I have a lowered 1975 corvette, and as with these cars it has a bump steer problem in both the front and the rear especially since its lowered. Are there any fixes that I could make to the suspension? It is the stock suspension with lowering bolts and adj. cam rods in the rear, and cut springs in the front.

 

How tight is your steering box and what's the condition of the rag joint? Any work to the front suspension, as in how are the tie rod ends and idler joint?

The reason I ask is because I have cut front springs, as well as a lot of other guys, whom may or may not have longer rear bolts. I don't recall many guys having the bad bump steer problem and I'm wondering if maybe your problem is related to a worn part.

 

My 76 is lowered a good inch ++ probably an inch and a half. I did it when rebuilding the front suspension and have no bump steer at all.

So basically, what's the condition of the front end?

 

How do you know? Guessing or measuring with toe plates? All C3s have bumpsteer, it's inherent to the steering linkage design. Lowering the car makes it even worse as you're migrating to where it gets cumulatively worse.

The simplest "fix" is a set of bumpsteer blocks, however far from perfect.

 

I agree with both.
First check your front end like Durango mentioned, a shot idler arm will make your steering very loose and unpredictable. rag joint, tie rod ends, ball joints ect. Doubt your frame is cracked or welds broken but worth a check.

But as VTwin mentioned, all c3's have bump steer, just the steering design geometry, some cars have less, some more depending on their suspension and steering setup/condition. Norval did a neat modification with his drag link to change the geometry of his tie rods and his tie rod ends to eliminate bump steer.
In my head I don't see how you can totally eliminate bump steer without limiting your wheel travel to a very small range of motion but you can certainly improve it various ways like the bump blocks Vtwin mentioned.

 

I did an advanced search in the C3 section for the word "bump steer" and posts by Norvalwilhelm going back "forever" (archives) The results were a bunch of threads from 2003-2005, the exact ones I was looking for but every post shows up invalid or a dead end.
Maybe the administrator can solve the problem or reactivate some of those old posts, alot of priceless info and pics there.

Here are a few pics of Norval's solution from those threads, but I didnt' save a lot of them that were on the threads I mention above.









Wish I had the money to buy his car, its got every trick done to it...and he pioneered most of the mods that are out in the marketplace improving the C3 handling.

 

Hi superstingray_1975 i have a 72 which i rebuilt some 13 yrs ago, i did complete strip down, and effectively re engineered the rear and front suspension to eliminate the inbuilt bump steer problems.
It is possible to eliminate 99% of the front bumpsteer but to do it, you have to be prepared to do some serious work/effort as it requires stripping all the front suspension apart, some bending of the relay rod using blow torch, possibly realigning - if necessary- of the bolt holes of the steering box.
I won`t go into a lengthy post what to do yet, unless you (or someone else) wish to go down this root.
I even have film of the bumpsteer from when i had the body off the chassis, demonstrating that the bumpsteer is as severe as 1/4 " at the tyre edge.
It is possible to get some of the bumpsteer out by some manipulating of the various shims in the front end
but you will find that when you settle for a combination while the steering is in the straight ahead direction, the bumpsteer will likely become more severe while the steering is at full lock!
And vise-aversa, the more bumpsteer you get out while the steering is at full lock, the more you will have when the steering is straight ahead.
So the only way to get rid of bumpsteer is to go the extra mile i`m afraid.

 

Chevy6600

I would be interested to see your suspension work. My C3 is stock and is staying that way, but I'm always anxious to look at an interestng mod.

What I've seen a lot of in suspension mods is that people do not consider that bump steer is designed in and has a purpose. The A- arms are unequal length for the same reason. The camber change in the IRS is also designed in and has a purpose, yet people spend a lot of money to eliminate it.

It is easy enough to design a front suspension with no bump steer or an IRS with zero camber change, but they don't neccessarily work in corners.

To do a proper design, you need to consider the suspension not only in a straight line, but in a turn. The body is leaned over and the inner and outer wheel are turned different amounts.

Shocks, springs, bushings, definately, but geometry changes should noly be done if the ride height or body roll is altered. Just my two cents.

 

Without completely re-designing the 50 year old design on the C-3, your pretty limited. I'm assuming your doing some pretty 'spirited' driving if you notice bump-steer at all, but it sounds like you want to keep it as stock as possible.

Probably the simplest thing you can do without wacking away at the car is to use a modified drag link (a la Norval) that lowers and centers the inboard tie-rod ends to reduce bump steer (a lot) in the front. I'd leave the rear alone because of the extensive mods required, other than just stiffening it up to reduce travel.

I've seen some pretty good drivers compensate for these older designs just by knowing what their cars will do in bump or rebound, so sometimes it's more about skill and experience with these old crocks characteristics than anything else...

Hans

 

Not to steal the thread, but I was reading some of the old threads on this issue, and still do not really understand exactly what bump steer is, can someone clarify the problem.

