My dad is bequeathing his 1972 to me soon, and he's recommending that one of the first modifications I make to the car is to correct a design flaw in the rear suspension that causes (effectively) bump steer. He explained that the wheels toe (in or out, can't remember) rather severely over bumps and it can cause spin-outs at high speed. The car is equipped with a big block stroked to 540ci and 6-speed transmission, and apparently even the weight transfer during accel and decel can cause enough change in the rear toe to cause issues. However, I come here to try and see if anyone else is concerned about this issue and all I can find is that people love the independent rear suspension and autocross it happily with stock geometry. He recommended the Sharkbite suspension to deal with the issue, but users here say it's poorly designed.

So my question is, should I actually be concerned about this? Is it a real issue, and if so, what should be done to address it?

Thanks in advance!

 

I'm kinda a new guy. I've only been in the Corvette hobby for about 50 years.
I've had my C3 for close to that.
I have done a few basic mods to my car. But my rear suspension is basically stock. Although I do run a better spring.
I have driven my car like a crazy man on the street all of these years. And on the track like an extra crazy man a fair few times as well.
I would suggest you take this car out and flog the heck out it! Slide it. And basically just go out and pound on it.
Then you decide what falls short.
This car is NOT A modern sports car. Don't kid yourself.
But for the time period. It's a lot fun.

 

When it comes to the chassis (brakes, suspension, and steering) other than springs, shocks, brake pads I leave those things stock. The factory engineers considered many factors that the aftermarket doesn't think about when designing their aftermarket junk. My car had rear bump steer when I first got it. Except for shocks and a spring I rebuilt the rear suspension with stock (rubber) components and did the alignment myself, I no longer had bump steer.

 

agree... check for worn parts, trailing arm bushings, etc.. first..

 

It's not an immediate spin out and crash issue, the car just becomes a bit more twitchy at that moment. Not the end of the world.

I agree with the others saying to go through and freshen up the suspension bushings.

The rear toe will do two different things depending where the axles are at during suspension travel. This is because the axle shafts are also stressed suspension members that control the position of the rear wheels.

When the axles are downhill (wheel end lower than diff end) toe in will increase as the suspension compresses. Once the suspension has compressed enough, the axle will cross horizontal and become uphill (wheel end higher than diff end). Once that occurs, toe in will then begin to decrease as the suspension continues to compress.

Notice I said toe in will decrease, not toe out will occur. Whether the rear end actually reaches a state of toe out depends on how much toe in you have at ride height and how far past horizontal the axles travel during compression.

Around stock-ish ride height this isn't as much of an issue since you have more compression travel available before the axles pass horizontal. The likelihood of the axles crossing horizontal increases as you lower the rear ride height.

You can do a couple of different things to help with this: raise the diff and/or increase the rear spring rate. Van Steel is one company that sells kits to do both of these things. They also sell new strut rod/brackets to decrease camber change as the suspension compresses.

Installing coilovers doesn't do anything to change the geometry unless you go with a Detroit Speed Decalink setup for big $$$.

 

I have been road racing my car for over 30 years and have never experienced the situation that you described. Don't worry about it. By the way, the car is probably faster than you are. Its best to learn how to drive fast before you drive fast.

 

Have you looked at this?
https://www.vansteel.com/index.cfm?f...BoCLbcQAvD_BwE

 

I've been driving my Corvette for 55 years now. As mentioned above, many crazy times with the Corvette.

The original 9 leaf spring and other components were not great. In 1972 I converted to all F41 components. Front/rear springs, sway bar & shocks. This improved the handling greatly.

Regarding the rear setup, I found "0 degree" camber is not good if you push hard in a turn. .5 degree negative camber in the rear makes for a much more positive rear.

About 10 years ago I switched to 17" wheels/tires, which gave another boost in handling.

 

I have autocrossed, at pro-solo levels, road raced, and designed my own suspensions.
The C3 will always have some lift throttle oversteer as long as you keep a trailing arm setup.
Nothing short of a completely re-designed rear suspension will cure that completely.
The DSE Decalink is the only bolt-in solution I know of that eliminates the trailing arms, and goes to dual forward links.

To live with the trailing arm, and minimize it's bad bump-steer toe-out side effects: Loose parts and a bad alignment can make it truly dangerous.

• Make absolutely sure your front trailing arm bushings are in good condition. Pry on them with a crowbar. This is the #1 worst offender.
• Make sure all the other rubber bushings in the rear are good, especially the strut rod bushings, which take a beating.
• Make sure the side yokes have limited movement in/out of the diff. Anything over an 1/8" you can feel/
• Makes sure you have a absolute minimum of 1/8" toe-in in the rear. Just slightly over is ok, not under that.
• Make sure the rear half-shafts are pretty horizontal at ride height.

