RCCAZ 1

iTrader: (10)

Here you go! Enjoy!!

RX7_Fanatic

OMFG my first post...

The only offsets offered on the 5-zigen fn01rc wheels that i want are +15 and +35. naturally i need the +35 on both the 17X9 front and 17X10 rear that i want but it yeilds a -.13 clearance front and rear. Could i solve this by rolling the fenders...probably a dumb question but i needed to know if fender rolling was taken into consideration. THANKS!

SpeedKing

iTrader: (4)

No. A +35 offset will not work w/ either a 9" or 10" wide wheel.

RX7_Fanatic

When you pop it into his calculator it yeilds that -.13" clearance, im wondering if fender rolling will make some clearance. I guess im wondering is, how much clearance can you get in addition to the clearance stated on the calculator from fender rolling =p

tom.jelly

I'm trying to put together a chart that will use the calculator and manufacturer tire and wheel size data to help me select 18" wheels and tires for my car. Please help me by emailing me data that I can add to the matrix, and/or correcting any errors. Someone who has dealer catalogs would be especially helpful. I'm sure there are plenty of mistakes and this is a work in progress, but once I get good data for 18's I'd be happy to do 17's and I will post periodic revisions as others add data. Otherwise feel free to use this as a starting point for something better, because I'm certainly no expert!

Tom

SAMIboarder

iTrader: (2)

Great thread!

On the spreadsheet I noticed that some of the inside clearences went into the negatives, but usually only around 1/4 inch. would running coilovers be enough to fix that clearence issue?

1234567

19x9.5 +17 & 19x10.5 +18 would that fit? and what do i need to do to make it fit?

rynberg

Why would you post a question like that in a thread containing a spreadsheet showing fitments???

No, they won't fit. Period. No way, no how.

kandycemitchell

My 17-year-old son is doing some mods...tokico illuminas all the way around with 'blue super-downs' my understanding is that the 7 will be lowered by 1 3/4". Will this affect tire/wheel size we should purchase? If we stick with 16" wheels will tires rub?

mcfly

really?

what would be the limiting factor?
fender or inside spacing between shock?

i know 18x10.5's will fit with a +30.

ImStillOnJava

iTrader: (30)

I'm running the spreadsheet to try and calculate what options I have with offsets and rim widths for 18" TE37s when used against Feed front and rear fenders, however ...I haven't been able to locate the actual addtl width the Feed rear fenders will add over stock, and info for the front Feed fenders I'm getting differing information of it being 25mm over stock or 30mm over stock. Does anyone know what the actual numbers are?

RX72NR

iTrader: (1)

Feed fronts 25mm rears 20mm

Necros

In light of this post, I'm going to ask a fitment question. Hopefully I'm not too much of a lazy ****. I'm just trying to wrap my brain around how this works.

I see on the spreadsheet that with a 18x9 in the rear (for example) that one should run offset of +31 with a tire size of 255/30/18s. I also see that for 18x9.5 that a +38 with 265/30/18s is appropriate.

Now, I know lots of people running 265/35/18 in the back. My question, is twofold: If I wanted to use this size tire, can I use an 18x9 wheel? Advantages/diadvantages?

Also, if the wheels do not exist in a +31 or +38 offset, is it better, with the 265/35/18s, to err toward the +31 side, the +38 side (for 9.5 wheels), or smack dab in the middle? It looks like there are 29, 34, 35 and 36 offsets (plus others, but those are the closes to the specified numbers).

I really hope I'm not being dumb.

Aeka GSR

iTrader: (4)

ignore me whoops

SystemsWizard

The wheel and tire sizing spreadsheet really helps me in experimenting and understanding. A question though, if sticking with near-stock sizes, say 17x8(F) and 17x9(R), which would fit either at the stock +50 offset or at the lower default offsets in the spreadsheet (which bring them closer to the fender), which option is preferable?

My understanding of lowering the offset to move the wheels out has benefits (wider stance for possibly better handling, better looks) and drawbacks (greater suspension wear, more bump steering, possibly worse handling).

