Gravel Tires or MTB Tires for Gravel Riding/Racing

Equipment
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Confused here. Looking for clarification. My specific use would be to run these tires, in gravel races on a full suspension XC race bike (don’t have a gravel bike, but might some day). It weighs 22 pounds with pedals, and Continental Race Kings (with sealant), 25 mm internal rim width, so pretty much any 700c gravel tire will fit. Not optimum for gravel duty, but it will do. Do the Race Kings have the best traction? No. but many of us have used them on longer MTB 100 milers and survived. Why would I (or anybody) use gravel tires on a gravel bike? According to bicyclerollingresistance.com they have higher rolling resistance than the Race Kings, I am not sure aerodynamics matter at the speeds I ride, especially since I will be on an MTB. Looking for the answer to why use a gravel tire instead of Race Kings, if they clear the frame/fork, as they might have superior tread, when not on a road.

Continental Race King, 29x2.2 18.0W at 35PSI (18MPH). measured width is 51mm

Continental Terra Speed, 700x40, 21W at 36PSI (18 MPH). measured width is 38mm

What am I missing here? Is it aero?

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Race King are great for chunky gravel courses. I have a Scott Scale hardtail set up with a rigid carbon fork and use the Race Kings on it. It’s great for courses with more than 50% gravel.

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Schwalbe Thunderburts are another XC MTB tire that is popular for gravel. I just put a set of 2.1" on 650b wheels - hope to get them out on a ride soon. I would have bought the Race Kings if I could have found them in stock. Nothing wrong with a light, fast XC tire for gravel if you have the clearance.

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it’s not all about the rolling resistance either, those Race King’s are almost double the weight of a set of Gravel King slicks.

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Good question. I’ll caveat my answer with: I’m 208 pounds and routinely carry 2-4 L of fluid.

I rode some new 44mm Cadex tires yesterday on a new Cervelo Aspero, as a test for someone, and I was daydreaming all day of XC MTB tires. Aspen, Rekon Race, or Ardent Race 2.25 &/or 2.4’s would have been dreamy, or at least my wife’s 50mm Maxxis Ramblers!

Context: Non-competitive but enthusiastically-paced ride hosted by SBT as a charity event in Calabasas, riding with (and getting my legs ripped off by) the front group.

I couldn’t have gone any lower psi (was in the 35-40psi range) because I was already rim-striking during descents. But I would have benefited by having them at 20psi for the bumpy climbs.

To heck with rolling resistance data (not saying it shouldn’t be considered, but yesterday was a striking reminder of a few things that matter more than lab RR data… and I love lab data).

There is zero doubt in my mind, a higher volume tire with lower pressure would have resulted in faster times on my 50-50 road/“gravel” (fire roads in calabasas area) ride yesterday. What matters most is rolling resistance when you’re operating at or above threshold because those watts are more expensive physiologically than below threshold.

Reasons I’d almost always opt for 50mm or larger for gravel, unless it’s absolutely glassine stutter-free gravel:

  1. More volume + lower psi = WAY lower rolling resistance when it matters the most (rough stuff, and during lumpy climbs).
  2. Bigger tires with bigger side knobbies would have provided massive speed conservation around every turn. Acceleration is enormously wasteful energetically.

The unevenness of the hardpack access roads, chunk rock, even slick rock, made me chuckle at the fact we were all on gravel bikes.

I’d have been faster on my Scott Spark, especially with the remote lockout feature.

Getting beat to death by a low-volume high pressure gravel tire is not the way to be fast.

Does anyone know of a tire that looks like a Maxxis Minion SS but designed for gravel??

Goal: slick/fast middle, and enormously larger side knobs that don’t affect rolling resistance when not leaned over.

Looking for 50mm or larger. Maybe 2.0-2.25".

Seems like there should be a growing market for this for any gravel racing that is more technical like that west of the Rockies. Virtually everyone at the event yesterday was under-tired, IMHO. And everyone who had >50mm tires had substantial middle knobs which would negatively impact rolling resistance.

Maybe something like the Maxxis Receptor, but with WAY larger side knobs?

