How far can offset be pushed before having to roll guards?

apsilon · Postby **apsilon** » Wed Mar 14, 2018 7:06 am

Wanting new rubber for the NA I picked up but as there's not a huge choice in 14" I'm looking at going to a 15" wheel at the same time. Wanting to keep it light I'm likely going to have to buy wheels sight unseen as I've already found a few of the ones I like have been discontinued. So, IIRC standard wheel is 14x6 +45. How far can you push that offset towards the guard, assume lowered 1 inch. Currently at stock ride height but may end up lower in the future.

Looking at 195/50-15 tyres to keep it around stock so thinking a 15x6.5 wheel. +35 should still clear correct?

StanTheMan · Postby **StanTheMan** » Wed Mar 14, 2018 8:53 am

you should have miles of room even when lowerd by 2"
its when you get into around 7.5" +20mm offset you start getting issues

I have the NB8B rims at 6.5" I think I would need to start rolling guards if i changed the offset from 42mm to 20mm. I'm at least 2" lower than stock. Thats on an NA6

apsilon · Postby **apsilon** » Wed Mar 14, 2018 9:43 am

Great, thanks. That's what I expected but wasn't sure. Now I just need to find something that I like and doesn't look out of place on the car.

hks_kansei · Postby **hks_kansei** » Wed Mar 14, 2018 11:07 am

Guard clearance is affected by both offset and width.

If you are keeping the same width of wheel/tyre you can simply measure from the wheel lip or tyre sidewall (whichever pokes out further on your car) to the inside edge of the guard.

That measurement can be subtracted from your current wheel offset to give you the lowest offset that will allow you to clear the guards (since the MX5 suspension allows it to camber inwards as it compresses you'll have more clearance as it loads up)

If you're also going for a different wheel width, you'll need to do a bit more maths, and add half of the width increase to the offset.

for example, current is 14x6+45

Say you measure 35mm from wheel to guard (that's a guess on my part, best you actually get the tape measure out), that allows for a 14x6+10 (45-35=10)

if you were to go for a 14x7 wheel the maths would be (45-35+12.7=22.7) giving you a 14x7+23 (rounded to nearest)

the formula to calculate a wheel's inner and outer barrel depths (backspace and frontspace if you will) is:

((width*25.4)/2)+offset = back space (rear barrel)
((width*25.4)/2)-offset = front space (front barrel)

for a 14x6+45 it would be:

((6*25.4)/2)-45 = 31.2
(152.4/2)-45 = 31.2
76.2-45 = 31.2mm from hub to wheel front.

also note that wheel widths and offsets are measured to the bead seat, not to the lip, so you're best to note that whatever you work out from the wheel specs mathematically you're best to assume the wheel will be about 5-10mm wider on each wide to cater for the lip.

apsilon · Postby **apsilon** » Wed Mar 14, 2018 11:46 am

Thanks, that's very helpful.

apsilon · Postby **apsilon** » Thu Mar 15, 2018 8:38 am

Any comments on my choice of tyre size vs wheel size? ie 195/50-15 on a 15x6.5 rim?

While researching I'm seeing so many people running 195 or 205 on a 7 or 8 inch wide rim I'm wondering if I'm looking at a rim that's too narrow at 6.5? I know not all 195 tyres are the same width but they're not that far apart either. FWIW I'm thinking of Bridgestone RE003 for tyres.

Is it just 7 inch and up rims are far more common widths? I'd think 6.5 would be spot on for a 195 and 6.5 has to be lighter than 7 or 8 everything else being equal.

hks_kansei · Postby **hks_kansei** » Thu Mar 15, 2018 9:30 am

7inch is a more common width for non-factory wheels.

Tyres are designed to be able to be mounted safely to a range of wheel widths, and most manufacturers will have a chart showing the maximum and minimum widths the tyre can be mounted on.

195 width tyres on a 6.5 rim is fine, the NB8A came from the factory with 195/50/15 tyres mounted to a 15x6 wheel.

Most tyre widths will have an ideal wheel width where they perform best, but unless you're a very good driver, and on a racetrack, you'll be hard pressed to find any difference (within reason)

apsilon · Postby **apsilon** » Thu Mar 15, 2018 1:03 pm

Thanks, that's what I figured but I'm seeing so many people say they're fine on an 8 inch, not stretched etc I was begging to wonder. Want to get this right, especially if I bite the bullet and spend some serious $$$ on Rays or something.

hks_kansei · Postby **hks_kansei** » Thu Mar 15, 2018 4:24 pm

while you can physically fit 195s on an 8, i dont believe any tyre manufacturer approves it.

My experience is that a 195 on an 8in wheel is just starting to be stretched.
205s fit better, but are about as narrow as I would personally go on an 8.

If you want, here's Toyo's data sheet for the C100 tyre (not a great tyre mind you), which shows the tyre sizes, and the wheel widths suitable for each.

http://toyotires.com.au/images/tyres/fa ... sC100v.pdf

Most tyre manufacturers will have very similar specs.

That chart shows a 195/50/15 is suitable for between 5.5 and 7inch wide wheels.

apsilon · Postby **apsilon** » Thu Mar 15, 2018 9:19 pm

Yeah I've seen that type of info before but it seems manufacturers don't publish it much anymore or hide it away where it's difficult to find. My rule of thumb has always been to use a wheel slightly narrower than the tyre that'll still give a fairly square sidewall.

Think I'll stick with 6.5"

project.r.racing · Postby **project.r.racing** » Fri Mar 16, 2018 9:36 am

195 wide tyres on a 6.5 wide rims is fine.

apsilon · Postby **apsilon** » Fri Mar 16, 2018 10:47 am

Thanks all, think I've made my decision.

Bronze TE37 Sonics in 15x6.5 +36 4x100 wearing 195/50-15 RE003s. First choice was actually the CE28N but they're no longer available in the combination of size, offset and colour I wanted but I've always wanted some TE37s so second choice is still pretty good. Not cheap but should be a very nice light set up that'll last a lifetime (well the wheels anyway) and I figure I made the mistake of selling my first MX5 so this one isn't going anywhere so worth the spend.