Bump Steer

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Bump Steer

Postby Loren » Sun Feb 11, 2018 10:32 pm

Bump Steer - What is it? Why is it bad? What causes it? How can we fix it? Let's find out!

Perfect World

Imagine a perfect world where you were driving a perfectly designed sports car with a dual wishbone suspension. A couple of things you'd want in the setup of that car would be lower control arms that were near parallel to the ground. And steering tie rods that followed exactly the same angle. You may have heard this kind of thing before. But, why?

The effects of the angle of a control arm or tie rod as the suspension compresses are fundamentally the same. If the arm starts out horizontal, as the outer end goes up (or down!) with suspension movement, the effective length of the arm decreases. (you can calculate how much by using some simple trigonometry, I'm not gonna do that right now) If it's a lower control arm, it's pulling the bottom of the knuckle/hub assembly inward, losing negative camber. If it's a tie rod, it's effectively pulling the steering arm inward. If the steering rack is ahead of the center of the hub, it's steering the wheel inward. If the steering rack is behind the center of the hub, it's steering the wheel outward.

The further you get from horizontal, the more extreme the effects of this change in effective arm length are. As you move closer to horizontal, the effects are much less. In the real world, suspension moves up AND down as you go over bumps in the road, as well as when we turn and introduce body roll. The goal is to have the arms (both lower control arms and tie rods) horizontal when the suspension is in its static condition so that the changes are minimized throughout the normal range of operation. Cool.

Imperfect World

But, we don't live in a perfect world. We're driving a car that is designed with compromises. Even if it's a sports car, it's designed to drive on "typical" roads. And now for whatever reasons, we've decided that we want our car to be lower. Easy. Fit a set of lowering springs, now the car is lower. Disregarding that whole "Roll Center" discussion, which is fascinating on its own, what bad things have we done with our control arm and tie rod angles?

While we haven't changed the "camber curve" that is defined by the suspension geometry (length of control arms, position of pick-up points on the chassis and steering knuckle/hub, etc), by lowering the car, we've placed ourselves in a different place on that curve. Instead of being in that happy middle zone where camber doesn't change as much with movement of the control arm, now we're a couple or three inches off of that, and the camber changes will be greater. The lower control arm is going to pull MORE negative camber away from you than it would if it were at an optimal static angle. That's not great... but, we're talking about bump steer.

SIDEBAR: Ever heard the double-wishbone or dual-A-arm suspension referred to as an "SLA" suspension? That stands for "Short Long Arm" suspenion. We want the lower control arm to be as long as practical and be relatively level to minimize its effect of having a "reverse" camber curve. We want the UPPER control arm to be much shorter, and perhaps have a little bit of upward angle because its effect as it moves is to pull the top of the knuckle INWARD, thus increasing our negative camber. So, we're trying to "control and minimize" camber changes due to the lower control arm... and trying to get the desired camber curve with the design of the upper control arm. SLA - Short upper control arm, Long lower control arm.

The tie rod does the same thing. By lowering the car, we've changed the angle of the tie rod from horizontal to significanly upward at the end. We're out of that "fairly consistent" middle zone, and into an area where we can get significant change in the effective length of the tie rod anytime we hit a bump. That's why it's call "bump steer". You hit a bump, it can steer the car in a direction that you never intended! And, even if it's not that bad, it can give your suspension a very inconsistent feel. It's kind of like having a variable front toe setting.

So, that's what Bump Steer is, what causes it, and why it's bad. Next... how do we fix it?
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Bump Steer

Postby Loren » Sun Feb 11, 2018 10:51 pm

Fixing bump steer is easy. We just need to get that tie rod back to a static horizontal condition to minimize the bump toe changes! You can do that in lots of different ways.

Steering Knuckle
You can replace or modify the steering knuckle to get a pick-up point that is in a correct (if you've lowered the car, that's going to be lower) position to make the arm level. If you're replacing the knuckle anyway (think "drop spindles" like you hear about in places like the lowered truck or hot-rod world), then it makes sense to incorporate this change in them. Otherwise, it's probably a pretty expensive way to go in most cases.

Steering Rack
You could work the problem from the other end and relocate the steering rack. If you've lowered the car, you'd RAISE the steering rack to get your tie rod angle back to normal. Some cars this might be easy. Others, not so much. Depends on how it's mounted and how much clearance you have to work with.

Tie Rod Ends
The more common way to correct bump steer is with a modified tie rod end. Remember, the goal is to move the PIVOT point down. So, we can't do something goofy like just put a dogleg in the tie rod or tie rod end. We actually need a "tall ball joint" kind of thing. It can be a custom tie-rod end that incorporates a ball joint with a longer stud. Or, it can be just a long stud that fits the knuckle and is threaded such that you can use a standard heim joint on the bottom to connect to the tie rod. The heim joint style would give you ADJUSTABLE bump steer, which could be handy.