Does that exist?

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I’m following along in case there are some more good tire recommendations, but I’m with you regarding bigger tires for gravel! I got a Salsa Cutthroat this year just for that reason and am currently running Schwalbe Race Kings 29x2.2. Still early in the year so this is the first set of tires I’ve tried, but the volume so nice to have.

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I put Maxxis Ikon 2.4 on front and 2.25 on the rear. They roll well and have enough knobs to handle chunky gravel to most the MTB trails I ride. Not a great choice if your course includes a lot of road. They’ll roll but other tires would be faster.

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How about the VITTORIA Terreno G2.0 XCR? No experience with it personally, but it looks like it might meet your criteria?

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Bingo! Thank you!!! :slight_smile:

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XC and gravel tires are the same thing basically. It comes down to the ruining the utility of the road bike end of things as well as weight, q-factor, aero, and frame clearance . Once you start making all these changes for a mass-market gravel bike, you end up designing a drop bar XC bike.

The widest point on a modern, fast XC tire is the sidewall, which puts that spot further rearward than the 50c gravel tires. The xc tire has about half the sidewall thickness as the gravel tire because they know you’ll be running the MTB tire alot lower psi. A 40c Terra Speed is .5mm narrower on the side than the lugs and fits small overall; the Conti XC tire is 2mm wider than the thread. The difference in how a super fast 54c Schwalbe (Thunderburt/Ralph ?) XC sits on the same wheel vs a G-One Bite 50c fit on my 2019 Trek Checkpoint isn’t 2mm of seat stay clearance either side - the 650b actually fit WORSE than the 700c 54c tire

Tire clearance vs chainrings / seat stays - You also severely limit the more road end of the gearing spectrum by making a bit more clearance for the tire— effectively to what you’d have the same gearing as a hardtail XC bike (for example Trek Procalibler takes 60c tires on a wider chainline vs. the Checkpoints 45c and 46t even with the dropped seat stays. ) . T
The MTB has 4mm more chainline clearance (+8mm tire right there) and +20mm more clearance at the crankarm. That gives you a lot more sidewall space.

In front, once you put a tire bigger than 40-45mm on there, you start worrying about toe/tire clearance (toe overlap). You need to move away from road geometry quite a bit to get that 47c plus tire on there in most frame sizes. (Trek added 20mm to the front of the bike in order to get 45c tires on the Checkpoint on the 2022 model). There are some fork adjustment solutions, but these don’t really sell fat tire gravel bikes, as the sales are on the road-bike/gravel end of things.

On the wind front, bigger tires have the same issue as aero wheels with crosswinds. a 54c on a 25mm deep rim has the same crosswind profile as a 25c tire on a 55mm deep carbon rim, only it’s not shaped as well.

The other thing, once you start needing tires that fat you are on terrain where you probably need a suspension fork to keep from going over the handlebars. With 50c tires, it’s pretty to get a scare even on a decent size root with road geometry. with the longer fronts, it should be much better.

Also BRR tests are not the same-
NOTE on BRRs test results… the test for the road tire <> gravel tire <> XC tire. For example the same tire rolls about 4w slower with the Cross test tube than the road tube looking at the GP5k results. Mini knobs add about 2w on the roller, but they probably don’t add any rolling resistance in the real world. Looking at BRR a Race King Protection tire probably rolls exact the same as the Conti Terra Speed; or I’d just make that assumption (the mtb test gives you a -2.5w lower result than the cross test). Having said that, the exact tire size matters because of the way casing overlaps inside of the tire as well as the load rating the MTB vs the gravel tire is rated for (most gravel tires are rated for 75kg-ish vs 110kg, which would cost about 2w on a test I saw). So tubes, knobs, casing rating, etc are going to throw off vs real world results.

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Interesting info! Appreciate the detail.

