Bump Steer

[Morris]

This one is probably hard to see but it's when we started and the numbers when we finished.....

Now it's important to say this is not something that takes 10 minutes....

[Morris]

This is on the Rear

Here's how many times we had to take the Upper control arm off to make an adjustment.....

Another thing that is important to note is that you want to go past your ideal setting and then come back to it.....

Making it Repeatable...... know that eveything is working like it should be....

The Right Rear actually worked out better with the numbers .....

[1795]

Thanks for that explanation Morris. It will be helpful to many people.

Jim

[Morris]

Here's a photo of the Right Rear and I just want to make sure everyone understands ....... that we are Screwing in the front Heim which is rotating the upright forward and allowing the two control arms to not induce any Bump Steer into the Suspension....

Of course this is done after we have first set our Camber and checked our Camber curve which is about 1*degree per inch of travel.....

And then set our Toe with 90*degree angle pieces ..... 1/16th.....Toe In...... maybe an 1/8"th inch depending on how the motor pulls....

[Morris]

Remember when you start moving the heims..... in and out of the control arms....... the rule........ is

1.5 times the diameter of the threaded part of the heim.....

In other words..... a 3/4" Heim is threaded at 3/4"X 16...... 1.5" long..... so you have to keep the heim threaded into the control arm or any piece the heim is threaded into 1.5 times ....=.... 3/4" + 3/8"...= 1 1/8" screwed into the control arm .....

That's a safety thing and strength.....

We don't want any heims or control arms breaking....

[Morris]

Well we got the suspension all back together and were rather excited about checking everything out....

Set the Castor.....
Set the Camber....
Checked the Camber gain....
Set the Toe In.....

And everything looked great..... so

Now it is time to pull out the Bump Steer gauges and set it all up.....

And we did....just like before.... The plate .....The stand.....The dial indicators....

[Morris]

With the suspension at 2 1/2" of bump .... I though I'd take you guy's thru the math one more time.....

As you can see in the gauge ...you can read down to a half of Thousands if need be and it's very repeatable.... which is very important..... because you need to know your measurement are exact..... and it'a very good way to check your work....

Here we are reading the front gauge and it's reading about .406" to .407" thousands of Inch from the 1" mark..... and the the word "IN" is next to the reading.....meaning the Plate has moved in a Toe In direction by .407"of an Inch......

[Morris]

Keeping in mind that the suspension is at 2.5" of Bump and we are taking the readings....

This is the rear gauge reading and it is reading .058" In.... meaning the plate is going away from the dial indicator.....

With both of the readings going in you would Subtract the rear reading from the front and you would get a total reading of .348" In .... Meaning the plate has turned in.....or has Toe In of .348" which is allot....

Remember the standard is .015" per Inch of travel ...... but fine tuning you can get it like we did in the rear where I think we covered 4" of travel with about .030" to .040" total.....

[Morris]

So if we look at what we got for our Bump Steer on the Front ....... it's Not acceptable......

We go from .348" In on 2.5" of bump to ....... .165" out on 2.5" of droop.......

That's .513" of movement that the wheel will steer in 5" of travel...... .... That's a Half Inch of movement.....

The best way to describe this is ...... your going down a road and you hit a bump..... and without you moving the steering wheel....the car changes lanes..... If it's a right hand turn and you hit a bump.... you drive into the ditch..... or curb or something stupid..... and you didn't even turn the steering wheel.....

If you've ever had this happen to you ...it will wake you up instantly....and you'll wonder what's going on..... then you'll try it again....and it'll do it again and you'll know something is not right with the suspension...but you won't know just what that is.....

It can be caused by many different things....and we'll get into that later....

Here's the first set of numbers we got on the new Morris Uprights....

[Morris]

So now let's look at some of the things you can change to make the bump steer better ....

1. Move the Rack..... moving the rack will change the arc of the steering arm and have a change on the Bump Steer. Up and down and also Fore and back....

2. Move the Steering arm Heim up or down to again change the arc of the Racks arm and change the bump.... this is usually just a fine tune for the major changes...

3. Change the Top of the upright so that the upper control arm is in a different position for it's arc there by changeing the steering as the upper control arm moves in with more camber gain....

4. Change the location of the Steering arm on the upright..... it's very important to have the steering arm on the upright in the right location....

