13 vs 15 Hoosier Radials

R

RonInSD

Well-known member
For next year I am debating running 13 inch versus 15 inch Hoosiers Radials and wonder what people have for helpful knowledge? I have been running the 23.0 x 8.5-15 Hoosier cantilevers but struggling with rear wheel inside tire lift and being able to get the power down mid to late corner. I have made some changes to the car that have improved the problem but it is still there on the tighter corners. The car came with 13 inch rims from the factory but I was talked into running the 15 inch tires years ago. I am looking at the radials because it would allow me to run a little more camber which the car seam to like when I ran ITB. The camber also helps the rear roll centers due to the semi-trailing arms IRS.

Car is a 1977 BMW E21 320i.

Car Specs:
Minimum weight with driver is 2216 lbs but closer to 2300 lbs at the end of this year.
2.0 ltr
Dog Ring transmission
Front suspension is strut.
Front Sway bar locates the lower control arm.
RWD
Semi-trailing IRS
Clutch Type LSD

Positives for 13 inch combo:
Improved roll centers (front and rear)
Rear end ratio change
Lower rotating weight
Possible to lower the car a little more.

Negative for 13 inch combo:
New rims - $800 to $1000 per set
clearance around brakes (front and rear)
clearance around tie rod end and lower control arm
Future supply???? (the big reason why I was talked into the 15" but I think that has gone away some what lately)
 
Ron I ran 15" Hossiers on the F 2002. I ran the 13" hoosiers on the H 1600. Then I switched to radials this year. The only thing that I would warn you about is that the 13" tires seem to go away sooner than the 15". The radials work great on the 1600. I was able to set a new track record at Portland this year with the radials. G
 
Gordon Jones":1kngvik8 said:
Ron I ran 15" Hossiers on the F 2002. I ran the 13" hoosiers on the H 1600. Then I switched to radials this year. The only thing that I would warn you about is that the 13" tires seem to go away sooner than the 15". The radials work great on the 1600. I was able to set a new track record at Portland this year with the radials. G
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I too have heard that the 13" go off faster then the 15" with a 2200+ car but you can get a national race out of them. I am really thinking the 13' would help the roll centers on the car but I am also looking into other options to help the roll centers on the 15" setup. I think the additional camber from the radial will help too. I think if I keep working with the 15" setup can get it to come around.

Gordon what you running for a rear diff? Welded or LSD?
 
struggling with rear wheel inside tire lift and being able to get the power down mid to late corner
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Lifting the inside rear wheel sounds like you either need less sway bar in the back, or more sway bar in the front. The inability to put down power early may be a rear geometry issue. If the suspension is toeing out under compression, the rear end is trying to steer the car while cornering (oversteer) thus limiting the power application until you are nearly straight. This is a common issue with semi-trailing arm suspensions.
 
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