This is something I have heard as a concern from a lot of people, and a few of whom, I consider very smart and educated on the matter of bike manufacturing. I have yet to ask my burning question to any of them, so here goes:

Why does toe overlap matter on road race bikes or gravel race bikes? I can’t think of a scenario where toe overlap would cause an issue in either form of racing. Sure in MTB, it would be disastrous for anything technical. But in gravel and road racing, if there are no near-track-stand hairpins, it seems like maybe this doesn’t matter?? Am I overlooking something?

This is a great point, and I currently contend that we’re going to see a LOT of that in future gravel racing.

I think the industry currently underestimates the cost of vibration and instability on human power output, substantially.

If I had to design a gravel bike to win some of the gnarlier gravel races I’ve seen it’d be shaped like a gravel bike, but with the suspension system of my Scott Spark with its 3-position remote lockout system. (twinloc, IIRC).

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Tire overlap -
Bike manufacturers want to give one value for all the frame sizes. On the larger frame sizes, it’s not as much of a concern because there’s more reach to work with. If you’ve got a big foot, you’ve got your cleat toward the middle of the shoe, and you’re running a big tire, your shoe can hit the front tire. When you do, it’s like hitting the front brakes hard while the bars are turned, and you’re in a funny position. This usually happens at low speed (because thats when you turn the bars more), so you just fall over.
From my experience, Trek said my bike was good for 45c. I put 700x50c tire on there. I have a 58cm/172.5mm crank / reasonable size 45 shoes / middle cleat position. The tire would just lightly brush my shoe during low speed turns, I could power past that rubs, it was ridable, but clearly could cause an issue. I put 650c for the fat tires after that… Also, then stand over height wasn’t good with the fat tires.

Blockquote
If I had to design a gravel bike to win some of the gnarlier gravel races I’ve seen it’d be shaped like a gravel bike, but with the suspension system of my Scott Spark with its 3-position remote lockout system. (twinloc, IIRC).

For fun or for … yeah obviously.
For competitive racing, right now, everything labled gravel is won with a avg speed close to 20mph and is over 3hrs. Aero still matters. The aero penalty of a proper fork and xc tires over gravel tires is probably 5w fork & 10w tire at that speed. I don’t see the Cannondale or Nine MCR winning a race with extended drafting sections.
You’re probably better off with the short travel suspension over the fatter tire still. The fat tire suspension advantage isn’t as big as you think because of the way air works. Air doesn’t like to initially compress, compresses linearly, then very non-linearly because air molecules don’t like to efficiently stack themselves and the effect. Plus, and more importantly of the combined volume of air relative (a 54c tire has 2.85x the air in it as a 40c despite being only 35% wider.) to the pressure you need to run the tire at is not 2.85x different (42psi vs 28psi maybe). The casing tension on a 54c tire might be 2x the 40c tire (which is why a 28psi mtb tire feels just as hard as a 110psi 23c tires) The fatter tire mostly gives you a bridge over the bumps rather. So at this scale, the fatter tire only provides float and more tire-suspension travel over a smaller tire.
The suspension doesn’t play this game, so it more likely to help you out. I did some math because I couldn’t find the answer, you’re probably looking at a spring rate of about 8n/mm (rising rate though) on a full suspension XC MTB (guess, based on sag) with ~100mm + 54mm travel . A gravel tire is about 90n/mm with 40mm of travel. Though, you can figure mashing the rim the the guideline here, so some more math says 23n/mm gives the same rim mashing soft tail mtb spring rate as the gravel tire, and 67n/mm for the hardtail mtb (I’m guessing that a really low pressure). Seat posts and frame flex play a factor here too, but the best of them are about the same as the gravel tire 90n/mm… except the new Trek Checkpoint, which is around 40n/mm (for 15mm maybe) if Tour’s test is good (this seems inconsistent with prior tests as it’s 50% lower) so that’d be about the same a hardtail MTB on really really low psi tires (18psi ). Anyway… over the gravel tire, the mtb tire gives you more gap bridging/float, rim protection, but not much in the way of ride quality.

Note: if you take BRR’s ‘15% sag’ psi * volume one the other website I have, you get their PSI value, so 25c @87psi = 40c@ 40psi = 54c @ 18psi

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I’m not sure I understand. I can run a substantially lower psi in a MTB tire, such that impedance is reduced, compared to a gravel tire. Not a small amount. Are you saying that’s just in my head?