Thanks

 

as the suspension travels up and down the wheel will actually turn slighlty left or right. i believe it is because the tie rod is not parallel with the lower control arm, which is why you can use those little blocks to lower the outer tie rod end to help eliminate some of the bump steer

i don't think it's the bump steer you're feeling. it's most likely sloppy suspension thats alowing the tires to move when you hit bumps

 

Just put bump steer blocks on

It will be fine

And the wheel turning left or right over bumps is most likely a loose steering box. Even if you got it rebuilt by a vendor here, I'm beginning to think they don't even rebuild them.

Bump steer is when the wheels toe in or out through the suspension travel

And zero bump steer in suspension does work fine, and is ideal

 

Like he said. Just look under the front of your car and imagine the wheel moving up and down. The end of the tie-rod at the wheel moves in an arc, pivoting at the inside rod end. As it passes through the arc, it "steers" the wheel very slightly in or out.

On the purpose built Formula Fords and Sports 2000 race cars I was involved with, we would start with a level setting on a loaded car (driver and fuel), then toe it out slightly so that we were at neutral half way up or down. It rarely toed in because of the stiff suspension and relatively smooth track surfaces. Also, initial toe-out gave us a fantastic light turn-in that the driver really liked. We used different settings for different tracks, but on rough streets and a fixed suspension your kinda' stuck.

Hans

 

Hi all, unfortunately i have no pictures or film that is pc compatible as i did all my work when pc graphics was not the norm, so that is why i mentioned earlier that i would go into more detail if someone was interested enough to go through with it, as i would need to digitise every thing and possibly do drawings. But i do not mind describing what i did as it is much easier to do and less work for me.
The reason i went to the lengths i did, was because i am more interested in engineering the mechanics...street rods being my passion. So when it cam time to do a rebuild, i had the chance to do something about the less than desirable front suspension, and i could see a way to improve things.

I take interest on how / why things are the way they are, so i do note that you need toe out on some vehicles and toe in on others. That when cornering, the inside wheel needs to turn more than the outside wheel, i will also add that you also have to compensate that extra angle to some extent because of tyre scrub being different due to the difference on tyre loading while cornering at speed. That toe in affects high speed stability. That different tyres will require slightly different settings. etc.

I believe that while the points mentioned above are needed, i do not think it would be desirable while cornering, to have the toe in setting jump wildly from one extreme to the other when you drove over a bump, especially if driving `on the edge` And while taking a blast down a straight road, it can not be ideal to go over a bump and have that wheel suddenly want to go in a different direction?

I think it must be said that if you have stiff suspension, the up/down travel is going to be negligible so you will not see any bump steering, but i have not got my suspension like a go kart, i like a little comfort. Also, i think you get more road contact over rougher roads as the tyres will follow the contour of the road surface better with some suspension travel, so in this situation, you will get the toe in jumping about...whether someone can feel it or not is another matter!

Food for thought...if the front suspension is jumping about from left to right (one wheel or 2 wheels) because of bump steer, together with the rear end which also suffers from bump steer, doing the same, then add in the fact your using a reciprocating ball steering box, what chance is there of going in a straight line?

as a side note, if you look at the early bog standard C3`s as they come from the show room, and also work shop manuals, you will see that the front suspension camber is set up so that the bottom of the front tyres (in contact with the road), are closer together than the tops of the tyres. This is opposite to what you find on modern cars and performance cars..in fact the only other situation i know of like this, is on a horse driven cart !.
However, this setup does allow the king pin angle to be pretty much straight up/verticle, and it is while the king pin angles and the angle of the steering box are all pretty much the same,i e. straight up, you will get 99% zero bump steer.
The high performance C3`s have the camber set the other way ie. the bottom of tyre in contact with the road being further apart than the tops of the tyres. You will not get bump steer, but only because there is no suspension travel on these versions!.

I`m not trying to force my views on any of you, so don`t take anything i say the wrong way, i just enjoy a bit of banter with similar minded folk.

 

You didn't say how low you went. Did you set up at Chevy Power recommended road race prep heights or go much further? Assuming you haven't slammed it beyond all recognition, installing bump steer blocks up front will improve (not perfect) the bump steer curve. They also increase Ackerman (IMHO a bonus).

Out back, the C2/C3's inherent rear toe steer issue is worsened when ride height is lowered to a point where the 1/2 shafts become lower at the diff than at the stub axle during suspension travel, as alignment moves progressively towards/into toe out once a 1/2 shaft goes below level. Short of redesigning the rear suspension, the fix/crutch here is to raise the diff x-member to minimize/eliminate this adverse geometry.

In any event, don't ignore addressing any slop in your suspension, especially in the steering box.

 

superstingray_1975,

For the front, I agree with the others ..... put bumpsteer blocks on and you'll love the results.

For the rear, your either stuck with raising the differential x-member or running alot of rear toe-in. Raising the diff is the best option, but it does take work and fabrication skills (a whole weekend). Increasing the toe-in will not correct the bumpsteer, but it will keep the wheels from going into a toe-out condition which is probably what you are feeling in the rear (of course this is at the expense of increased tire wear).

What are you using your car for? spirited street driving or track use?