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Formula for tire width for gravel

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Formula for tire width for gravel

Old 06-07-21, 10:50 AM
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scottfsmith
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Formula for tire width for gravel

Noonievut suggested on a recent thread here that there should be a formula to pick how wide a tire to run on gravel. I thought it would be fun to take a stab at it.. here it is.
  1. Look at the Cycling Tips 1-5 scale of gravel difficulty and pick the number which is the max you will frequently be riding on (i.e. you will be under-biked if you had smaller tires on that).
  2. If you are rarely on gravel (less than 10% of the time) subtract one from your number from step 1 -- you don't want to compromise your road riding too much and should be willing to be a bit under-biked sometimes on the gravel. Otherwise keep number the same. Lets call this your RGG, your raw gravel grade.
  3. Now, take RGG * 3 + 30, call that RGW, your raw gravel tire width. This is the initial approximation of what size tire to run and we will now go through some potential tweaks to your RGW.
  4. If you want a nice compromise of speed and smoothness, make no change. If you value smoothness over speed, add 3. If you really value smoothness over speed, add 6. If you really value speed over smoothness, subtract 3.
  5. For every 20 lbs over 160lbs you are, add 1, and similarly for every 20 lbs under subtract 1.
  6. Apply all the tweaks to your RGW and that is your gravel width, GW, the size of tire to run.
Applying this to me, I am about at a 2.5 in terms of the max gravel I ride by the CyclingTips rating system. I have special gravel rims which are mainly on gravel so no minus one for me. So my RGW is 30 + 2.5 * 3 = 37.5mm. I don't have any of the other corrections so my GW is 37.5mm. This is pretty close to what I run, my Terra Speed 35c are 37.2mm mounted.

Of course this is all just a stab in the dark with numbers pulled out of my head. But I did a couple calculations imagining various scenarios and it didn't seem too far off. Feel free to punch up your numbers and see. It is also mainly for fun and maybe for beginners to get a start, don't take it too seriously.
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Old 06-07-21, 10:56 AM
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I use the following formula:

(Weight of Rider + Bike) (in kg) divided by (cycling tips gravel rating x 3) + (riders age divided by 3)

Finally, multiply the answer to the above by zero, and add 40.
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Old 06-07-21, 03:14 PM
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Hmm, let me plug into your formula Clyde .. it will take awhile because I have to convert to kg. .. (crank grind crank) .. It looks like I end up at 40mm there. I think my formula is better for me, from experience I like 35-38mm tires the most. My formula is winning!

I think we can declare my formula the best gravel tire width formula ever invented based on the data in so far..
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Old 06-07-21, 06:53 PM
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Originally Posted by scottfsmith View Post
Noonievut suggested on a recent thread here that there should be a formula to pick how wide a tire to run on gravel. I thought it would be fun to take a stab at it.. here it is.
  1. Look at the Cycling Tips 1-5 scale of gravel difficulty and pick the number which is the max you will frequently be riding on (i.e. you will be under-biked if you had smaller tires on that).
  2. If you are rarely on gravel (less than 10% of the time) subtract one from your number from step 1 -- you don't want to compromise your road riding too much and should be willing to be a bit under-biked sometimes on the gravel. Otherwise keep number the same. Lets call this your RGG, your raw gravel grade.
  3. Now, take RGG * 3 + 30, call that RGW, your raw gravel tire width. This is the initial approximation of what size tire to run and we will now go through some potential tweaks to your RGW.
  4. If you want a nice compromise of speed and smoothness, make no change. If you value smoothness over speed, add 3. If you really value smoothness over speed, add 6. If you really value speed over smoothness, subtract 3.
  5. For every 20 lbs over 160lbs you are, add 1, and similarly for every 20 lbs under subtract 1.
  6. Apply all the tweaks to your RGW and that is your gravel width, GW, the size of tire to run.
Applying this to me, I am about at a 2.5 in terms of the max gravel I ride by the CyclingTips rating system. I have special gravel rims which are mainly on gravel so no minus one for me. So my RGW is 30 + 2.5 * 3 = 37.5mm. I don't have any of the other corrections so my GW is 37.5mm. This is pretty close to what I run, my Terra Speed 35c are 37.2mm mounted.

Of course this is all just a stab in the dark with numbers pulled out of my head. But I did a couple calculations imagining various scenarios and it didn't seem too far off. Feel free to punch up your numbers and see. It is also mainly for fun and maybe for beginners to get a start, don't take it too seriously.
Well done!
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Old 06-07-21, 10:16 PM
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I use the largest tire that'll fit
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Old 06-08-21, 08:24 AM
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Originally Posted by Steve B. View Post
I use the largest tire that'll fit
Honestly, this is the best solution.

THe flaw with the formulas given above (except mine, of course) is that they (a) assume having a tire wider than 'ideal' is somehow a disadvantage on smooth surfaces, and that (b) the type of gravel you plan on riding is the only type you will ride. In fact, any drawbacks from wider tires is miniscule, especially if you add pressure for rides expected to be smooth or paved, and on rough or soft surfaces the wider the better. Anyhoo, isn't the whole point of gravel bikes to be able to take on roads and tracks with which you are unfamiliar? Wider tires mean you can turn down any road or trail without worry. 35mm tires, for instance, are pretty multi-purpose, but on the roughest of roads you need to proceed with great caution. 42 or 45mm tires are damn close to what we used on rigid mountain bikes in the 80s and 90s - they can go anywhere.
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Old 06-08-21, 08:44 AM
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My tire size choice (and PSI) is fully dependent on the surface and the specifics of the ride ( Racing v. Bikepacking v. Gravelling).


