scrub radius question

[chimp koose]

OK I see this on another board and there are a few different opinions . How much scrub radius is too much ?  I realize zero would induce some twitchy steering but I think more than a few death wobble issues are caused by excessive scrub radius . What is a good number , or is it really not that simple ? I tend to build my own stuff so as an example I have made the ackerman on my car right on the money when I made my steering arms . I will be using rims on the front that I can cut and move the centers on so once the front tire size is nailed down I would like to again go the extra mile to ensure the best geometry I can get with wheel offset . I would reason that the lighter the front end , the less it may matter but what do we aim for as a good range ? Lets hear what you have to say .

[enjenjo]

I think scrub radius might not be as important as you might think. Many new cars have a horrible scrub radius, as much as 3". But since most driving is done going straight more or less, it's rarely noticed. I believe if you can keep it within one inch it will be about ideal.

I have had cars with a standard wheel offset that later had reversed wheels and never noticed a difference. I think most hot rods that have death wobble have the front spring set up wrong without enough tension on the spring shackles allowing the front axle to oculate on the shackles. A Panhard bar can correct this in most cases, but a shorted spring would correct it too, without the extra hardware.

[chimp koose]

I had thought somewhere under 2" as thats what I had read somewhere . I suppose the lighter the front end , the less it would matter . I will likely string mine out on the floor and measure what I have from bolt flange to steering axis and figure out a wheel offset when the time comes .

[jaybee]

All of that above, and plenty of positive caster. It doesn't just help the wheel self center. It also tends to tilt the wheels when turned in a way that's quite beneficial in terms of camber. With a straight axle that's the only way to get any camber gain to prevent tire distortion from trying to pick up the inside edge of the outside tire...and the opposite on the inside tire of course.

[chimp koose]

Jaybe , when I taught mechanics years ago  I used to take my front engine dragster axle into the classroom to teach that very thing . That axle was light so it was a great demo tool . It really helped kids to understand the rise and drop of opposite spindles due to caster when they could see it happen . With a bubble level on the axle they could see the axle rise and drop as the spindles steered . You could also use an exaggerated caster angle to better demonstrate .

[jaybee]

That's a pretty cool demo to run, too.

[chimp koose]

I ran the dragster for 3 seasons with my students as the crew . We were season champions in '96 . I met a student who was on the crew in '95 just this summer when I was visiting relatives in another province . My sister in law was having photos developed and when she left contact info the old student who runs the photo department asked if she was any relation to the teacher in Saskatoon . When I visited this summer I made a trip to the store to say hello .