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Let's talk tire clearance

Old 02-28-21, 10:15 AM
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Let's talk tire clearance

I have an endurance bike (Roubaix) I am using for easier gravel (class 1 and 2). I don't do enough gravel riding to warrant a whole bike for it, but I still want to get the biggest tires I can fit. So, I have been reading up on tire clearances including some old posts here.

What I want to know is, what is a good gauge for minimum measured clearance for different situations? I know the ISO standard is 4mm minimum measured clearance. There was a recent GCN Tech video on tire clearance where they spouted 2-3mm. I have heard others state 6mm.

The whole tire clearance thing is a big ball of mud thanks to the fundamental inability to put precise numbers on any individual thing. Tire widths vary based on pressure and inner rim size, and different manufacturers use different standards when putting one number on the side of the tire. Frames could have a wide height but a narrow width clearance, be more or less flexible so more or less clearance needed for that, heavier riders need more clearance because their frames are flexing more, etc etc down the rabbit hole.

Even if you have measured the minimum clearance, it is still a big ball of mud (literally) as different treads and riding conditions will lodge bigger or smaller chunks of stuff in your tires which could then hit the frame and chip the paint or worse. And different riders and frames have different tolerances for chips or damage due to rocks.

Still, I thought I would throw out some numbers of rough targets for what would "work" in terms of minimizing problems with chipping paint, frame damage, jerking on your ride, etc. These are all for the minimum measured clearance anywhere on the tire/bike interface; in fact you may be able to have a bit less clearance on the sides in some cases but let's leave that out.
  • Slick tires (nothing rocks could lodge in) - 2mm minimum clearance
  • Small tread, Gravelking SK kind of tire - 4mm minimum
  • Big knobs, MTB kind - 6mm minimum
Yes I just made this up, no experiments done. But it is based on what I seem to hear from roadies vs MTB on clearance.

It is also important to have a good and easy way to measure clearances. The best way I have found is to use a set of long metric hex wrenches. The 6 for example is 6mm wide side-to-side. Just work it all around the wheel and the biggest size not catching is it.

So far for me I have found that the measured numbers are conservative. I recently got some "35mm" Schwalbe studded tires which have 6mm clearance on my "33mm max" Roubaix frame. So I feel good about those. I just ordered some "36mm" Donnelly X'plor MSO WC which may or may not have enough clearance. I am hoping I can use those as my regular gravel rider tires.

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Old 02-28-21, 01:44 PM
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I have a late 80s road frame that I've run tires with just over 1mm of space between the seat tube and front derailleur band. They were 25mm gp4k that measured out to 27mm.
I only ride that bike on pavement in good weather so I'm not picking junk up when riding.

My gravel bike though?...more stuff will be picked up by tires, even in dry conditions. It's inevitable- between the tread and road surface.
I would want more clearance.

And if ever riding in sloppy conditions, more clearance is needed. Wet gravel is abrasive and will gouge thru paint and even carbon.


p
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Old 02-28-21, 06:51 PM
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Avoid jerking on rides, you'll go blind. Otherwise my big knobs don't collect stones and i ride with 2- 3mm of clearance - in the dry. Small patches of mud are ok and fortunately rare.
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Old 02-28-21, 07:30 PM
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It's all down to personal comfort, riding conditions, wheel strength a reliability, etc. There are (as I understand it) standards for bike manufacturers when it comes to listing specs, but those are not commandments.

Example: my custom gravel frame is listed as having 38mm tire clearance in the rear triangle; the builder, when asked, said I could squeeze in a 42. I am currently running a tire that measures out at a little over 44mm wide...And that is with about 4mm clearance on each side. I am okay with that, because I run strong 32h 3x wheels and am not too worried about knocking one that far out-of-true.
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Old 02-28-21, 08:02 PM
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My Renegade is rated to run 700x40 tires, and I have run Schwalbe Marathon tires with a good (5mm?) amount of clearance on each side, for a long time. I tried some Surly Knard 700x41, and had very little clearance. In fact, in high temps, I had some rubbing on the chain stays. Not good.
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Old 02-28-21, 08:16 PM
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Thanks for the observations, it sounds like things are working a mm or two tighter than my little table above, which is good news for me. I was going to require 4mm clearance on my new X'Plors but I'll probably keep them if they are giving me at least 2-3mm clearance, but for dry days only.