That is about all you can do, and should make it pretty decent. It should really only be noticeable near the handling limit, when you get very strongly on the gas, or on the brakes. If the above is OK, it should be manageable.

Some guys like to run way stiffer rear springs on big blocks, but mostly when they are racing them. On the street it should not be needed.
But did you say you had a 540? With that much power & TQ full throttle could sit the rear all the way down on the bump stops. You may want a stiffer spring if you intend to use that power frequently.


Here are several bump steer curves on the rear IRS that I measured.
1. Raising the rear diff1/2" helped with Van Steel Solid diff mount bushings.
2. The Van Steel Smart Bracket lowers the inside strut rod mounting point and helps the curve. The C3 bracket is graphed above. The C2 bracket is even worse. The VS Smart bracket is the best.
3. Hein jointed rear strut rods should also help. The above curves are with Zero bushing compression.
4. 1/8" static toe-in is mandantory to prevent toe-out from appearing.
5. A stiffer spring & roll bar setup lowers the stock body roll of 2.5" down to 1.0" and helps tremendously.
6. Ideal rear u/j(ride) height is 1/2" lower on the outside, vs the inside.
6. Doing all the above you can decrease the factory .180+" toe-out at 2" bump down to .030" at 1.0" bump. A huge improvement. Stock is near 1/4"!!!! .030" is pretty acceptable, even on a race car.

 

Wow, thank you everyone for all the responses. Leigh, you're a legend. Let me see if I interpret your graph correctly: Starting from the center (0,0), the yellow line (which represents the Smart Strut bracket middle setting), experiences .050" of toe-in at roughly 1" of rear suspension compression. With some serious weight transfer forward and suspension decompression, they all perform essentially the same (rapidly toeing out). Very great info!

Was this data recorded with the solid diff mounts installed? Or is it even better with the solid mounts?

You also say 1/8" static toe-in is necessary to prevent toe-out. Do you actually recommend this, or is some toe-out somewhere during suspension travel acceptable?

Also, can you clarify point 6 regarding the u/j(ride) height? I'm not familiar with the abbreviation (or maybe the whole concept there).

It looks like the Smart Struts and Bracket, plus the solid diff mounts are no brainers. I don't know how old all the rubber is under there, but it's likely it'll get the parts cannon for all the bushings (unless he's had it done in the past 30 years). I like the idea of stiffer sway bars, so maybe the whole Slalom kit from Van Steel is the call.


Thanks again Leigh! And everyone else, too!

 

Glad it helps. And thanks for the compliment!
1/8" toe-in is recommended by Van Steel and almost everyone including GM.
1/8" total toe-in is .062" on each wheel, and most of that could go away on bump. With a factory strut mount you would have double that at 1" and already have toe-out.
You want to prevent the IRS from going into toe-out at any point in it's travel. Especially on a bump when that tire is heavily loaded.
Sudden toe-out would cause the rear to steer outwards in the turn.
It is the sudden change from toe-in (at ride height) to toe-out (as outer settles) that causes the issue.
It feels like someone bumped the car in the rear, It feels twitchy.
My data was all recorded with the solid & slightly raised diff mount. And all based on one wheel.
Setting the ride height as specified by GM, pts the outer half shaft u-joint, 1/2" below the inner. The half-shaft swings in an arc. It would be best to start 1/2-1.0" below the center of that arc.
If you look at the curve, that puts it in a better location to start, and minimizes the change, as the car moves 1" into bump.
IE: 1/2" below and then above the u-shaped arc.
Doubling the roll rate with gymkhana bars & springs is one of the best things you can do,
It just cuts all the changes in half right away.
At 2.0" of bump the factory toe-out change would be near .250" At 1.0" of bump, with the smart strut, it could only be .060" of toe-out, which makes the initial .060" of toe-in, just go exactly neutral.


Van Steels recommended alignment settings. I especially like the advanced ones. Unless you are just a cruiser. I doubt that with a 540!

 

Got it. Makes perfect sense!

 

No perfect set up!!!!!!

To cure toe,trailing arms kill that idea along with half shafts that act as a suspension upper link. Like suggested especially with a big block, heavier springs with proper married shock selection and settings that control springs in both compression and rebound, would greatly help tire plant, and weight transfer would have less effect, that also would limit suspension travel and therefore less travel means less chance of toe change., but if you can't aim to miss your mystical bump phobia, I used to soften things up on rough roads, but now everything is spherical, heim, rotational after eliminating suspension deflection issues, as attempting to let the tire plant search to conform to the road irregularities (within reason)! I run extra large heims, added security to hopefully avoid shearing a link, a joint snapping!

When I ran trailing arms, I would set rear toe neutral or actually 1/8 out depending on how many straights into tight turns, or accelerations onto straights and the tracks ratio! Example is like Laguna Seca's corkscrew, I would never set up the car for the corkscrew, that would be my throw away section in favor of better set-up for the majority of that course!

