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Well I got finished with the adjustment of my front suspension. I think I got every thing lined up pretty well and got bump steer to about 3/8" of toe in at 2" of bump. I think this should be good, I don't think most sevens are any better. Of course I could be wrong, I'm no expert. I'm not sure what a lot of bump steer is?? or a little for that matter. But it stays here unless I decide to correct it at a later date. I'm just happy to be this far. Now on to mounting the rear axel again.

I put pictures of the whole process at http://www.texaslocost.com steering button.
Take a look, comment if it strikes your fancy.

wayne

Wayne

The 3/8" bump steer is considered a lot. You should shot for less then .010" per inch of travel. Jounce is more important then rebound.

Spend the extra 45 minutes to get it honed in. These lite Wt cars are darty enough when you hit a dump, without adding to it.

More thin shims please, Dave W

Hi Wayne. Do you mean 3/8 toe in per front wheel in bump or total of both wheels?

3/8" on each wheel would add up to 3/4" of toe-in total when you hit the brakes hard. You didn't mention the toe out on rebound, but that might be bad too.

Basically, as you travel down the road the wheels will move up and down a good bit. Whenever the weight is unequal from left to right, the car will generate turning forces. This unequal weight will be caused by the contour of the road surface as the car moves. Crown on the road and undulations will cause the car to pull, reducing traction.

Might seem a pain now that getting the car working is in sight. But it's something that could irritate you for years. or maybe not.

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I posted a way to make a setup tool a few months ago. The tool holds the rack in position during testing and supports the rack so the frame can be brought to it.

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Thanks for the replys. I measured the amount of bump at 3/8 inch per wheel out at the dia of my tires. I'm assuming that is the right place to measure.
I could still bring the rack up a bit, or as much as I like, I guess but I don't know if it will get much better unless I get a new rack that is just the right length. Mine is about 1 1/2" too long still even after cutting it down. I was worried about cutting it more. I will probably drive the car some first so I can tell if I think I even have a problem worth correcting. The guys I've talked with who have a finished car say they dont' even notice a problem even though they took no real steps to correct for bump. I still have the feeling that this is as overrated problem since sevens seem to be notorious for bump steer yet most people don't seem to notice or make corredtions. These are just my observations. The saga may not be over yet. after I drive it some I'll post my impressions again, and I may still make more adjustments this week.

Well I think your doing the right thing by knowing the numbers and being willing to be observant. This is enough toe change that you should be able to see it when you hit the brakes hard. A parking lot might be good for this.

How many degrees does that toe work out too? That would give you an idea of the amount of force being "balanced" between the front wheels.

So here's your problem scenario. Instead of cautions street driving, because your car bump steer and scares you a bit., you opt for days at the track or autocross. You start getting better and going faster ( and getting braver ). Some day you soon, you land up going a little faster and deeper then you had been, you use more brakes and land up starting your turn in. As the nose dives more and your weight moves onto the outside wheel - you find it's turned in more then you expected, because the force is unbalanced now.

At this point you realize the car has already started swapping ends and your going to have to leave those brakes clamped on for the duration...

If your going 60 on the highway, at 0 toe, and then turn the wheel right or left to get that 3/8", how fast does the car respond? Damn those fenders on my street car... Is that about a mild lane change? As long as the weight is the same on those tires you won't notice it, but stands to reason you may feel some or all of that amount of steering as your suspension moves to the road surface.

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Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.

Thanks much for your insight. I think it may be you who has posted some of the most useful information on this subject. It will be at least next summer before I can finish the car and drive it. I will look for the things you mentioned and post some observations then. Maybe I'll buy a custom racing rack sometime to get it the right length?
Wayne

Nate - Seems like you keep running into issues right before I run into the same ones. I just wanted to say "Thanks" for clearing the way!

I was fooling around with my inboard shocks when I got sidetracked. I put some frickin' laser beams on my front hubs and watched the bump steer on the wall. The laser dot moved 8 inches on the wall 8 feet away. That means the front of the rim moved out 0.6". If I understand correctly how toe out is expressed, that means I've got 2.4" of bump steer! (is that correct?) YOWZA!!!

Before mounting my rack, I came up with a nearly perfect solution in Wishbone. But when I went to build it, it was impossible. You guessed, it, the Miata tie rods wanted to pass directly through the LCA. I was "in a mood" at the time, so I just eyeballed it and put the rack where it looked good. Dumb bass.

