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

Pics or bs.

"Doesn't run 275s with offset bushings" crew is meeting elsewhere. :jerkit:

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 ;)

5mm spacer with 10" 6uls, Works fine. Also we have custom top hats that don't stick "out" as much as stock.

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

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.

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.

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.

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

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?

Img

https://www.miataturbo.net/attachmen...smo-billet-jpg

Nice!

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

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

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.

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

Lol original 90k mile ones.

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.

Whining about how your 275s hit your _________ at full lock makes you a tremendous baby. Just FYI :party:

Dont you know, you need all of the steering angle to drift with 275s.*






*not really, just put heat cycled out Rs on the back and decent As on the front and it'll drift with just gentle caresses on the steering wheel

I think mine are like their 4th redesign due to strength issues they had. I still don’t like some of the geometry and weld locations they have for attaching the outer end plate metal parts onto the large round tubing. I believe they have some fairly substantial load paths going through short sections of weld on tube walls. Looks like they got all the strength issues sorted but still may have some longer life fatigue issues yet next to some key welds.

My original thought on the stock ones was to just move the holes. But after having the V8R ones I come to realize the benefit of having the outer adjustment if you ever want to do any self-alignment with primitive tools so I might would try and get the cam bolt features incorporated.

That outer cam bolt and the other bolt both need to be tight as hell because with that much grip they tend to slip. Probably a good Idea to use higher strength steel like 4130 for the hole slots to keep from yielding it under the washer with repeated use.
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Hitting the tire on the sway bar at full lock isn’t a concern. just nice to have it go away completly. Having the A-arm machine away the rim lip within a few revolutions at full lock I think is an issue and should be addressed with some steering stops if you have that situation. 10’s with no spacer will just kiss the rim lip on stock A-arms at full lock. 11’s would have about ½” of interference.

Or you could just not use full lock, like I've done in every Miata I've driven since 2007 :party:

9s will contact the A-arm at full lock if you have the offset upper bushings. At least, they will if you have the steering rack one spline off-center the way I do (I really ought to fix that one of these days).

I've done it a few times when maneuvering the car onto the trailer, and it's not that big a deal. It does scar up the painted edge of the rim, but it's not going to destroy anything as long as you don't hold it there.

From a conversation I had elsewhere, it sounds like most CSP Miatas have offset bushings and 15x10s with a spacer, so maybe there's hope there yet. I guess I just need to find some more wheels. :)

--Ian

FWIW it is easy to make steering rack stops from a piece of SCH-40 PVC pipe (2" IIRC). Takes less than 30 minutes to fab and install. Easy peasy.

I have a solution that should work well. It involves moving the ball joint out 1/4"-3/8". If my math is correct that's about .5 to .75 degrees of camber. More that enough IMHO.

I will send you some pics if you want to take the same approach.

Still having a strugglefest here myself. I finally have some sticker hoosiers, wow theres a lot more rubber here than the scrubs I've been buying. I added packers until it didnt rub at the stock height xida bump stops. That resulted me being on the bump stop before even hitting static ride height. Took the 1/4 of a bump stop I had left over from making the rears match emilio's installation instructions and put that in with a spacer. I'm at ~5/8" of bump travel now with it setup to not destroy the tire on shock tower, AND also a 6mm spacer making my total offset +13. Any more than +13 and it will rub significantly on course on the back of the wheel well. If I went to like +6 offset then it would clear the shock tower and I'd be able to run over 1.5" of bump travel. I'm beginning to think my car is more bent that originally thought, lol.

I may have to do this. the 11" wheels hit the 2.25" ID springs before full wheel cut. Rubs through the powder coat real fast.

But even with the 10" wheels they rub on the springs, just enough to take the powdercoat off.

Mine are rubbing on my swaybar. Anyway it is a really easy fix. I think there is a thread about it somewhere around here. I might be wrong on the PVC diameter, but essentially you just cut the pipe to the length you need for the bump stop, cut out about 1/4-1/3 of its circumference so it is c-shaped, pull off the inside end of the tie rod boot, clip the pipe onto the rack, put the boot back, done. The pipe hits the inner tie rod on one end and the rack housing on the other, shortening the rack travel. Add the spacers on both sides obviously. Works great. Only disassembly required is pulling back the boot, which is easy.

I'm new to this, can some one quickly explain the offset bushing, or post a pic of what it looks like?

Thanks

The hole in the bushing is drilled offset.

Offset bushings allow you to effectively shorten (or lengthen, but there's little reason to do that) the upper A-arm, by moving the pivot point closer to the upper ball joint. There are some useful photos in the ISC installation instructions.

--Ian