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Old 04-19-2013, 07:25 AM
kso kso is offline
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I'm not quite the old car guy that could give specific advise as in that really-great article you linked, but I can offer a little info about bump steer that applies generally...you could take the plan laid out then see how it compares to what you're doing. You would think it was a simple thing that was always worked out, but more often than not things like packaging, incorporation of anti-dive geometry, and the fact that sometimes bumpsteer was dialed into the scheme by the factory make that not-so. I gotta say though, that your efforts to keep the original style suspension in there and not just swap over to new stuff is a cool thing, my hat's off to you for that. For sure there are going to have to be ball joints there, not king pins.

I couldn't seem to open the Mustang II pdf's, but I'm not sure how much anything about those is going to help you as they are of-course r&p and have the steering arm up at a different position than most, having been originally designed for a car with tiny 13" wheels. The thing said about their a-arms being parallel, I don't understand...because they're usually not, at ride height...but "whatever", this is not about that.

Two things...first realize in working out steering geometry, it's all about pivot points and their location. Don't be fooled by the shape of components. For example, whatever the a-arm looks like and if it's straight or curved, what matters is where the center of the inner bushings are, and where the ball joint pivot is (which may be up, down, or even w/ the rest of the part). Same w/ tie rods, etc.

Next, it helps to separate things into "views", like top-down view and view from the front. Especially since I can't give you drawings here.

Given that your basic a-arm components and location are already set, and given that normal ride height as viewed from the front means the line between lower a-arm inner- and outer-pivots is (or should be) parallel to the ground or nearly so (uppers should be inclined slightly down toward the center of the car to give it a "roll center"), and given that most stock steering arms put the tie rod pivot at close to the same height as the lower a-arm ball joint...

...all you have to do to start is have the tie rod parallel with the lower a-arm, again as-viewed from the front and considering pivot points not component shape. That is the single most-important thing in minimizing bump steer.

Then you have to figure where the line of the tie-rod is going to be, as seen from above. You could look at the lower a-arm pivot on the spindle, then it's steering arm pivot and draw a line between those, then figure the tie-rod should shoot out from there (the steering arm)at a 80-90-degree included angle. A better plan is to, starting with the spindle steered straight ahead, steer it halfway one direction and make a mark (say, on the floor under the pivot where the tie rod goes using a plumb-bob if you're doing it for real and not on paper). Then steer it past center halfway over the other way and make another mark. The line between the two marks is where your tie rod should run along, again as seen from above.

Now you can determine tie-rod length. As seen from above:

Draw a line from the front lower a-arm bushing to the rear...this will be more-or-less in line with the rest of the car, depending on model and how they designed or packaged the suspension originally. Now draw a line from the outer ball joint toward the center of the car, making it parallel with the tie rod line from earlier (that's important). At some point this second line will intersect the first one. The distance between those two points (ball joint and intersection) is the optimum length for your tie rod.

Drag link length is just a fill-in between where the inner tie-rod pivots wind up, of course. Pitmann arm and idler arms usually go out away from the axle c/l from where everything else is, for packaging and ackermann angle.

That simple (I hope you read it slow)...if you imagine or actually draw along those points, that would give you steering relatively free of bump steer (without considering other intricacies which aren't that important). Now if you take those point and apply them to the setup you already have, and see the differences...you can make a plan for what can be changed and then just do your best. Again, the very most important thing is to have that tie-rod angle match the line between your lower a-arm ball joint and inner bushings, as seen from the front, at ride-height. It's amazing how far off people can get on that.

Last edited by kso; 04-19-2013 at 07:51 AM.
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