07

RX-7 Toyo R888 335 tires on SSR SP4 wheels

Project V8 RX-7: Wheel, Brake and Tire 3 PART MEGAPOST!

Part 1 - Huge Wheels and Tires from SSR and Toyo Tires

By Jeff Naeyaert

Two installments ago we put the Rocket Bunny wide body kit on our Project V8 RX-7 and if you remember it was looking pretty damn stupid on the stock wheels and tires inboard at least 6 inches on each side.

Rocket Bunny RX-7

Here's what your Rocketbunny FD kit will look like with stock wheels and tires--depressing!

It’s hard to imagine how a wheel and tire combo will end up looking on a car if it’s not something someone has already done before so we were naturally a little worried about how our wide body FD would turn out.  The funny thing was we never intended to go wide body in the first place.  

The decision came about by accident because of one tiny assumption we (I) made.  Once the suspension installation was completed in our last installment we only needed tires to replace the well worn dried out pair on our RX-7 and the project would be roadworthy.  The question came down to how big of a tire could we get to fit in the back to get all the torque our E-ROD LS3 was putting to the ground!  To avoid rubbing the inside of the stock fenders in the front the tire could not be much larger than 25 inches in diameter.  One of the few tires that satisfied that constraint and offered us some fat options for out back was the Toyo R888a DOT legal R-compound tire.  Up front a 255/35ZR18 R888 has an overall diameter of 25.1 inches—close enough, and pretty wide.  For the rear, the 295/30ZR18 that was also 25.1 inches in diameter.  A 295 is cutting it pretty close though as far as width.  We had 285 width Potenza S-03’s before and they fit fine but these are R compound tires and those sizes can be a little more variable—you need to look at the tire manufacturer’s specifications for overall width to be sure.  

Here at MotoIQ though we are fortunate enough to have a lots of tires laying around.  Though not R888’s we did have an R compound tire of the exact same spec (295/30-18) mounted to a set a wheels for a different [to remain unnamed] project car that happened to have the exact same lug pattern and offset needed to fit our FD perfectly!  

These tires are the Yokohama Advan A048 in the same size as the R888 we are interested in (295/30ZR18) and according to their website they are 11.6 inches in overall inflated width.  The R888’s compare at 11.8 inches overall inflated width—.2 inches wider.  

 

ADVAN A048 ON FD RX-7

We mounted the wheels and they fit great!  Assuming we got a wheel with the same offset we should be able to fit that extra .2 inches of the R888 under our FD’s hips no problem.  
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Comments
Supercharged111
Supercharged111link
Wednesday, July 08, 2015 7:52 AM
On page 1 you mentioned wheel spacers increasing scrub and seemed to imply that proper backspacing was the fix, but on page 4 you indicated that the wheel's offset would affect front scrub. Just to be clear, either one affects scrub equally. That said, I am curious about the effect of scrub out back if you can call it that. Seems to me it'd really only affect wheel rate and long term wheel bearing wear. Any reason you're not targeting a square setup? I swapped from the stock 265/295 stagger on my C5 Z06 to 295/295 and the car was much more manageable. The front was more in command and I could actually get turn in on throttle lift, whereas prior to that there was none. Car really wasn't much faster, but the feel was night and day. My 4th gen Camaro also wears a square 275 setup and I just can't imagine running staggered. I always suspected that the Camaro had a ton of scrub given its mid-corner push (largely fixed now) and its tendency to rip the wheel out of your hands every time you hit a bump, but when I measured it was only 1/4". Mike advocates 3/4"-3" of scrub, so taking a little bit there might not be as bad as you think.
Mike Kojima
Mike Kojimalink
Wednesday, July 08, 2015 10:08 AM
We know either one affects scrub. We did not infer that one did and one did not. Rear offset affects mostly motion ratio and load on the suspension components. It can make a rear problem like toe steer worse. Scrub most affects turn in and not so much mid turn. The tendency to rip the wheel out of your hands can be due to a lot of factors like caster induced bump steer, bump steer in general, KPI induced bump sensitivity, just having a big tire in general.
warmmilk
warmmilklink
Wednesday, July 08, 2015 1:01 PM
This quote sound like the article is saying what supercharged said:

"Ideally we’d like to find wheels that extend to the full width of the fenders without a spacer—especially in the front. Running a wide spacer screws up the suspension geometry, mostly by making the scrub radius too wide."
Jeff Naeyaert
Jeff Naeyaertlink
Wednesday, July 08, 2015 5:55 PM
I apologize for the confusion, one of the editors added that sentence before seeing i already covered it on the last page. ..no doubt setting up for something you the reader aren't supposed to know about yet!
tanyeewei
tanyeeweilink
Thursday, July 09, 2015 3:10 AM
You mentioned you were going for a target offset of -17 mm.

