Hey guys, Im going to be in the market to buy my wheels and tires, im probly going to get the coker smoothies 16x6 with 750's on the back and 16x5's for the front. heres the question.... what size tire do you guys run on the front to get your scrub radius correct, and do you guys run a neutral offset rim? im under the impression that the wheel and tire sizes and offset plays an important role in the scrub radius. i tried searching but didn't dig up much as far as an actual sizes and offests go, there must be a general number to go by, oh and im using a superbell 4" drop axle and disk brakes (I know, i know but ive already got all the stuff ready to go) thanks in advance guys!
I'm under the impression that ideally a line drawn through the Kin Pin to the ground, should intersect a line drawn from the center line of the tire to the ground.
Yup...line drawn through the king pin or upper and lower balljoints should intersect at the ground with the center of the tire for perfect scrub radius. But unless you're using deeply offset wheels that push the tires outward, it's not a problem. Even then, it's not a big deal.
How does the steering feel now? If it's just right, you're probably best off maintaining the same scrub radius as you have at present. If its a bit vague and wanders and doesn't load up as much as you'd like to give you feedback in corners, increasing the scrub radius might help. If it's heavy in bends, tends to kick the steering wheel about a bit in your hands and/or tends to tramline on ruts and cambers, you might want to try decreasing the scrub radius a bit.
Scrub errors, with other issues mixed in, can induce things like death wobble. Try to get the line under the tire, at least part way IMO Also large errors will add wear and stress to components. Not a bad thing to think about with the absolute gamble taken on, in using repro parts with no known test data or component materials data. Although it won't change the line a huge amount, a shorter tire will get you going the wrong way, and a taller tire brings the line towards the tire a bit. Also plan for caliper clearances when using a custom offset wheel
I can vouch for this. I had some low profile tyres (B60x15) on the front of my '32 when I first put it on the road and experienced all of the above. Clearly the problem was excess scrub radius. This was solved by the simple expedient of replacing the tyres with a taller 185x15 tyre. The change was quite dramatic. No longer having to grip the wheel with two hands having to concentrate on every bump and rut on the roads to the point where it would simply drive through all of that and remain true and only one hand on the wheel. As a consequence of that experience I always look askance at beam axles with wide tyres and lots of offset. Not a good mix, not to mention the look.
Thanks guys appreciate the info, the car is in the assembly stage at the moment, so haven't driven it, but I'm gonna buy the wheels and tire so I can mock up the suspension, and just wanted to make sure I check and get the right stuff so I'm not posting on here about death wobble later on haha, but from the responses it looks like as long as I just get neutral offset wheels and some 500's for the front I should be in the ballpark thanks guys!
Plus .5 to Plus 3 will probably work with 1.5 - 2 being better. If you end up with symptoms you will know what to chase.
Actually, the lines should intersect below the surface because the tire deflects (bulges) as it sits on the ground.
If I read what you say correctly, then you mean the tyre bulge results in a slightly reduced tyre radius from the weight of the vehicle. That being so then, would not the lines need to intersect above the point of contact with the ground when the tyre is unladen so that when the tyre is loaded it will push the point of intersection down to ground level? I would be surprised if taking that into account is going to make much difference.
All I know is that when Detroit (and everyone else) design front end geometry, the two imaginary lines intersect below the road surface. And yes, it does matter as otherwise the tires are scrubbing - but - most importantly, if the scrub radius is not set right, then as you turn the wheelbase gets longer on one side and shorter on the other. This is like the steering on a Radio Wagon, and what happens when you try to turn them at speed? Obviously the amount of scrub is less in a car, but the mass and speed are higher, too. This is a reason why the 4x4 mall-crawling crowd running wide wheel spacers end up with vehicles that handle evilly at higher speeds. EDIT: If you look at the diagram in post #7, you see that the lines intersect well below the road surface.
Yes below is normal for RWD cars. When we say 1.5 inches positive it means at the road surface the steering axis line is 1.5 inches inside the wheel centerline at the road surface, so they would intersect below the road surface. A negative scrub radius is used on FWD cars and some other applications, the steering axis crosses the wheel center above the road, so it measures outside the wheel centerline.
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