There is no panacea.
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Old 06-08-21, 08:55 PM
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I think for a lot of people the widest tire that fits is indeed a good way to go. But many gravel bike owners are in fact doing a good fraction of their riding on the road, and if they also are interested in going fast they will need to make a compromise somewhere. Either that or get another set of wheels. My formula gives a pretty good idea where to compromise. It might be good to be more explicit about the road % in the formula.
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Old 06-16-21, 05:16 PM
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Originally Posted by ClydeClydeson View Post
I use the following formula:

(Weight of Rider + Bike) (in kg) divided by (cycling tips gravel rating x 3) + (riders age divided by 3)

Finally, multiply the answer to the above by zero, and add 40.
Hey I use that exact formula myself!

Only I add 50 at the end...
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Old 06-17-21, 11:26 PM
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I rode a cyclocross bike with canti brakes as a "gravel bike" for about 10 years before they invented "gravel bikes". That bike can only handle a ~35mm or so tire easily. I found that tire satisfactory, for all of the non-paved riding I do, and have a fairly new pair, so I moved them over to my new disc brake GR300. That's how I've picked my gravel tires. I'll probably try a 38- 40 or so when i need new tires. The frame will take ~45mm with 700c wheels and ~2.1" with 650B. I might end up with ~45 or so tires on a set of 650B and keep ~38's on the 700c. I don't see a need for really big tires on a gravel bike. i don't use it for technical or overly rough stuff, that's what my mountain bike is for. I don't use it for dedicated road rides, that's what my road bike is for.

if I had one bike (which is the case when we're traveling... using the above CX bike), I would have two sets of wheels: 35-38mm for non-paved and general use, and 25-28 for dedicated pavement riding when I want to ride in a road group. That's proved to be very useful for the several years we've been traveling that way.
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Old 06-18-21, 08:34 AM
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Originally Posted by scottfsmith View Post
I think for a lot of people the widest tire that fits is indeed a good way to go. But many gravel bike owners are in fact doing a good fraction of their riding on the road, and if they also are interested in going fast they will need to make a compromise somewhere. Either that or get another set of wheels. My formula gives a pretty good idea where to compromise. It might be good to be more explicit about the road % in the formula.
At higher speeds when riding alone, there will be added aerodynamic resistance from a big fat tire, and probably a bit of a weight penalty, but otherwise there is little or no downside to wider tires. Only if someone is using there bike 98% on road but needs to occasionally cross a bit of known smooth gravel would a narrower tire be a net benefit, IMO. Otherwise, any rough or soft or unknown surface is faster on a wider tire, and hard surfaces are not significantly slower.
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Old 06-18-21, 09:29 AM
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Originally Posted by ClydeClydeson View Post
Honestly, this is the best solution.

THe flaw with the formulas given above (except mine, of course) is that they (a) assume having a tire wider than 'ideal' is somehow a disadvantage on smooth surfaces, and that (b) the type of gravel you plan on riding is the only type you will ride. In fact, any drawbacks from wider tires is miniscule, especially if you add pressure for rides expected to be smooth or paved, and on rough or soft surfaces the wider the better. Anyhoo, isn't the whole point of gravel bikes to be able to take on roads and tracks with which you are unfamiliar? Wider tires mean you can turn down any road or trail without worry. 35mm tires, for instance, are pretty multi-purpose, but on the roughest of roads you need to proceed with great caution. 42 or 45mm tires are damn close to what we used on rigid mountain bikes in the 80s and 90s - they can go anywhere.

Cycling is great sport for those who love to continuously tinker with things whether it's tires, handlebars, saddles, gear setup, etc. There is a never ending list of things to tinker with. I certainly do plenty of that. I've tried plenty of tires and finally settled on my Specialized Pathfinder Pro size 42 which are fairly speedy on paved when needed. If I felt like I needed to be faster on my gravel bike on paved I would go with my Terreno Zero size 38 tires.

I think it just depends on what your goals are and preferences are. For most people in day to day riding wider is better and there are plenty of gravel tires that will at least do okay on paved surfaces.
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Old 06-18-21, 09:40 AM
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My wider gravel tires are slower on the road than my wide road tires which I assume is because of the tread/rolling resistance. But my wide road tires are perfectly capable of keeping up with the fast Wednesday night ride.

The motor, on the other hand...
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Old 06-18-21, 11:12 AM
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Originally Posted by scottfsmith View Post
I think for a lot of people the widest tire that fits is indeed a good way to go. But many gravel bike owners are in fact doing a good fraction of their riding on the road, and if they also are interested in going fast they will need to make a compromise somewhere. Either that or get another set of wheels. My formula gives a pretty good idea where to compromise. It might be good to be more explicit about the road % in the formula.
I think that is true for me, as I do ride a fair bit of tarmac on my gravel bike, and dislike the feel of wide tires on pavement. The problem is probably a mix of tire weight, inflation pressure, and handling characteristics (esp. when turning), but in any case, I compromise gravel road optimization in favor of acceptable paved road manners. I don't have a lot of experience, but 35c seems to be the sweet spot, running around 60psi for my 255lbs.
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Old 06-19-21, 12:27 AM
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My formula starts with wearing out the tires that came with the bike, and then probably buying something slicker and one size bigger
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