Tyre down or rim/wheel down (size)

I’ve just started a new build and have hit my first quandary … tyre clearance.

The frame is meant to clear a 2.4” tyre – I appreciate tyre sizing is a dark art.

I have an Ikon 2.35” tyre that on its current rim (IIRC 30mm internal) sizes up as 2.4”.

All okay so far … but … a real world 2.4” when in the frame is, IMHO, tight. With my squints on I’d say under 5mm on each side at the chainstay.

I generally avoid the claggy mud but around the wilds of Suffolk it’s pretty sandy and creates a weaponisable level grinding paste that has a tendency to self clearance …

I like the Ikon on the rear so is it worth sizing down to a 2.2” and hoping the rim gets it to closer to 2.3” or rim/wheel down size to a narrower rim?

Part of the appeal of this build was to run low pressures too 🙁

Borrow a wheel with a 25mm internal rim and stick that in there, in the first instance?

Smaller tyre will obvs be cheaper than narrower rim, but 2.35in on a 25mm internal should be better than on a 30mm anyway.

Smaller tyre will obvs be cheaper than narrower rim, but 2.35in on a 25mm internal should be better than on a 30mm anyway.

It probably won’t lose 5mm width, unfortunately. The rim internal width is part of the circular cross-section of the tyre; assuming a perfect circle (for the sake of argument) you’ll lose a couple of mm in overall width tops

650b might be better for clearance, but 2.4″ will lose overall tyre diameter, volume and lower the gearing. Fatter tyres on 650b will sort that and give you lower pressures too

I did a load of measurements for this a few years back and lost it all, like an idiot. The bottom line is a change in rim width very nearly adds the same amount to circumference. It’s not quite so simple- rim internal height makes a difference too, because tyres aren’t round, they’re sort of lightbulb shaped. But in terms of actual size 5mm of rim will usually reduce the diamater by a little over 1.5mm. It might just be the magic amount but it’s not much. I mean, I don’t think I’ve ever looked at a tyre and thought “that’s just not enough clearance but if it had .75mm more on either side it’d be grand”?

Equally I’ve had plenty of frames that had only 5mm clearance and didn’t stress it, especially with a less knobbly tyre that doesn’t haul up mud. Helicopter tape will stand up to quite a lot of wear and doens’t lose you significant clearance really.

Tricky one tbh, if you definitely want more clearance then you need a smaller tyre but it’s going to cost you one of the things you wanted.

Helitape the chainstay and see how long it lasts.

Then decide based on that if you’re willing to put up with the hassle of replacing that often.

Thanks folks.

I’ve been mulling/stewing over this for a day now (full drama llama style).

On a revisit to the rims they are 30mm ID / 35mm OD so perhaps I’m asking too much.

Looking at photos of the complete bikes almost all are being shown with a 2.4” rear but often with an mis matched pair of rims …26mm rear, 30 front.

Sadly at the minute I don’t have another tyre or wheel to try 🙁

I may break out the heli tape first. The thought of buying a new wheel to still not get the clearance and then still have to go down a tyre size will drive me to madness!

I may break out the heli tape first

My thought would be to run what you have and see how it goes.

You didn’t specify 700c or 650b, but either smaller wheels or narrower tyres to gain a bit of clearance will cause a “reaction” elsewhere in your ideal bike build