landapanda

Some quick maths for ya'll today so just some input on the specs I want to run compared to OEM and my current

I am planning on owning two sets of wheels. One track and one street (currently being built). I have been told that the diameter and circumference have to be within +/- 3% from OEM specs to not throw off alignment and for handling purposes. I'm sure width plays some role but not entirely sure tbh

Here are OEM AP1 wheel/tire specs:
F: 16x6.5 +55 (205/55) --> diameter: 631.9mm; circum: 1985.2mm

R: 16x7.5 +65 (225/50) --> diameter: 631.4mm; circum: 1983.6mm

My current setup is:
17x8 +42(235/40) --> diameter: 619.8mm; circum: 1947.2mm

17x9 +38(255/40) --> diameter: 635.8mm; circum: 1997.4mm

Using some quick percentage error maths

For my current setup --> Diameter and circumference percent error = 1.91%(front) and 0.70%(rear) from OEM specs.

Question is: if my new setup(street wheel) is 18x9.5 +42(265/35) and the diameter percent error = 3.69%(front) from my current setup but only 1.71% from OEM. would that be okay to run and swap wheels without messing alignment or handling? I'm not worried about the rears as the offset and width will poke basically the same as my current setup. Any one with similiar specs in the front please chime in

engifineer

Who is telling you that changing wheel size changes your alignment???

Your alignment angles do not change with a change in wheel size. If there is a fitment issue, you can sometimes remedy with an alignment change, but that is a different story.

Certain alignment settings may work better with one tire vs another, but that is due to tire compounds, sidewall stiffness, etc and is not based on size alone.

And a change to tires will always affect handling in some way (could be good or bad) but is not due to alignment being off.

Now, if you have a much stiffer sidewall tire, you can maybe get away with less camber since the tire does not roll over as much (if rolling onto the shoulder was an issue before) but again, that is a characteristic of the tire design, not just a change in size.

I have to worry that you have a tire and alignment shop trying to make some easy money off of you. If that is who is telling you this, then you should find a new shop.

landapanda

A shop was not telling me this, it was a family friend. Well in this case I'm referring to not just simply changing tires and changing the different size of the tire.

I'm more worried about going from the 18x9.5 to 17x8/9 for a track day and my alignment being all wonky. I'm thinking in terms of geometry of the suspension. How the entire face (i.e. the diameter) affects where the steering axis, due to the increase/decrese in width and/or diameter.

My friend was trying to make a point in saying, Honda used the wheel/tire specs for a reason. And that if I want new wheels/tires with a larger wheel base, I would want to match the diameter and circumference as much as possible my manipulating tire sizes. That's why I can only run a 35 series on the 18" as opposed to the 40 series on the 17". Hopefully that makes some sense.

Chuck S

Makes sense. I think. The late 17" wheels and lower profile tires are the same circumference as the earlier 16" wheels with slightly higher profile tires. They can be interchanged on whim. With aftermarket wheels you need to pay close attention to wheel offset. S2000 offsets are close to unique.

-- Chuck

rrounds

Well I run 235/45-17 up front with 245/45-17 out back on my '06 and the only other tire I would run out back is a 265/40-17. Have well over 125k miles on this size tires and I will not be going back to the stock size tires any time soon. Best traction I've had on the roads I drive(all back roads) over the 13 western states and Canada.
For the track I'd run different size tires but for the street I like these tires. Still have to get a good summer tire though, stay away from A/S tires.

ROD

Car Analogy

You said wheelbase but likely you meant track width. Obviously you can't change wheelbase due to wheels and tires. Wheelbase is distance from axle center to axle center front to rear (or tire contact patch front to rear, which will be the same distance). Track width is width left to right. Often different value front track width vs rear.

Note that changing width can have an affect on things like Ackerman. That is a geometry of track width and wheelbase that determines how much more the inside wheel should turn than the outside wheel. In a turn, inside wheel and outside trace different arcs. You need Ackerman for the same reason you need a diff.

If you alter Ackerman due to different wheels, one of the front wheels is going to scrub in turns more than with stock wheel dimensions. Is it a big deal? No. Is it going to ruin your handling? No. There is only so much change you can make to Ackerman just from changing wheels. Unless you are making stupid offset choices, its not a big factor.

Another thing to note, changing track width via wheel offset also means that wheel has different leverage on the suspension spring, so it can affect ride height to a small degree. That of course will affect alignment. But again, to a small degree. Unless you are making stupid offset choices, its not something to worry about. But useful to know about.