The fact that your 40's had 5mm and 41's had near-zero clearance is part of the challenge of the lack of standard. I did look around on the X'plors and they are supposed to size about as measured. My Schwalbe 35s are 33mm on my calipers and have a 6mm gap with them, so a true 36mm should have approximately 6 - (36-33)/2 = 4.5mm left on each side. Plus or minus a couple mm reality factor.

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Old 03-01-21, 12:38 AM
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I don’t think this line of thinking is correct at all, and that’s speaking as a cyclist with over 30 years of MTB, trail, and gravel riding.

Sure, you need clearance for mud, but in my experience, one doesn’t need clearance for rocks, because a) they don’t get picked up and carried around by the tires hardly at all, and b) the bigger the gravel the less clearance is needed because that kind of gravel surface is less likely to be muddy and bigger rocks are less likely to be carried, flung, or otherwise find their way to a place where they can jam or damage the frame.

So, I think we should choose clearance for fenders and mud, not based on surface gravel size.
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Old 03-01-21, 07:07 AM
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I don't think anyone was suggesting that bigger gravel is worse. But I think you are correct that some mud needs to be thrown in for there to be a real problem, and perhaps mud plus wetness plus some big enough gravel in the mud is the worst combination.. some of those mud-encrusted rocks are too big to squeeze through. If the gravel is too big or too small, either it won't attach or it won't have a problem getting through. It is that bb to pea size stuff I would guess that is the concern, with small tread more on the bb side and bigger tread grabbing peas. Also dependent on the type of mud, see picture above for example which looks like someone rode through cement.

I have over 40 years trail riding experience (got a Specialized Hardrock when they first came out, was trail riding on 10-speeds before that), but I never paid attention to what was hanging off my tires.
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Old 03-01-21, 09:26 AM
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Originally Posted by scottfsmith
I don't think anyone was suggesting that bigger gravel is worse. But I think you are correct that some mud needs to be thrown in for there to be a real problem, and perhaps mud plus wetness plus some big enough gravel in the mud is the worst combination.. some of those mud-encrusted rocks are too big to squeeze through. If the gravel is too big or too small, either it won't attach or it won't have a problem getting through. It is that bb to pea size stuff I would guess that is the concern, with small tread more on the bb side and bigger tread grabbing peas. Also dependent on the type of mud, see picture above for example which looks like someone rode through cement.

I have over 40 years trail riding experience (got a Specialized Hardrock when they first came out, was trail riding on 10-speeds before that), but I never paid attention to what was hanging off my tires.
Sorry, I did misread you. Re-reading, I see I mistook millimeters of suggested clearance as the measurement of gravel size. Whoops!
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Old 03-09-21, 07:36 PM
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"How much tire clearance do you need?"

- Yes
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Old 03-10-21, 05:12 PM
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Things don't have to be so complicated. The builder says my frame will fit 50s, so I mounted a pair of GravelKing SK 48s and that's the last time I gave it a thought. I'm not going to measure anything, I'm not going to worry about frame flex, and I'm certainly not going to care about the paint on a gravel bike. Eyeball it and go for a ride!
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Old 03-10-21, 05:36 PM
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Originally Posted by Rolla
Things don't have to be so complicated. The builder says my frame will fit 50s, so I mounted a pair of GravelKing SK 48s and that's the last time I gave it a thought. I'm not going to measure anything, I'm not going to worry about frame flex, and I'm certainly not going to care about the paint on a gravel bike. Eyeball it and go for a ride!
The complicated part for me is my builder says my frame will take only up to 33mm. I bought 33mm tires and found cavernous gaps so I want a bigger tire. I don't care about paint on a gravel bike, my bike is a carbon endurance bike with no paint on it. I care a lot about carbon, I don't want serious scratches in my frame which could weaken it. Your builder should have already figured that part out by putting in plenty of clearance in that 50 number.

...Following up on my original post I have some good news, I got some Gravelking SS 35's and they measure at about 35mm inflated and have 4mm clearance all around. I did a couple rides including lots of rocks and mud and everything went swimmingly. The only issue is the SS are not great mud tires so I am going to get another pair for mud season.
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