Technical Scrub Radius

I have a 32 3window with a 4" dropped tube axle, 175x65x15 front tires, polished American 5 spokes with a 3.250 back space. I did have165x80x15 but it was time for new tires so I bought the 175's It never rode all that great but I chalked it up to short wheel base, straight axle frontend, hey it's a hot rod right. Now that I changed to a smaller dia. tire it's worse and you really have to pay attention when steering at speed it wanders really easy. I also should include I installed those SO CAL ( please don't start swearing at me ) Buick / Wilwood brakes . I started checking around here on the HAMB and read a lot about scrub radius and back spacing, I think I need a wheel with more back spacing to bring the center of the wheel closer in to king pin maybe like 3.750 but that would have the wheel very close if not touching the fins on the drum. I could go to a 16 in. rim but I have fifteens on the rear and that wouldn't work. Sorry for such a long post I wanted give all information I have. Any ideas, help I would really appreciate Thanks

 

pictures would really help us to help you

 

Shorter tires means less caster. How? It's not the degree you set it at but where the contact to the pavement happens as shown by n.

 

Covered pretty good in here

https://www.jalopyjournal.com/forum/threads/buick-drum-scrub-radius-question.1236243/

 

Hey @greg37

The smaller 175/65r15 tires are 5" less in diameter than the original 1932 tires. This will lower the spindle height by 2.5", which will give a 5/16" increase in positive scrub radius over Henry's original setting. This should not cause the problems you describe.

I think the brake setup has probably pushed the wheels outwards, and this will be the cause. Post up a pic of your wheel/brake setup and let's have a look.

That is, of course, if scrub radius is the problem.................

 

Thanks everyone, I will post pictures later when I'm home from work.

 

Hard to tell accurately, but if you drew a line down through the center-line of the king pin, it looks like it would be at the inside edge of the tire at ground level. If this is the case, you would have around 3.5" of positive scrub radius (half of the 175 wide tire). That is quite a lot, and may be your problem.

Try to take a pic from directly front on. You may have to be slightly to the inside of the tire so we can see the king pin boss.

 

Thanks everyone for your helpful answers, I'm taking the 175x65x15 off and putting 165x80x15 back on also getting new rims with a little more back space A few day's ago I never even heard about scrub radius. There's more to consider then just buying some tires and rims that is if you want your rod to handle and drive well. Thanks again everyone for your help !!