old Mopar Super Stock springs with the second leaf all the way to the spring eye and possibly some clamps in the front half.

http://www.guldstrand.com/scripts/prodView.asp?idproduct=166