I'm going to explore doing something with my tie rod ends on the Mirage sometime soon. So far, I've lowered the front of the car by about 2.5" and brought the lower control arm back to horizontal with a 1" taller ball joint. But, the tie rod is WAY out of whack, and it DOES feel "weird" in certain bump situations. And, because my steering rack picks up on the back side of the hub, my bump steer is OUT. So, when I turn sharply, the outside suspension compresses and the loaded tire steers OUT of the turn... it gives it a very imprecise and inconsistent feel in sharp turns. I'd like to fix that.

(Curiously, the more traditional "front steering" car would toe-in under compression, a problem you could assist by using the traditional autocross "toe out" setting. Makes me wonder if that's where the whole "toe out" thing came from? And if it DOESN'T really apply to "rear steering" cars? I might research that.)

Stay tuned...
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Bump Steer

Postby Loren » Mon Feb 12, 2018 1:03 am

Getting deeper into this.

What I said above is a bit oversimplified. It's not incorrect, but it leaves some things out. I didn't think they were important things... but, in looking further into it, they are.

If you want a more thorough understanding of how bump steer works (as well as plenty of photos and drawings), here's a Good Article

Basically, what I glossed over was the fact that the angle of the tie rod should point toward the instantaneous roll center. What the heck is that? That's the mythical point at which the angle of your upper and lower control arms would meet. On a properly designed car, it should be well outside the wheelbase of the car. On a McStrut car, the "upper control arm" line can be considered to be perpendicular to the strut.

So, on my McStrut car, as long as the lower control arm is "close to horizontal" (as alluded to above), that instantaneous roll center would be WAY out there somewhere. And the lower control arm and tie rod are close enough together height-wise that they're essentially going to have the same angle. You could do the math and/or do some drawings to figure out where the IRC is and what the exact angle needs to be if you want. I'm just going to try to make it match my lower control arm... or at least be closer to it. I've never been trying to make my car "perfect", just "better"!

There are other things to consider if you're designing a car, or doing more extreme modifications to the steering system. But, I'm not going that far. So, everything else stays the same... all I'm willing to do here is modify the outer tie rod end. That should solve my bump steer problem. If there are other problems (like incorrect "ackerman" geometry), they were probably design compromises from the factory, and I'll live with them.
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Bump Steer

Postby CaptainSquirts » Mon Feb 12, 2018 10:15 am

Easy solution for this. Get some crazy high spring rate springs and a fat sway bar so the car has no body roll so bumpsteer can't be induced :lol: . Anywho, how about just adding shims or something to the outer tie rod if I'm assuming the tie rod is pointing down from outer to inner to get it more level?
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Bump Steer

Postby CaptainSquirts » Tue Feb 13, 2018 12:52 pm

Can bumpsteer be a good thing in some cases? I know a little about it(not the hardcore technical stuff) but I guess in a case of bumpsteer in the rear. For my car for example, the arm that controls the rear toe is on the front side of the front of the knuckle. So correct me if I'm wrong, compression would toe in and toe out on extension. So in a straight line, acceleration would give the car a little more stability, braking it should cause a little instability. If the arm was connected to the rear of the knuckle then I would assume it would do the opposite. Now are there scenarios where you could purposely have some bumpsteer to manipulate how the car reacts in certain situations for a positive outcome instead of it being a negative?
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Bump Steer

Postby Loren » Tue Feb 13, 2018 1:39 pm

Yes, sometimes a little bump steer can be designed into the car. Particularly on the rear, as you've noted. Probably more so on high-powered cars where they're trying to tame a potentially wild rear end.

Miatas (at least the early ones that I used to play with) are designed with NO toe change in the rear. Full range of rear suspension motion, no change in toe.

What I'm more talking about in this thread is the bump steer that is created in the front by lowering the car. That's the common problem, and one that is worth considering.
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Bump Steer

Postby Rpwolf » Tue Feb 13, 2018 8:58 pm

Loren wrote:
Tue Feb 13, 2018 1:39 pm
Yes, sometimes a little bump steer can be designed into the car. Particularly on the rear, as you've noted. Probably more so on high-powered cars where they're trying to tame a potentially wild rear end.

Miatas (at least the early ones that I used to play with) are designed with NO toe change in the rear. Full range of rear suspension motion, no change in toe.

What I'm more talking about in this thread is the bump steer that is created in the front by lowering the car. That's the common problem, and one that is worth considering.
interesting, thought that it was always bad, binding up the suspension travel in a bump steer case would case the spring to be ineffective and the car would react to the harshness of the binding parts. Sounds like you have been doing some research though, many props as you are the few that do.

other simple solutions in solving bump steer (pending your oem design) could be flipping the tie rod or making spacers for the steering rack.
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Bump Steer

Postby Loren » Tue Feb 13, 2018 9:08 pm

Tie rod ends usually have a tapered hole in the knuckle assembly, just like a ball joint. (as I learned in my ball joint project: it's a 7-degree taper) So, while you "can" sometimes install the tie-rod end upside down, it might not be as secure.