I would agree.

Okay agree here. But if there’s no single-track and it’s just a gravel race bike, then cool with loads of toe overlap right? Just a personal preference here, right? Seems like for a LOT of folks (gravel racers who don’t do MTB single track and who don’t do CX… as in, all the roadies moving to gravel) don’t have to worry much about this when considering tire choice.

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I can get onboard with those numbers.

And I still think the tradeoffs might put things in favor of the use of a shock for any race over 100mi or any climby gravel race that contains beat up roads. The difference between floating over potholes, rocks, roots, and random slickrock sections, or just large-chunk gravel sometimes close to 100W depending on just how beat up the road is. In rough areas where it’s a 13-20% grade it can be the difference between standing still and continuing forward motion.

Rides are popping up all over that have 100 feet of climbing per 1 mile, and in those races, aero is going to matter much less. Traction and impedance reduction are more important. You’re either climbing slow enough that it’s a small components of your losses, or you’re on a descent where you’re braking half the time anyway.

Further, I think the thing that has not yet been measured well is the actual energetic cost, or muscular fatigue created by, putting out the same power with varying levels of instability.

What I know for sure from the literature is: The difference in maximum power output ability between unstable conditions and stable conditions can be remarkable. I’d wager there are remarkably big differences in human energy cost when operating on a non-smooth bike environment, and that even if higher drag and/or higher rolling resistance on smoother sections does occur, they’ll be outweighed more than most folks think, by the reduction in instability-induced energy cost by using suspension and/or bigger/softer tires.

My perception is also biased to the gravel riding I’ve seen. I’ve only ridden gravel west of the Cascades/Rockies (WA & SoCal primarily) and east of the Appalachians in the foothills of VA. That is… not a smooth gravel road in sight. Maybe I’m just riding gravel that is really MTB territory.

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I stand corrected. It has been investigated and it’s massive.

https://www.journal.aesasport.com/index.php/aesa/article/view/140

I did not look into their methods.

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It might be worth a test. The FS spring rate difference is massive though on full bumps. Add that to the control you get with dampening.

The MTB tires are probably a lot better a moving around bumpy spots, while they’re going to be equal on ripples. The mtb tire is still putting more tire on the ground and has an advantage on soft surfaces. The gravel setup is going to be a lot faster on the road and groomed surfaces while handling better.

You can get the same gearing on both.

A better case for the mtb tire gravel bike might be something like a cross Michigan race where the trails are mostly smooth, but there are muddy spots and some rocks. The FS gravel bike would work better on a bumpy, even rocky road trail - think washboard dirt roads, high desert, or long distance adventures, maybe even Leadville that are mostly road.

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Yeah that’s where my brain is at, at the moment. :slight_smile:

Totally agree. It’s a fun experience to take a gravel bike up gnarly access roads.

I’m just brainstorming for my wife’s more competitive future racing schedule.

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After I asked this question months ago, I went and bought the Terra Speed and Terra Trail tires. They mounted right up on Enve 525s, without any sealant. They have been holding pressure, no sealant, in my basemnt fornover a month! I will ride them once I get my shocks back from annual factory service with the right conditions. I am wondering how they will climb, if I pump them up rock hard, and depend on the actual suspension to do what suspension does, absorb bumps and imperfections. If they still hold traction on gravel climbs while heavily inflated, might be super fast on the descents and flats. I guess it will be a flat bar full suspension gravel bike for me. Might look weird, but its what I got. If conditons warrant a higher traction tire, I also have options.

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I skimmed thru the study, and it is a study. Methodology looked promising, sort of standardized, 22 subjects. No data tables. 80w power demand, vibrating crank set. I was hoping for more. The company that makes the erg bike used in the study is Power Plate, which sells excercise equipment that vibrates to burn more energy…

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Way late but check out the Schwalbe Thunder Burt 2.1, pretty much exactly what you’re looking for an I can confirm, they rip!

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