Tyre/wheel calculation problem - pls help

[klutz_100]

Hmm... I have a problem on my S7 and I'm not sure what to do.

I bought a Gunze Lotus Elan to change the Tamiya rims to those correct for the version I am building.

I immediately noticed that the Gunze rims are visibly smaller and was intending to fix that.

However, I did a test fit of the two different wheel/tyre sets and, comparing to ref pics, it seems (optically at least) that in fact the Tamiya wheels are slightly over sized in proportion to the wheel arches.

So here's my question.

The original S7 had these rims: 4 1/2" x 13" Lotus Elan type bolt-on rims
and these 520 x 13 (crossply) tyres.

What diameter would this work out to in millimeters in 1/2 scale?

The Gunze tires are 23.75mm diam
Tamiya tires are 24.75mm diam

Gunze rims are 14mm diam
Tamiya rims are 15mm diam

Thanks for any help you can give in resolving my quandary.

[Hiroboy]

Maybe this might help a bit ? I wrote this a few years ago.

[MPWR]

Wellll....

The Tamiya wheels work out to just over 14", which is correct for 13" wheels/tyres. This seems to have been the most popular tyre size for 7s for years. 3 1/2" seems really narrow- for that matter, so does 4 1/2". We're talking bicycle tyres. The kit tyres work out to about 6" wide. 520mm is 20". (20-13)/2 = 3.5. Again, that seems kinda short for the sidewalls. I could be wrong, but I'm not terribly sure about these numbers.

Either way, in a few days you'll have a solution that

A. Fits the Tamiya tyres
B. Fits the kit hubs
C. Looks like the Elan style wheel

If you like 'em, you can even cast another set, get another set of tyres from Tamiya, and use them on your Elan.

[mikemechanic]

Originally Posted by Hiroboy

Maybe this might help a bit ? I wrote this a few years ago.

Except there is a small flaw in your system. When a manufacturer says that a rim is 18,19 or 20 inch they are talking about the tire size that fits on it. Our Dodge truck rims at work are called 20" but if you measure the dimension of "x" they measure out at almost 22". Quite a few people are confused by the difference between rim size and tire size. Hope this helps.

Mike.

[robrex]

Originally Posted by mikemechanic

Except there is a small flaw in your system. When a manufacturer says that a rim is 18,19 or 20 inch they are talking about the tire size that fits on it. Our Dodge truck rims at work are called 20" but if you measure the dimension of "x" they measure out at almost 22". Quite a few people are confused by the difference between rim size and tire size. Hope this helps.

Mike.

I was stunned when I read this so measured my rims on my car and you're right!
They are also 20" rims are just under 22" diameter.

So the tire size must be determined including the bead??

[mikemechanic]

Originally Posted by robrex

So the tire size must be determined including the bead??

Yes Rob. When you buy tires the last number is the rim measurement for example 255/50/16 fits a 16" rim, but that 16" is for the diameter of the bead which is where the tire sits on the rim, whereas the the rim itself at dimension "x" will probably measure 17.5 or 18"
A lot of reviews I read always state that the rims measure out to be incorrect, but I am sure that the rims really are the correct size they are just being measured incorrectly.
I imagine quite a few people are outside measuring out there rims right now

[hirofkd]

This has been discussed several times, and it's one of the basics of car modeling, but it's also a tricky one, because normally you'd think X inch wheel should have the diameter of X inch, but the published wheel size doesn't include the flange height. That's why Fujimi's Lexus IS wheels are too small, because the kit designer didn't know that.

The diameter of 13 in wheel in 1/24 scale can be obtained by...
13 in x 25.4 mm/in = 330.2 mm
Add a flange height of say 18 mm (that's JJ) to both end of the diameter, you get...
330.2 + 18 + 18 = 366.2 mm
Divide by 24, and you get
15.258mm
So the "visual" diameter is a little bigger than 15 mm.

Typical flange height varies from 14 mm (B) to 19.5mm (K), but most wheels have 17.5mm (J) or 18.0mm (JJ).

[klutz_100]

Sorry to be a PIA about this but if we take the rims out of the equation for a moment, what is the outside diameter of 520 x 13 (crossply) tyres?

[gionc]

Well you all discuss of matematics but actually we have another problem (or better isn't a problem?)

Tamiya ever offered oversized tyres and rims: just check in your models. The rim's diam is often 1 inch (so in 1/24... 1,something mm) plus and also tyre is ever very wide: have a look in the street NSX, in the 360 modena, almost ever: other manufacturer like Fujimi and Hasegawa offer more "right" sizes but we ever liked "badass" Tamiya appeal. Agree?

Obviously this fact may give troubles when you try to mix Tamy stuff with other manuf. stuff.

My opinion is "inside the limit of good taste" huge is great LOL (also my wife though ... ROFL)

[gionc]

PS sorry for the 8AM english, little brain is warming up

[Hiroboy]

Originally Posted by mikemechanic

Except there is a small flaw in your system. When a manufacturer says that a rim is 18,19 or 20 inch they are talking about the tire size that fits on it. Our Dodge truck rims at work are called 20" but if you measure the dimension of "x" they measure out at almost 22". Quite a few people are confused by the difference between rim size and tire size. Hope this helps.

Mike.

I'm sure that is what I wrote on a 20" inch wheel the O/D of the wheel is 21.5"

[mikemechanic]

Originally Posted by Hiroboy

I'm sure that is what I wrote on a 20" inch wheel the O/D of the wheel is 21.5"

Sorry Steve, I misread your whole chart. I was reading the 23" Pegasus sleeve and didn't notice that you had adjusted for the dimension "x" in the left side of the chart. My humble apologies for doubting you.

Mike.