My concerns are performance first and looks second. I'm thinking of a slightly wider back tire to deal with upgrades I expect to bring the car to the 310 RWHP range. If this increase in HP is insignificant to the stock 8 inch width, I'd like to know that, too.

Some relevant info may be that the Spirit cars had 17x8 and 17x8.5 rims at +50 offset and a link, that while possibly inconclusive, was fun to read:

http://forums.nasioc.com/forums/arch.../t-385857.html

Thanks everyone. This is my first post, so be kind. I have read the forum for a couple of weeks and tried the best searches I knew how on the subject...

rynberg

You guys need to read better.....the offsets that Jim lists are the ABSOLUTE MINIMUM, NOT THE RECOMMENDED. The recommended offset is always +50 (as close as you can get to that), to maintain stock scrub radius.

SystemsWizard

Rynberg - I'm new, you're a moderator, and I'm pretty sure I've learned stuff from your posts on many of the threads here, so I don't wish to argue. But, I quickly read over the posts of this thread and the spreadsheet (this being probably the fourth or fifth time over) and if I really missed that one of the main points was to stick near the stock offset, I keep missing it - maybe a mental block... A counter-point is in the spreadsheet itself, which defaults to giving everything about 1/4" outer clearance and states near the top, "please note that the numbers shown are recommendations".

You, Jimlab, and a lot of others here have more experience with suspensions, in particular with this vehicle. I've seen the benefits/drawbacks of stock offset vs. pushing the wheels out argued inconclusively (on this forum and others) to my non-expert mind, that I thought I'd ask people with experience having tried the different choices.

- So, is having a moderate-sized wheel at stock offset, as opposed to being out a bit more (lower offset) absolutely clear in performance terms, when either option fits well?

- Also, should one go to a larger width rear for 310RWHP? It's surely a matter of opinion, but I guess my goal is to keep the stock oversteer/understeer feel.

fastrx7man

iTrader: (63)

If I used the tool right I shouldn't have any problems with 18X8 40mm upfront and 18X9.5 44mm in back...

jimlab

Yep.

As stated in the spreadsheet text box for Tire Clearance, Outside: "Offsets have been calculated to maintain about 1/4" clearance so that rolling fenders should not be necessary unless the car is excessively lowered."

True.

For those that don't know, the scrub radius is the distance between the extended centerline of the steering axis (where the centerline of the shock would hit the ground) and the centerline of the tire contact patch on the road. To maintain the factory scrub line, using the same wheel offset as stock (+50mm) will put an equal amount of wheel/tire inboard and outboard of the scrubline (maintaining the stock tire contact patch centerline) regardless of the increase in width.

However, that's not always possible due to available offsets and available clearance, so it is more important to come as close as possible to maintaining the stock scrub line while staying within the safe margin of inside and outside wheel/tire clearance. Since most people seem to have trouble understanding the effect that wheel offset has on where the tire and wheel are located on the car, I threw together a quick diagram to illustrate.



In the picture above, the blue lines show the centerlines of the wheels and the red lines show where the wheel mounting flange is located based on the amount of offset for each wheel (the drawings are to scale). The green zone is the "safe" clearance zone. In this case, I used the rear wheel clearance zone for the FD, which is about 7.5" of clearance inboard and about 4" of clearance outboard of the wheel mounting flange.

You can quickly see that having a +50mm offset on all three wheels would put the wheel centerlines (and therefore tire contact patch centerlines) in exactly the same spot, maintaining the stock scrub radius despite the increase in width. I chose to vary the offsets to illustrate where that places the wheel in relation to stock and to illustrate that a small change in offset does not significantly affect the scrub radius (not to mention the fact that aftermarket wheels with a +50mm offset are pretty rare).

The goal of the spreadsheet was to allow selecting a wheel size and offset that is within the safe clearance zone. Once again, the calculated offsets were intended to maintain about 1/4" of clearance on the outside to avoid having to roll fenders. Use as much offset as you can (up to +50mm) by replacing the offset value with a larger number and watch the effect it has on inside and outside clearance as they are recalculated.

