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Fitting 275 Hoosiers on an NB with offset upper bushings

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Fitting 275 Hoosiers on an NB with offset upper bushings

 
Old 03-02-2014, 05:52 PM
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Default Fitting 275 Hoosiers on an NB with offset upper bushings

So I tried to test-fit some 275 Hoosier A6s on my car today. It's a 99, fenders are rolled and fender liners removed, but no flares yet. I pulled out the spring, put the shock back in, and ran it through the suspension travel.

The first thing that hits is the tire against the upper spring perch (the NB "top hat" on the shock). From what I've read from various other reports, most people have it rub on the underside of the shock tower, but I haven't heard of it rubbing on the hats.

I have the ISC offset bushings installed to add camber (currently running 3.25 degrees). Since this moves the upper arm inboard by about half an inch, they seem likely to be the cause of the rub on the top hat. Has anyone else tried this with the 275s?

Any suggestions as to the right fix here? Pull out the ISCs, go back to poly bushings, and live with not having as much camber? Spacers? I normally run a single 5mm spacer up front, plus the 2mm or so that comes from the aluminum hats vs stock rotors. Putting two more 5mm spacers on there (for a total of about 17mm) moved the tire just about far enough outboard to clear the hat, but that sounds like a lot of spacer.

On the fender question, the rolling looks OK, but I'm concerned about the flange where the fender bolts to the front bumper cover. Have other people needed to trim this?

thanks,
--Ian
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Old 03-02-2014, 06:40 PM
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Pics or bs.
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Old 03-02-2014, 08:09 PM
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Originally Posted by thirdgen View Post
Pics or bs.
"Doesn't run 275s with offset bushings" crew is meeting elsewhere.

Originally Posted by codrus View Post
Any suggestions as to the right fix here? Pull out the ISCs, go back to poly bushings, and live with not having as much camber? Spacers? I normally run a single 5mm spacer up front, plus the 2mm or so that comes from the aluminum hats vs stock rotors. Putting two more 5mm spacers on there (for a total of about 17mm) moved the tire just about far enough outboard to clear the hat, but that sounds like a lot of spacer.
10" 6ULs should need 5-10mm of spacer in front depending on exact camber figures. 9" 6UL will need more. A little rubbing here and there is to be expected when putting rubber that's 90mm wider than stock on the car
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Originally Posted by codrus View Post
Basically I've come over to the camp of "If something is a reliability problem on the track, just ask Andrew and do what he says".
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Old 03-02-2014, 09:33 PM
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5mm spacer with 10" 6uls, Works fine. Also we have custom top hats that don't stick "out" as much as stock.
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Old 03-02-2014, 11:39 PM
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Originally Posted by Savington View Post
10" 6ULs should need 5-10mm of spacer in front depending on exact camber figures. 9" 6UL will need more. A little rubbing here and there is to be expected when putting rubber that's 90mm wider than stock on the car
The test-fit tires are mounted on 15x9s. If I decide to buy a set, mine will likely go on 15x9s as well. I know the 15x10s are faster, but I already own 15x9s and there aren't any 15x10s available at this point.

Some rubbing is fine, but the top hat is half an inch lower than the underside of the shock tower, suggesting that it wouldn't just be "some rubbing", but rather "tear a hole in the tire when the suspension compresses". Spacers are fine too, but 20mm plus sounds like a lot? Doesn't that have undesirable effects on the suspension geometry?

FrankL: are you using offset bushings? Did you make the top hats yourself? Are they modified stock ones, or are they OTS somewhere?

I didn't take any pics of the test fit process -- not sure there's all that much that's interesting to take pics of.

thanks,
--Ian
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Old 03-03-2014, 12:53 AM
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Yes. We built the offset bushings and we built the top hats from scratch. Check out our fb page in my sig for pics. About to make a new design too. All aluminum pieces as we are very weight conscious in Street Prepared.
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Old 03-05-2014, 12:44 PM
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An effective wheel offset of 20mm is about the most inboard that the 275 purple crack will work good with. 15X10 6UL needs a 5mm spacer. 15X9 6UL needs a 15mm spacer. Offsetting the upper bushings further inboard might require more spacer than that.

