Wheel math

[933233311602]

Using the stock 225/55 R19 ET-47.5 7 inch rim & tire for reference.

I test fit a Ford 235/60 R18 ET-52.5 8 inch combo. It fit but with very little space left between tire and shock tower.

The wheel-size.com calculator says the tire was 10mm closer with the 235/60R18 combo. With so little space left between wheel and suspension, I figured 10mm was the maximum extra intrusion possible.

But member Trooper says his 255/50 R19 ET-45 8.5 inch combo fits with some mm to spare. The wheel-size.com calculator says this combo is 13mm closer to the suspension.

How can this be? My test was 10mm closer to the suspension and left very little space. The 255/50 R19 ET-45 8.5 inch combo calculates to being 13 mm closer and Trooper reports that there is still space remaining with his set up.

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[mkhuffman]

What is your question?

 

[933233311602]

What is your question?

How does a wheel combo with 13mm more intrusion fit with room to spare into a space where a wheel combo with 10mm more intrusion barely fits?

 

[Maquis]

Do all models have dimensionally identical shock towers?

 

[HayaCruza]

Your wheel choice is about 17.5 mm further in than stock.
5 mm by ET.
12.5 by rim width
Compared to Trooper, he is about 16.5 mm in from stock.
Out 2.5 mm by ET
19 mm by rim width
Then figure in tire bulge.
Numbers are approximate since I mostly used 25 instead of 25.4 mm per inch.

 

[markboris]

There are several variables here. One, is you say the 235/60 R18 ET-52.5 8 inch combo you tried has "very little space". What does that mean? Very little space to you might mean something completely different to Trooper. This needs to be measured to be accurate. In some of the wheel/tire setups I have had, very little space to me means I can only slide one credit card in between the tire sidewall and the shock/strut body. Two credit cards would not fit. Is that YOUR definition of very little space, because that is mine. Another is tire bulge. Some tire sidewalls have a rounded profile (bulge out) while some are more square (flat). In the scenario I said above where I only had enough space to fit one CC, I was using Michelin Pilot Sport 4S tires with a square/flat sidewall profile. Had I used most other tires, that setup would not have fit. The wheel width does not have much to do with the fitment of how close your tire is to the suspension. It is mainly the width of the tire, profile of the sidewall and the offset of the wheel.

Use your wheel size calculator to plug in 275/40 R20 ET-38 9.5 inch and see what you come up with compared to your reference setup. That is what I had on my car last month and later this month I will have a setup that is 2mm more inset than that.

 

[markboris]

Do all models have dimensionally identical shock towers?

I assume you are talking about the front struts and from what I have seen, yes there are very close if not identical. The only one that could be different is the GTPE because the strut piston/rod has a larger diameter compared to all the other models so the strut body might be a bit larger.

 

[AtomicInternet]

You're neglecting the event horizon which occurs right at the edge of the tire and allows for the disruption in space-time which makes it fit.

 

[HuntingPudel]

I’m another one using a similar setup to @markboris with 275/40R20 tires on 9.5” rims. I have a bit more than a credit card’s thickness between the tire bulge and the strut. ??

 

[933233311602]

Your wheel choice is about 17.5 mm further in than stock.
5 mm by ET.
12.5 by rim width
Compared to Trooper, he is about 16.5 mm in from stock.
Out 2.5 mm by ET
19 mm by rim width
Then figure in tire bulge.
Numbers are approximate since I mostly used 25 instead of 25.4 mm per inch.

Can you show me how you calculated this?

 

[HayaCruza]

Remember offset or ET is from the centerline of the wheel. Us old school people used to talk about backspace rather than offset.
Stock is 47.5 mm so the mounting face is pushed out that far.
Your choice of wheel is 52.5 so another 5 mm.
Stock wheel is 7” and your new wheel is 8”.
So you have to account for 1/2 of that as additional offset, 25.4 / 2 = 12.7 mm.
So the mounting face is about 17.7 mm further out.
Which moving the mounting face further out makes the back edge of the wheel get closer in.
Trooper is 45 ET and 8.5”
From stock is -2.5 ET instead of your +5.
Then adjust for the width, 1.5” x 25.4 / 2 = 19.05 mm.
In total, -2.5 + 19.05 = 16.55 mm

None of this accounts for tire width or shape….
Hope that helps.

 

[Jerrytball]

This is a nice website for tire size comparison stock, tire outlook changing it to a different tire the dimensions not sure you’re looking for this, but I’ll share.
https://www.tacomaworld.com/tirecalc?tires=245-45r20-255-45r20

 

[933233311602]

Remember offset or ET is from the centerline of the wheel. Us old school people used to talk about backspace rather than offset.
Stock is 47.5 mm so the mounting face is pushed out that far.
Your choice of wheel is 52.5 so another 5 mm.
Stock wheel is 7” and your new wheel is 8”.
So you have to account for 1/2 of that as additional offset, 25.4 / 2 = 12.7 mm.
So the mounting face is about 17.7 mm further out.
Which moving the mounting face further out makes the back edge of the wheel get closer in.
Trooper is 45 ET and 8.5”
From stock is -2.5 ET instead of your +5.
Then adjust for the width, 1.5” x 25.4 / 2 = 19.05 mm.
In total, -2.5 + 19.05 = 16.55 mm

None of this accounts for tire width or shape….
Hope that helps.

Thanks. This was helpful.

 

[Jerrytball]

Speaking of doing the math, I forgot to bet and I picked the winner of the Kentucky Derby darn it

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