Different approachs to reduce bumpsteer
I'm hoping some experienced builders can help me with questions I have on tie rods and center link design to minimize bumpsteer.
I'm designing these components for my 37 Buick similar to wiki: http://www.crankshaftcoalition.com/w...ension_upgrade
I'm using primarily GM truck components in my build.
I'm using a Saginaw gearbox instead of R&P described in the referenced wiki because I need more travel than a R&P can provide.
One circle track website I found discusses determining a tie rod length based on geometry and then aligning that tie rod so it passes through an "instant center" (point whereupper & lower control arms cross) to reduce bump steer:
Heidts describes how the Mustang 2 front suspension works and a different expalnation for preventing bumpsteer. It says the upper & lower control arms are parallel which means there is no "instant center". It describes a specific length of tie rod that must be tied in at very specific lines.
1) Are both methods valid?
2) Does anyone know which approach is used in design of a 70's to 80's 2wd Chevy truck?
2) The concepts are the same, but the Chevy truck did not use a rack and pinion. The tie rod ends attached to the steering linkage at points in alignment with the control arm mounts.
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.
Thanks very much for your detailed description.
I'll try to lay out as you described and see what I get.
Have an additional question on set up for reduced bump steer:
I may be using upper & lower control arms from a 58 Pontiac on my build. Looking at pictures of this suspension, (see attached) the upper control arm appears to slant up towards the center of the vehicle. If this is the case, then did cars of this era have a different approach than modern suspensions to reduce bump steer?
From the pics it's kinda hard to tell...the differences in control arm length and the angle from being at full-droop tend to fool the eye. It would be better to have it tied-down to normal ride height and then get a more straight-on view.
Did they do geometry a little different back in the day? Oh yeah, and it had more to do than with just bump steer. High aspect ratio bias tires, customer preferences for ride quality with the bad roads and lower speeds of the day and finally the safety-driven belief that the car should always understeer even if it had to be induced by (what would be today) bad geometry were factors. Hey, at the limit, better to have it plow straight forward than get a grip and roll over...
Still, I doubt that at ride-height with the lower a-arm parallel with the ground there would be an upward angle on the upper one (and remember it's about the line between pivot points, not the shape of the component). If it turned out there was, I'd not want that present in your new setup.
I had not considered that under load of engine and front sheetmetal the upper control arms would be deflected upward.
Thanks for your insight.
In the photo you posted and in regards to the upper control arm angle, the suspension is unloaded, that is why it looks as it does. At designed ride height the upper arm will either be parallel to the lower or angled slightly down toward the center of the car. In comparing the angle of the upper control arm between various suspension systems, modern theory has the upper arm angled down toward the centerline of the car at ride height. That way camber goes more negative in bump and the roll center tends to be lower.
Thanks for your help
What you posted is pretty much how I do it.
A most excellent explanation without makin' it too long.
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