Last edited by bbundy; 03-05-2014 at 01:31 PM.
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Old 03-05-2014, 12:57 PM
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Originally Posted by codrus View Post
The test-fit tires are mounted on 15x9s. If I decide to buy a set, mine will likely go on 15x9s as well. I know the 15x10s are faster, but I already own 15x9s and there aren't any 15x10s available at this point.
9s sit in approximately the same spot as 10s do (within 2mm) in relation to the inner shock tower, but they pinch the outside of the tire in, which effectively moves the tire itself closer to the shock tower. 15-20mm is definitely an excessive amount of spacer, but if you had the right wheels for those tires you wouldn't need to run so much spacer
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Originally Posted by codrus View Post
Basically I've come over to the camp of "If something is a reliability problem on the track, just ask Andrew and do what he says".
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Old 03-05-2014, 01:57 PM
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Originally Posted by Savington View Post
9s sit in approximately the same spot as 10s do (within 2mm) in relation to the inner shock tower, but they pinch the outside of the tire in, which effectively moves the tire itself closer to the shock tower. 15-20mm is definitely an excessive amount of spacer, but if you had the right wheels for those tires you wouldn't need to run so much spacer
Do you have the right wheel for the tires? I'm still not convinced I do.
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Old 03-05-2014, 02:08 PM
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Originally Posted by FrankL View Post
Do you have the right wheel for the tires? I'm still not convinced I do.
15X11 ET 19.05mm. I will have some in the next couple weeks. the 10's work real good with 245's. 275's on 10's feel sqirmy with snap brake away charicteristics compared to 245's on 10's

Last edited by bbundy; 03-05-2014 at 04:42 PM.
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Old 03-05-2014, 02:25 PM
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Originally Posted by bbundy View Post
15X11 ET 19.05mm. I will have some in the next couple weeks. the 10's work real good with 245's
Now we're talking. Put some 275s on 11s for the back of a Porsche. That looks like Hoosier intended. More info on the 11s?
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Old 03-05-2014, 04:36 PM
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Originally Posted by FrankL View Post
Now we're talking. Put some 275s on 11s for the back of a Porsche. That looks like Hoosier intended. More info on the 11s?
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Attached Thumbnails Fitting 275 Hoosiers on an NB with offset upper bushings-kosmo-billet.jpg  
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Old 03-05-2014, 05:09 PM
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Nice!
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Old 03-05-2014, 06:22 PM
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OK, I was confused by the offset & pinching discussion so I went and drew some pictures and now I think I understand what Savington and bbundy are saying.

The 15x9's 36mm offset and the 15x10's 25mm offset place the inboard edge of the wheel in almost the same location relative to the shock body, because that 11mm difference is almost the same as the extra half inch of wheel that the 15x10 has on the inside. This means that the extra width of the wheel is entirely on the fender side, not the shock side.

However, the tire's edge inboard edge isn't located by the edge of the wheel -- it's located by the center point between the two edges of the wheels. When you put the 275 on the 15x9 it sticks out further beyond the edge of the wheel than it does on the 15x10, so even though the rim is the same distance from the shock body, the tire isn't.

So 275s on a 15x9 don't fit on my car without a ton of spacer -- the 15mm Bob was talking about, plus another half inch or so to compensate for the offset bushings. That's about a 30mm spacer, or an effective -5 offset on the wheel. Is that actually a good idea to run, or will it totally screw up the scrub radius/geometry/roll center/whatever?

It sounds like in the short term I probably need to convert back to the non-offset bushings and try to get the camber in some other way. Are you using the V8R lower A-arms, Bob?

Got any 15x10s you want to sell to make room for those 11s?

--Ian
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Old 03-05-2014, 06:45 PM
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I was hitting the NB mounts a little. My solution was to pull the mount out of the car and attack with the cutoff wheel. I also had to extend the xida bump stops to stop the wheel from hitting the rest of the chassis, but this is an NA. 15x10s with +19 offset (think 6uls with the 6mm spacer built in).
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Old 03-05-2014, 07:01 PM
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Originally Posted by Leafy View Post
I was hitting the NB mounts a little. My solution was to pull the mount out of the car and attack with the cutoff wheel. I also had to extend the xida bump stops to stop the wheel from hitting the rest of the chassis, but this is an NA. 15x10s with +19 offset (think 6uls with the 6mm spacer built in).
What upper bushings do you run?