In this set of numbers we changed the rack and lowered it by 1/8" from spacers that were 5/8" down to spacers that are 1/2"..... these are the spacers under the rack.....

You can see that moving the rack 1/8" made the numbers worse...... but that's a indication of how sensitive this adjustment is.... as small move will make a large change in suspension geometry....

Now it goes from .473" In ...... in bump ...... to .275" in droop.......

Which is almost 3/4"...... now if you hit a bump..... the car would jump over a fence....or another car if one was next to you.....

Obviously lowering the rack is not the thing to do..... but can we raise it enough to make it work....

[Morris]

So here we are with a suspension that wants to through us off the road..... not good.....

And making small changes won't get the Bump Steer anywhere it needs to be......

What do you do..... well a couple of things

We need to remake the top piece of the upright to raise the upright.... but in reality that not always a good thing to do because you are also changeing the Instant Center point which in turn is moving or raising the Roll Center.... And remember concensus is that the Roll Center is the most important issue when dealing or compromising all of the suspension adjustments......

The other thing that could be done is to move the Steering arms down on the upright..... that way you could leave the Instant Center point and Roll Centers alone and change them when you get to a track to see if and how much they have an affect of control of the car....like turn in... But that means re-making the upright.....

So like any project we are at a stand still as to what will be the next move..... I'm running our own computer program and changing many varables to see what is the best thing to do...... and much of it has come down to exactly what we thought.....

Raise the upright .....
Lower the Rack.....
Lower the Steering Arms.....

That is what are program said to do..... Tom K and I will figure this out and move on down the road.....

It is an important reminder for us .....and that is check and double check everything you do and what other do.... not that anything is done intentionally...... mistakes just happen and our goal is to catch those mistakes before they catch us..... and that goes for everything....

In the begining I said everything would be gone thru and disassembled and checked to make sure when we get done the car will be capable of doing anything we want it to do....and this is no different.....

I just hope it doesn't take us a long time to resolve the issue.....

As the world turns .... so do our projects..... and the good news was that My Wheels came in at BBS after one year.... but at least they are here in country.....

I'll keep you updated on the progress of KMP 259

[Morris]

Remember this all took place back in 2004.... I'm just showing you guy's that building a car is not just hanging parts on it and off you go.... which is the case with many folks...

When building a car you have to be concerned with Safety as well as Speed and many times these cars will kill someone because it was built by someone that didn't have the knowledge to sort out a car before it gets on the road or racing surface.

A foot note is that Tom K and David K reworked the front uprights to help out in this problem that all of their cars have. I don't think Shelby Inc has ever changed this situation because I worked on my brothers CSX 41XX and it was way off... much like KMP259....But we (Tom,David & Myself) fixed the issue on our vehicles.

But, how many cars are out there that have a issue and don't even know it.... Then you see a Wreck an wonder what caused the car to change directions. .... you just never know.
Morris

[eschaider]

Morris,

Thank you for the tour-de-force on bump steering one of these cars. It was excellent!

Until you take the effort to do the job it is difficult to appreciate how much nicer the car is to drive — anywhere but especially on surfaces that cause the suspension to move through its range of travel.

This thread has become part of my tech archive. Again many thanks!


Ed

[ACHiPo]

Morris,
This is great. I've flagged it so I can come back and digest more thoroughly.
Thanks!

[xb-60]

Great thread, Morris. I will be doing the checks and measurements before taking my car in for a “professional” check-over.

I remember on my first car...quite a number of years ago...I changed out the front drum-braked hubs for later model hubs with disc brakes. I found that under relatively mild braking, my front tyres were squealing quite a lot, and the tyres were wearing quickly on the inside edges.
Investigating, I noticed that the tie-rods from the steering rack to the steering knuckles were at a significant downward angle whereas they had previously been approximately horizontal.

Back then, I didn’t know what “bump steer” was, but I sure realized without too much thought what the problem was and what was causing it. The tapers were reversed on the later knuckles, so I replaced the disc brake steering knuckles with the old drum brake knuckles. Problem solved, and lesson learnt.

Thanks for posting.

Cheers,
Glen

[ERA 626]

wow, great read... if my car has this issue I will likley be bugging you for help or better yet Pay David Kirkham to fix it... I have a CSX 6000. I hope not