Just don't save the spreadsheet after changing the values in calculated cells or you'll need to start over with a new copy.

jimlab

True, but you should go with a larger offset in the front if you can.

SystemsWizard

Jimlab - thanks for more great info. Searching on the net for scrub radius after reading Rynberg's post, one good explanation of the effects of moving the wheels out (decreasing offset) is paraphrased:

-----

Place two sheets of paper side-by-side, like the front wheels. Take two pencils, treat them as the line through the left and right front shocks, touching down to the center of each sheet of paper. Move the sheets forward with the pencils - they move relatively straight forward. Now, keep the pencils where they are, but move the sheets outwards to the side, like decreasing offset. This time, when you try to move the sheets forwards via the pencils, they want to turn outwards on their own.

-----

This would be like bump-steer, requiring a firmer hold on the steering wheel, decreasing straight-line stability, but possibly increasing turn-in ability, right? I've read some people espousing a little of this as a Good Thing (TM), even one guy on this forum - though a lot of people disagreed. I guess also with the front, but more especially with the back, when you move the wheels out significantly, the effects of changing the contact patch (the weight pushing down through the suspension at an angle) could really decrease the benefits of a wider tire. Do you know if anyone has measured the 50 offset and found it to truly center the contact patch or if the number may have possibly been a safety compromise for greater stability?

Just trying to really understand - thanks!

jimlab

Well, somewhat.

Bump steer is added toe-in or toe-out of the front wheels as the suspension travels through its range of motion. It is caused by misalignment of the suspension and the tie rod linkage which results in them traveling in different arcs. Something has to give, and in this case, it's the wheel and tire; pulling in when the steering linkage's arc is too short and pushing out when the linkage's arc is too long. Both wheels pull in or push out by an equal amount, assuming the rest of the suspension is properly aligned.

Bump steer causes erratic tracking when driving in a straight line as the builders of the "Supra 7" found out when trying to get a few time slips. The car was all over the place at the end of the track because they'd had to drop the steering rack so far to clear the 2JZ-GTE and hadn't compensated for the change in steering geometry. They finally gave up and as far as I know, the car is now a "shop car" only.

Scrub radius, on the other hand, changes camber as the suspension travels through its range of motion. It is far more critical on front wheel drive cars, in my opinion, but can contribute to a car's tendency to push or pull in a corner while braking, or if one wheel loses traction. However, many race cars run positive scrub radius to give more wheel feedback.

Not to my knowledge. I'm sure the FD was designed to have 0 scrub radius (as are most, if not all, passenger cars), and +50mm is the offset which places the tire contact patch centerline precisely on the point of the shock centerline. If that's what you mean by a "safety compromise", then yes.

DamonB

scrub radius discussion split:

https://www.rx7club.com/suspension-wheels-tires-brakes-20/wheel-offset-vs-scrub-radius-619446/

wanklin

iTrader: (2)

Latest Revision:



An XLS File Based on tried and tested setups. The code is based off of the tried and tested 18x10+50 CCW setup with 2.5" coils and Rotary Extreme Trailing arms, and the proven 18x10+42 on all corners with stock springs and trailing arms. The code will adjust according to your wheel widths, offsets trailing arms and coil setup to provide you with setup verification, and or an explanation of why it won't fit referenced to a explanatory diagram. The XLS will also automatically convert your wheel into metric and provide you with "true" front/back spacing. Please keep in mind that these numbers are based on extremes so don't try to push your offsets to the limit.

We're all tired of the same questions day-after-day. It doesn't get any easier than this.

Latest revision based off this feedback: My father and I both run 18x10, 45mm offset, 285 series tires on the back of our stock suspensioned FD's. They fit perfect and according to that excel sheet they don't. But it is a good tool to get you down to the right area. I'm only running a 245 series tire up front. My father is running a 255. We both have stock trailing arms, 18x8.5" +45 offset.

jimlab

Looks like someone doesn't know how to read directions.