--Ian
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Old 03-05-2014, 10:23 PM
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Originally Posted by codrus View Post
OK, I was confused by the offset & pinching discussion so I went and drew some pictures and now I think I understand what Savington and bbundy are saying.

The 15x9's 36mm offset and the 15x10's 25mm offset place the inboard edge of the wheel in almost the same location relative to the shock body, because that 11mm difference is almost the same as the extra half inch of wheel that the 15x10 has on the inside. This means that the extra width of the wheel is entirely on the fender side, not the shock side.

However, the tire's edge inboard edge isn't located by the edge of the wheel -- it's located by the center point between the two edges of the wheels. When you put the 275 on the 15x9 it sticks out further beyond the edge of the wheel than it does on the 15x10, so even though the rim is the same distance from the shock body, the tire isn't.

So 275s on a 15x9 don't fit on my car without a ton of spacer -- the 15mm Bob was talking about, plus another half inch or so to compensate for the offset bushings. That's about a 30mm spacer, or an effective -5 offset on the wheel. Is that actually a good idea to run, or will it totally screw up the scrub radius/geometry/roll center/whatever?

It sounds like in the short term I probably need to convert back to the non-offset bushings and try to get the camber in some other way. Are you using the V8R lower A-arms, Bob?

Got any 15x10s you want to sell to make room for those 11s?

--Ian
Yes the outer tire shoulder (part of tire that crashes into the chassis) doesn’t change profile that much when mounted on a 9 versus a 10 most of the difference is in the sidewall shape. So on the 949 wheels 10mm more spacer makes up for the 11mm difference in where the tire is centered relative to the mounting face between the 9’s and 10’s.

I’m using the V8 roadster arms which effectively move the lower ball joint out to get enough negative camber. I have also considered modifying the stock arms to achieve the same thing as I'm still a little uneasy that V8R got the design quite right. Having the adjustment point at the ball joint makes camber changes much easier without screwing with caster and actually could make reasonably accurate camber changes at the track now to try things using just hub stands. The added bonus of the V8 roadster arms is they should still allow full wheel cut even with 11” rims. An ISC sway bar or something similar is also needed for full wheel cut as the tire will crash into a racing beat or similar bar pretty hard at full wheel cut I have the 3 piece nascar style front swaybar.
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Old 03-06-2014, 12:24 AM
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Originally Posted by bbundy View Post
Im using the V8 roadster arms which effectively move the lower ball joint out to get enough negative camber. I have also considered modifying the stock arms to achieve the same thing as I'm still a little uneasy that V8R got the design quite right. Having the adjustment point at the ball joint makes camber changes much easier without screwing with caster and actually could make reasonably accurate camber changes at the track now to try things using just hub stands. The added bonus of the V8 roadster arms is they should still allow full wheel cut even with 11 rims. An ISC sway bar or something similar is also needed for full wheel cut as the tire will crash into a racing beat or similar bar pretty hard at full wheel cut I have the 3 piece nascar style front swaybar.
What are your concerns about the V8R arms? How would you modify the stock ones? Just lengthen them by some fixed amount and lose the extra adjustment or would you put the eccentric bolt in as well?

I have the RB 1.25" sway bar and even the 225s crash into it at full lock, so yeah I don't expect to get the 275s that far over. Fortunately I don't have to parallel park the car at an autox.

--Ian
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Old 03-06-2014, 12:54 AM
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Originally Posted by codrus View Post
What upper bushings do you run?

--Ian
Lol original 90k mile ones.

Originally Posted by codrus View Post
What are your concerns about the V8R arms? How would you modify the stock ones? Just lengthen them by some fixed amount and lose the extra adjustment or would you put the eccentric bolt in as well?

--Ian
I've seen the v8R "bubba" arms break at csp grip levels, actually I was the last person to drive that car before it broke... Not that stock arms are really any stronger since they also break at more or less the same grip level. The arms I'm designing use a larger diameter tubing with smaller wall thickness that equates to the same weight/foot but more than double the strength in bending. Of course for reference the raw materials for my arms cost as much as a set of v8r arms retail for, good sphericals are expensive.
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Old 03-06-2014, 01:51 AM
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Whining about how your 275s hit your _________ at full lock makes you a tremendous baby. Just FYI
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Originally Posted by codrus View Post
Basically I've come over to the camp of "If something is a reliability problem on the track, just ask Andrew and do what he says".
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