Sure, but dimensionally there’s bugger all difference.
There are XC rims that are plenty strong for MTB and as light as a gravel rim. Just like there’s XC rims that wr not working in ally up to the task of rowdy trails on a normal sized human.
I realise that this thread is about gravel racing, but perhaps people following this thread may be interested in some analysis I did of my cyclocross results this past season, for which I occasionally used my drop bar MTB. There are no tyre width of bike rules for my local CX races, so anything goes.
For the six races I did, I looked at my finishing place in the field, as a percentile, and also my time behind the winner as a %. For two of the six races I chose to use my drop bar MTB, with 2.35” Thunder Burts. The other 4 races were too muddy, so I used my cyclocross bike with 33mm mud tyres (Challenge Limus or Baby Limus, both HTLR casing). Most of the field used CX bikes and 33mm tyres for all the races.
Ignoring for a moment my rather mediocre results in these races, it’s interesting to see my best results were on the bike with MTB tyres. There are obviously multiple factors affecting race results, so it’s far from conclusive, but I think it’s still a strong indication that the MTB tyres are faster for those drier races.
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So you’re faster on your drop-bar MTB relative to the rest of the field, for sure.
Are there other differences between the two bikes? Maybe geometry, bar width, other things that could affect the handling?
This is an interesting way to look at the data. I compiled the events done on my dropbar MTB and compared the relative placing to my gravel bike. These are the same events year after year, many years I would use the MTB for a race I did the prior year on the gravel bike or vice versa. I went into this with an experimental mindset. N for MTB Tires is 13, N for Gravel Tires is 24.
Being cognizant of my relative placing while using the dropbar MTB is what motivated me to change back at the end of 2020, but this is the first time I’ve compiled the data this way.
This thread has rekindled my desire to experiment and after looking through many of the events I did in the past I think positioning and gearing selection, as well as using above average gravel tires and below average MTB tires; were the main contributors to my overall lower relative performance.
I don’t have the budget for a gravel bike that will fit MTB tires - When this thread started I had an old MTB frameset that I had outgrown, sitting under my porch with plans for the scrapper. Since then I’ve pulled it out, rebuilt it as a dropbar bike and have spent the past two weeks experimenting with gearing and tires. Right now running 700cx42 Pathfinder Pro in the rear and 29x2.35 Mezcal in the front. Both average tires in their respective class.
I have ordered Thunder Burts (super ground) and once it dries out and gets closer to the season I’ll switch over and race a few events and see how it shakes out. Right now the bike seems very fast, I’m working on getting the gearing exactly the same range as my gravel bike, my estimation is that I don’t need the low-end gearing in the 20s but I do need the high end gearing around 115-120 gear inches. One of the major speed losses was getting dropped from the front group(s) in gravel races trying to spin 38x11 against 50/48/46x11 geared gravel racers. As well I was loafing up longer strenuous mountain climbs in 26x36 when I should have been at 1x1/@30 gear inches limited since I am fit enough to handle that gearing and possible higher on the climbs in our events. 42/28 right now but I expect to be able to use 46/32 once I swap cranksets, 11-32 cassette.
Not sure if I’m going to do anything position wise, the bike handles very well and while it seems possible to use a -25°/-35° stem to get 11mm/22mm lower I don’t think that’s going to make much difference for aero considerations. This bike has 0cm saddle to bar drop, my regular gravel race bikes have 6-7cm bar drop. And not expecting to run fenders, for dry events.
Thanks to everyone who posted in this thread, it’s been both motivating and extremely informative.
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How wide those bois?
2.1
So the tire clearance for crux is 700c @ 47 mm or 650b @ 2.1″.
What you all choosing? Two wheel sets?
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Yeah, the geometry of the drop bar MTB is longer (the wheelbase), taller and slacker. I was worried that would make it less agile through tight corners, but surprisingly it’s been fine, seemingly no slower than my cyclocross bike. Perceptions can be misleading, so I use the speed of my regular competitors relative to my speed as my gauge for whether I’m slower or faster through corners. Bar width is similar for both bikes at 42cm.
The most noticeable difference is the BB and saddle height, which are both almost an inch (22mm) higher for the MTB. It needs more of a jump to remount the MTB after any running sections. In comparison, the CX bike feels like I barely need to jump, casually throwing my leg over the saddle to remount.
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Very nice. That’s a good sample size, which helps statistically, as long as your fitness and level of your competition has stayed fairly consistent over that period.