 

Oh yeah I get it.
I used to run toe-out also, when racing. Sometimes.
But like you said there were other items that were just as important as toe, when racing.

I assumed this is a street guy, new to Corvettes, and looking for a street setup.
If he was racing, he probably would not have even asked the question.

There is just so much bad advice, and so many bad opinions around, since the internet, and you-tube videos, that I enjoy presenting some actual facts, to counter some of it.

I let the facts speak for themselves.
I let the readers decide to believe it or not if they want to, or would rather believe a video...LOL

In my opinion, (LOL) the whole C3 "bump-steer" terminology thing has taken on a life of it's own, due to the internet, and is really not that bad on a C3.
A bad steering corvette is much more likely to be due to lack of maintenance, on like trailing arm bushings, etc.

 

You are one of the top engineering type minded guys around here, anyone you share with should be appreciative to get the type of in depth tested in the real Word experience. Most guys do think the thing to do is go full race, and operate it on the street. In my younger days I kind of liked to brag up the parts and mods, but in my old age, I have softened up the suspension like when I do my occasional Canyon run, on a very non-smooth road (Turnbull Canyon- coming out of Whittier up into Hacienda Heights. For smooth race tracks, I set a completely different set up as you would do. Thanks for sharing, I even enjoy the the level that you share in posts, diagrams and explanations.

 

And thanks for sharing your extensive racing experience!
With some of the real legends in this hobby.
That is becoming all too rare these days.
Way too many want-a-bees / posers / pretenders / crazy drivers these days with no experience.

Back when, I once ran against a twin turbo Ferrari Boxer V-12.
In my "Just a Camaro" as he called it. (a heavily modded Street Prepared 70 Z28 / LT-1)

Before the runs he was doing all his strutting, posing and bragging. You know the type.

I would have been glad to give him some pointers, etc. but he couldn't listen to anyone else.
I asked him to join us for a walk-thru.... but no.

I had designed the course, was the safety steward, one of 4 driving instructors, and set FTD for the day of 125 cars.

After his run, he did not even stop, he just sulked off and vanished!

I annihilated him by some 10 seconds, on a 60 sec course.
He was almost the slowest car there.
Honda Civics beat him.
I can not imagine how bad his ego hurt!

 

There is not much I can add to leigh's excellent post!!!! The man knows what he is talking about through years of corvette experience, technical theory, and very importantly, racing heritage........

I did find that changing the rear strut rods on my 78 L-82 to competition adjustable ones with heim joints, years ago, no solid urethane or the stock rubber bushings, which deflect under load, improved rear steering response and reduced unwanted suspension movements. In addition, the adjustable competition strut rods are MUCH STRONGER than the stock OEM strut rods with rubber bushnings which will deflect under load. I bet your rear struts are moving around quite a bit, causing unwanted suspension deflection, from that BB 540.

The other issue that will require a lot more work to address, especially with BB TQ, is that the stock C3 frames when new would flex quite a bit under load and stiffening up the chassis will pay dividends, especially at the limits of adhesion....not a simple fix, though

 

I bet just a few outfits even have spreader bar kits (easy to make thou). Many older Companies avoided a kit for big blocks! I have always added one, on all of our cars. I weld the lower a-arm attachments just for safety and there are a lot of locations where gusset plates can be welded to strengthen the frame in areas without pulling the body! My. Opinion is if you are racing without a roll cage, you aren't race capable, as that singularly would give the most benefit related frame twist and rotation. The raising the diff cross member is very advanced and welding it in. So much that can be done, to get the car better!

I only auto crossed limitedly, so only participated in course setups a number of times, as the imports always want a slow course that is tight. More of them out, so hated the Politics, luckily like with my time with the almighty Country dominant NCCA in Norcal, they had the clout, and usually were the primary organizers, so we could get a horsepower car capable course! My Asebring San Diego days, seemed like POA Club (Porsche) ruled the roust; as I said, hate the politics. Vintage racing organizers didn't like my level of mods, even though I told them to give others the accolades. But the venues I see in the modern look less interesting to me, like the super tight parking lot stuff with all of the Promod, non-vintage cars. The vintage racing I have courted going back for another try, but now a money thing for the rich guys, who will rub on you, nothing is vintage or is rarely vintage anymore! So my question, where are you going to use it, if you build it?

I kind of take Zen runs thru my private Canyon where I named all of the turns, labeled the course formally in my neighborhood. It is a 2.1 mile stretch equal to many race tracks in length with far more turns and is highly technical due to it being a real road with all of its flaws, and issues, and is lightly traveled! I will recon it for Law Enforcement presence, flip it around to return to the bottom, then send it, and exit out the top Back ends side into my neighborhood! It satisfies the speed addiction bug, with no fanfare, no politics!