Relocating the steering rack can work. Some cars it's just a matter of installing spacers and longer bolts.
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Bump Steer

Postby Loren » Sun Feb 18, 2018 12:10 am

Soooo... the most typical "bump steer kit" looks something like this:

Image

The black tapered pin fits the steering knuckle, and the spacers on it allow for height adjustment of the heim joint. The big black part is a chunk of aluminum that threads onto the end of the tie rod. Pretty simple stuff. Prices range from about $130 to over $200. But, they're all pretty much the same in their basic construction.

Of course, none of the cheap-and-plentiful ones will fit my Mirage! That would be too easy.

A little measuring showed that my tie rod thread is M12x1.25. And research says that the tapered hole on the knuckle is a standard 7 degree taper. Interestingly enough, this is the same as Miata. But, nobody makes a bump steer kit for a Miata because even when you lower a Miata, the bump steer usually isn't enough to worry about. However... it's ALSO the same an FC/FD RX-7. And there ARE a few low-volume kits out there for those. But, they're pricey.

I'm having fun fabricating stuff, and I'd rather do some fab work and spend less money, so I started looking into the options.

Everything centers around a standard 5/8" heim joint. That's a 5/8" hole and 5/8-18 threads. Very standard, and available from about $10 to $50 each. For tie-rod duty, a super-high-load joint really isn't required. Though as much range of motion as possible is a good idea. I settled on a mid-priced one from a reputable company. Word is that no matter what, these things will wear out and get clunky LONG before an OE tie-rod end would. At $15 each... they are super-easy bolt-in parts, readily available, and cheap enough to be considered expendable.

From there, we've got the pin end and the threaded adapter end.

The tapered pin with adjustment spacers is a readily available part. Available as cheap as $9.99 each in a style that uses a locknut instead of spacers. But, for a variety of reasons, I wanted the style that used spacers, and those are more like $23 each. Includes the spacers and nyloc nuts. (and looks exactly like the one QA-1 uses) No additional parts required for the pin end.

The threaded adapter... The goal is to have 5/8-16 threads on one end for the heim joint, and M12x1.25 threads on the other end. It also needs to be of sufficient length to make the whole assembly match the length of the OE tie-rod end. That will make it roughly 2.5-3" in length. (I'll measure and verify all that before I start fabrication) I considered different ways to do this. There are off-the-shelf steel bungs available with those threads with 1" OD. Could get some of those and weld them into a chunk of 1" tube. Would certainly work, and be overkill as far as strength goes. But, it's a lot of work and some expense to round up all of those parts. And I can't weld pretty enough for that, so I'd have to pay someone to do it. Meh.

But, everybody's just using aluminum. (and, yes, I did some research on what kind of aluminum to use) And... I've already got an M12x1.25 tap! If I get the tap for the other end, all I need is a chunk of aluminum! So, an $8 tap... and a $15 piece of 1.125" diameter aluminum bar and I'm mostly there!

So, I ended up buying a tap for $8. But, parts and materials for this project? Right about $85. And the only fabrication is cutting an aluminum bar and drilling and tapping 4 holes. Maybe grinding some flats in it so that it can be held with a wrench. That's it!
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Bump Steer

Postby Loren » Sun Feb 18, 2018 12:12 am

BTW, my stock tie-rod end mounts bottom side. So, I don't need to worry about flipping anything. This will mount in the same direction, and just push the end of the tie rod down.
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Bump Steer

Postby Native » Sun Feb 18, 2018 8:39 am

You know, there may well be a market in, where was it...Thailand, Cambodia (?)...for your parts...
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Bump Steer

Postby Loren » Sun Feb 18, 2018 10:07 am

:thumbwink:
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Bump Steer

Postby Loren » Mon Feb 19, 2018 11:32 pm

Most of my parts and some new tools showed up today. Gotta love Amazon Prime!

It's possible I could have this done before Endurocross. :happyblob:
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Bump Steer

Postby Loren » Wed Feb 21, 2018 4:53 pm

I spent some time and wasted some material yesterday attempting to make the aluminum tie-rod adjuster/connector pieces. Quickly learned that my drill press is really not up to the task of drilling a straight enough hole through something that's 2" long. That, and I don't have exactly the right size drill bit for either of the taps. This is really a job that is more suited to a lathe.

Fortunately, "I know a guy."