There's actually a -19 mm available, using the 144 mm lip width and Hyper Disk. Why was that not considered?
Rockwood
Rockwoodlink
Thursday, July 09, 2015 11:01 AM
@Supercharged: Wider than 295s will probably turn the steering wheel into a worthless video game rudder: no feel or indication on what's happening until after it happens. :)
Wrecked
Wreckedlink
Thursday, July 09, 2015 11:23 AM
Hmm.. new control arms in the works?
sethulrich
sethulrichlink
Thursday, July 09, 2015 12:42 PM
Your 11 +37 is going to be sunken 30 mm compared to the 12 +20 wheels that onpoint is using. Is that still going to look ok? Or can you massage the fenders to make it look better? Seems like it would look fairly sunken, but maybe it's not as bad as I am imagining. I'm sure you've got some wizardry planned to address it without messing up the geometry or handling.... you always do!
Jeff Naeyaert
Jeff Naeyaertlink
Thursday, July 09, 2015 2:31 PM
@tanyeewei -17 was for the same poke as the onpoint car had... looking at all the pics we've seen of the car we knew we could afford the 5mm extra width--perhaps even more. Other than that, the 157mm lip was chosen for maximum street cred.

@supercharged I needed to use the 295's for the front, and honestly even 295's on the rear look really skinny and weird on THIS wide of a kit especially with how the backside of the rear overfenders terminate on the rear of the car. To be sure though I consulted my chassis engineer (kojima) and he's confident he can tune this setup to work "bitchin".

@Wrecked & @sethulrich you're gonna hafta wait and see, but yeah Seth the front wheels are not gonna be any sorta flush without a spacer or SOMETHING.
BillyJ
BillyJlink
Sunday, July 19, 2015 7:29 PM
Scrub radius isn't detrimental to the performance of the car and I doubt it'll hurt the steering feel of the car much.
Jeff Naeyaert
Jeff Naeyaertlink
Sunday, July 19, 2015 11:21 PM
on the street it does..
BillyJ
BillyJlink
Monday, July 20, 2015 7:07 AM
In my experience, not as much as you'd think; even on the street (especially in a car with power steering) I don't think you will notice much of a difference from the increase in scrub.

I'm surprised you were considering a 255 up front since I drove an LS1 FD at Buttonwillow with 285 18s all around. Still one of my all-time favorite engine swaps. I think your car can break the street RWD record!

You need to let me drive it again Jeff!
Jeff Naeyaert
Jeff Naeyaertlink
Monday, July 20, 2015 1:11 PM
@stuntman thanks billy! I MAAAY let you drive it to get an idea how much potential it has... but probably not cuz then everyone will know how little I have! Plus you're arguing with me--save your energy for Kojima and your gt350 article you're writing! ;) Driving impressions aside though, additional scrub makes the wheel take a bigger sweep lock-to-lock requiring greater fender clearance which I'm already out of.
BillyJ
BillyJlink
Monday, July 20, 2015 2:38 PM
Did you know the Record-holding FXMD NSX has one of the worst scrub radius out there with no power steering!
Jeff Naeyaert
Jeff Naeyaertlink
Tuesday, July 21, 2015 2:12 AM
well fix it so you can go even faster you dummy! #foryourhealth
Mike Kojima
Mike Kojimalink
Tuesday, July 21, 2015 5:42 PM
Scrub radius is something that is really an important factor to consider when designing geometry. Too much scrub causes poor turn in, tramlining, steering pull on split mu surfaces and exaggerates the effects of stuff like bump steer. There is a reason why modern performance and race cars have much lower scrub by design than before. The trend for lower scrub in race cars started in the mid 80's and was considered to be a breakthrough in Trans Am and GT cars of that era for instance.
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