I'll spend some more time in Wishbone, but I'm pretty sure I know what I'm going to do. Going by the rubber-band-and-ratio method that every book describes, I think moving my outer tie rod ends above the Miata steering arm will almost perfectly match where my inner tie rod ends currently reside.

I'm inclined to bore a taper from above and use one of those bolts pictured above. Seems like I'll be able to dial out the bump steer even if I don't nail it right off.

Anyone have any advice on this? Is there a universal taper for this? Anybody recommend a good source for a reamer?


-dave

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...nowadays people are so intellectually lazy and lethargic that they can't build ANYTHING with their hands. They'll spend hours watching whiny people marooned on an island, but won't spend a second adding anything to the world. -weconway
Visit my [Locost 7 build log]

Dave

I modified my steering arms "Toyota Hi-Ace" by welding up the bottom 1/3 of the taper and then drilling thru the top with a drill slightly less then a half inch. Finished it with a 1/2" reamer so I could use a standard 1/2" striper bolt and a high angle rod end. I think it will be more difficult with the Miata steering arm because the taper it is not vertical. It would probably require a jig bore to do the I.D., plus surface cuts on both the top and bottom. I would try to mock up the steering and see if you have enough clearance with the Miata angled arm and a high angle rod end mounted on top and see if it would bind as it goes thru the suspension travel. If it is OK, then you could probably do the drill and ream job on a home drill press. Reducing the bump steer is worth the effort, if you drive SLIGHTLY over the speed limit.

The angled Miata steering arm is the killer part. Dave W

Lasers won't work because the path of the upright is curved due to the arcs of the A-arms. When the dot moves sideways on the wall, it's impossible to know whether it's bump steer or scrub. That said, 8" at 8'... yes, that's saying something's not right.

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Both available from https://www.lulu.com/

Hmm...thanks, Kurt...I hadn't considered that. Maybe I'll make one of those bump-steer boards with the hinge and a (two?) dial indicator(s). What if I put one laser on the hub and one on the end of the control arms, and look to see if they converge?

But, yes...it's really bad. The reason I broke out the lasers in the first place is because I could *see* the bump steer when I started playing with the shocks.

I'm also considering moving the rack out front, under the radiator. This will eliminate the tie-rod/LCA interference and let me get the rack low enough. No problem clearing under the little Civic radiator, and fixing the steering shaft shouldn't be too much trouble. The big buzz-kill is that the rack would be about 1.5" too long a that point, and I've already shortened it 5.9", so I don't think I'll do that.

Anybody know of some sort of taper inverter? It would be nice to bore a taper half way from above, drop in a taper bolt, then insert a hollow cone from below that had an inside and outside taper to fill the old taper. Probably no better (maybe worse!) then weld-filling and boring, but sounds slick. I don't have a taper attachment on my lathe, or I'd give building one a shot.

-dave

_________________
...nowadays people are so intellectually lazy and lethargic that they can't build ANYTHING with their hands. They'll spend hours watching whiny people marooned on an island, but won't spend a second adding anything to the world. -weconway
Visit my [Locost 7 build log]

I've always thought the dial indicators unnecessary. Does it matter what the number is? What matters is that you have none.

G

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Thanks for the advice, Dave. I was worried about that, too. At a glance, it looks like the lower ball joint lower surface and the steering arm lower surface are on the same plane. If that is true, jigging will be easy. In fact, I wonder if that's why Mazda put that crazy angle on the steering rod in the first place...just for manufacturing ease?

BTW, what is a striper bolt?

-dave "only time I seen a stripper bolt is when I asked her to marry me" hempy

_________________
...nowadays people are so intellectually lazy and lethargic that they can't build ANYTHING with their hands. They'll spend hours watching whiny people marooned on an island, but won't spend a second adding anything to the world. -weconway
Visit my [Locost 7 build log]

That's right. If two pointers are set up so there's zero gap at rest, that's all that's needed. No one cares about how much toe there is exactly , just as long as it's minimized.

_________________
Midlana book: Build this mid-engine Locost!, http://midlana.com/stuff/book/
Kimini book: Designing mid-engine cars using FWD drivetrains
Both available from https://www.lulu.com/

Dave H

A stripper bolt is a shoulder bolt that is hardened and then ground to the exact dia. Typically used to locate fixturing. Should fit the rod end and the reamed hole much better then a standard bolt.

If you have a Fastenal in the area they will have them in stock. I got them for less then $2 each.

HF has cheap China made dial indicators; well worth the $13, plus if you work on the engine it will also come in handy there. You'll only need one, can use a hard stop on the other side of your bump steer plate. I wouldn't tell your wife about the stripper Dave W