Spent some quality time at Joe's Garage today. Joe and his little baby Chinese lathe did a good job of drilling and enlarging the holes in my 1-1/8" aluminum rod to perfection. Using the lathe as a rig to start the taps perfectly straight made tapping the holes quick and easy, as well. We even got creative and sort of used the lathe as a mill to cut the wrenching flats for a 23mm wrench.

The threaded length of the rod end was way too long, so we ended up cutting a half-inch off of that, too. (possibly may end up cutting more off of it pending how long the tie rod on the car is... we can easily cut another 1/4" or more off of it if we need to)

In the end, what we have is this:

Image

And the "adjustable bump steer tapered pin" parts should be here tomorrow. Then, we're ready to install!
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Bump Steer

Postby CaptainSquirts » Wed Feb 21, 2018 5:12 pm

Is there a greater chance of getting some debris in the ball of that joint and cause some bad wear since there isn't sort of boot to keep crap out?
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Postby Loren » Wed Feb 21, 2018 5:20 pm

Yup.
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Postby CaptainSquirts » Wed Feb 21, 2018 6:36 pm

wouod be cool to see a before and after difference from a bumpsteer gauge or something.
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Bump Steer

Postby Loren » Wed Feb 21, 2018 6:52 pm

I don't have a proper bump steer gauge, but I do plan to try to get some kind of measurement of the toe change before I start changing parts. And then again after.

Given what I feel at the steering wheel, I should readily be able to measure the toe change.
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Bump Steer

Postby Loren » Sun Feb 25, 2018 9:00 pm

Finished Product:

Image

Installed:

Image

Verdict:

Definitely reduced bump/roll steer by A LOT. It's almost non-existent. Certainly not noticable.

The only caveat for this installation, possibly because everything on this car is designed to be as small and light as possible (so the stock tie-rod end is short in length), is that the threaded end of the heim joint needs to be shortened considerably. We took a half-inch off of it already, and it's currently set to where the heim joint end is in contact with the end of the tie-rod. Can't adjust it in any more, and I've got 5/8" of total toe-in. That's only 5/16" per side at the tire tread, so maybe a couple threads at the tie-rod end. Plus a little more to allow for some toe-out adjustment. There's plenty of length left to cut off the the thing. Just gotta take it apart and do it.

Another oddity, maybe somebody makes a "lower profile" heim joint? The tapered stud adjuster kit included three 1/4" spacers and one 1/8" spacer. You'll notice that I used two 1/4" spacers... and there's ZERO room for more than that. I've got the nut all the way at the end of the stud. So, I really only got about a half inch or so of correction on the ball joint where I was hoping to get closer to the same 1" change that I put on the lower ball joint height. But, even with a lower profile heim joint, it couldn't allow for more than about an additional 1/8" of height adjustment.

Not gonna sweat it. It's a definite improvement. I just need to fix the toe, and I'll be happy.

Worth of Note:

I checked range of motion of the heim joint. No binding at full droop. And, at static height, the angle is still slightly below horizontal. I have about the same compression travel as I do droop travel. So, there's no way it will bind at full compression, either. This is VERY important to consider when doing this kind of mod. If you max out the heim joint under compression, you can break it and lose steering!

Actual Numbers:

I did take some rough measurements. I couldn't get measurements under compression travel without removing the spring (or, I guess I could have just loosened the adjusters and lowered the ride height a couple inches to get into that range of motion) and that wasn't on my agenda for today. I just measured at static height, and at half-inch increments as I jacked the car up. The car doesn't have a steering lock, so things could have shifted as I jacked the car up, making these measurements highly suspect.

The curve seemed surprisingly linear in the "before" condition. That's with the car lowered 2" and lower ball joint adjusted 1" down. The angle of the tie-rod was slightly upward, the angle of the control arm was a little more than slightly downward. They should be close to parallel with each other.

There was a LOT of bump steer, right about 1/4" per inch of droop travel, consistent from 0-2". Given that the tie-rod angle was already upward a bit, I would expect it to get worse under extreme compression travel. But, even at that rate, at full roll, the car would be seeing at least a half inch of toe out! I don't need accurate measurements to see that verifies what I've been feeling the car do.

After installing the kit, I measured again. It is significanty less, and I didn't spend time trying to get perfect measurements, but it's definitely at least half what it was. If my mseasurements are right, it's right about 1/8" per inch, or 1/4" over 2".

I did a tiny bit more research, and I think it's possible that while "zero bump steer" maybe be race car design nirvana, a little bit of toe-out on bump may actually be designed into most street cars because it essentially makes the car understeer if you yank the wheel too hard. And, who knows what other subtle suspension geometry is designed into the suspension to work with that? I sure don't.

So, I'm not going to pursue a perfect "zero bump steer" condition. Both because I'm not sure that a little bump toe-out is a terrible thing, and because the car feels SO much better now.

Closing the book on this one!
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