I think these analyses of race results are interesting, because that’s ultimately what we care about. I still think though, the best way to make decisions is to do a controlled tyre test to determine the fastest setup. Virtual elevation tests are my preference, but admittedly it takes a lot of time and not everyone has the inclination to do that. It would be interesting to see any results of any more experimentation you do, if you’re happy to share them.
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I’m doing Rock Cobbler, 80+ Miler in Bakersfield, CA on Feb 10th, on my Scalpel HT this year. So I’m here for this thread.
So far for that race it’s looking like Nano 2.1 (52mm) or Maxxis Ramblers 50mm.
Both roll very well on tarmac, Nano’s have a leg up on dirt, but Ramblers in 60TPI have Silk Shield, Nano 2.1 only comes in a 60Tpi, but no reinforcement at all.
Got an email from the organizer today with this nugget…which as me leaning toward, Ramblers…
IRL is a slow tire, they also promote these small races, so is advertisement.
While the IRC Double Cross is a plug for a Race Sponsor (Not so sure how “small” The Rock Cobbler is, nor that other race they sponsor, Unbound, really are.)
Double Cross is “okay” for gravel, but no go on an MTB as 42 is as wide as you’re gonna get. I’ll run 1.9"/50mm on an MTB, I raced a 2.1 front/1.9 rear for YEARS, so while 1.9 isn’t odd for me, 1.65 is right out.
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Now the question is? Gravel vs MTB inserts? The difference is about 100g extra total
I wrestled with this question for a while and ended up on the gravel inserts. From how it reads, the MTB is meant to allow you to run ultra low pressures, and makes regular contact w/ the ground (through the tire, of course), whereas the gravel is more a safety item for when you really hit something hard.
I ran gravel inserts most of the time, I didn’t really test its runflat capability though.
Another new wrinkle is the Vittoria Air-Liner Light, which was launched sometime this year and is only 8g more than their gravel insert. From how it reads, it’s supposed to function more like the gravel insert (i.e. not run with ultra-low pressure and only there as an emergency), but let me know if you find out different.
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Vittoria Airliner Light (MTB) are only 55g each. What I’m using with good luck so far combined with race kings.
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Just chiming in on the super race vs super ground discussion. I had a chance to run the Kenda SCT casing (feels like super ground) vs. the TR casing (feels like a road bike sidewall…super thin) and didn’t find a difference. I really expected the TR to be measurably faster but it’s a little more info that makes me think SCT or super ground is probably the way to go vs super race or TR.
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I think I was at Unbound last year and the folks at Vittoria said they were going to make an Air-Light liner for Gravel, but that hasn’t seen light of day…
Earlier this week, I did a re-test of the Race King versus Thunder Burt test that I did last year (see results in post #74 of this thread). As the original question from @oldandfast was about the Race King, I thought my latest test results might be of interest here.
I used the roller technique again, which is a kind of poor man’s version of drum testing that organisations like BRR do. I’ve made a couple of improvements to my testing though. Firstly, I used a Power2Max power meter, which measures total power now, whereas previously my Stages single sided PM was subject to variability coming from potential L/R leg imbalances. Secondly, I now have a spare set of wheels, so ABAB type testing was easier to do, with less hassle and delays between changes.
Results are shown in the plot below. A few notes:
- For reference, my previous tests results are shown in grey/lighter colours (numbered 1-3 in the legend). Note that those were performed with a butyl inner tube though.
- The new tests were done tubeless. The BRR values (open square symbols) are also updated to showed their latest tubeless rolling resistance coefficient results.
- The new Race King data is shown with the solid blue symbols and solid blue trend line. The Thunder Burt data is shown with the green symbols and trendline.
- I did an ABABA test (A=T.Burt, B=R.King). Repeatability seems to be good, with the exception of one rogue data point for the T.Burt.
- Annoyingly, I mounted the Race King in the wrong rotation direction for the first two runs (#5 & #7) before realising my error. I repeated the test the day after, which gave similar results, not only for the tyre mounted in the wrong direction (#9) but also, surprisingly, once I had remounted it in the correct rotation direction (run #10 - darker blue symbols).
- Note that both tyres are old (2020 for the RK, 2022 for the TB), but had little wear. Probably 100-200 miles of usage for both. There is an uncertainty coming from aging though.
Overall though, this test once again shows that the Thunder Burt I tested is a faster tyre than my Race King. The gap is bigger than seen in the BRR results, although the BRR tests used the Super Ground version of the TB (not the Super Race), and with a 2.25” width.
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