Russ Roth

Bit of a late reply since the thread was bumped. This fact is nothing new. My first two seasons of CX in 98 and 99 was on my 93 Giant Road Bike. Did have one particularly muddy course where I have to clean out the Ultegra calipers part way through but the bike survived the New England/NY cross seasons and is still my back up if needed.

mattscq

So old thread but I guess it still pops up if I Google the topic but the update:

I haven't really done much gravel but I did get 32mm Gravelking knobbies which fit with a bit of room to spare. 32 is fine but I'd really want something fatter when trying to do something looser. I think if I really stretched it I could maybe do 35 slicks but this is all on 700c wheels.

Typically, what do you think I could gain in terms of tire width if I went down to 650b? Maybe 38s or even 40?

chas58

On my bike, I gain nothing, as my chainstays are parallel.
It is tricky if they do widen, because then they are not parallel, and on a smaller wheel/fatter tire, the wide point of the tire is in a lot different position. Its really hard to calculate or even guess.

Personally in your shoes, I have run 32 rear, used a good cush seat post (a small thud buster is amazing, but may be over kill), and put a fatter tire in the front - as there is typically more clearance on the front fork. In an extreme case, I've just spent $100 to get a front fork that would take a bigger tire. Like putting a shock on a mountain bike, the front is where you need it most (and will help with flotation in the soft stuff).

Keep in mind, the ETRTO requirement is 6mm on each side. You can go smaller, but if you do a broken spoke may put you out of business, and any mud on the tires is likely going to wear your frame down, so be wary. I went down to 2-3mm once, but on sprints my tire would rub against my frame - and rubber is stronger than aluminum (whoda thunk it?)

msu2001la

Discussed earlier in this thread, but I'd be surprised if you gained any tire clearance by going to 650b. It may actually be less clearance than what you'd get with 700c. Also, even if you could fit a 38mm tire on 650b, the diameter of this would be significantly smaller than 700c with a 35mm tire, resulting in some gearing and handling challenges. Typical 650b conversion involves going with a much higher volume tire on the 650b wheel size (like 48mm+), which keeps the overall diameter similar to what you'd get with a 700c x 38-40mm.

For reference, here is a shot of a Canyon Ultimate chainstays with 32mm tire installed. You can see how the stays remain parallel with each other for several inches behind the BB, so there's really no additional clearance to be gained from running a smaller 650b wheel size:

mattscq

Helpful photo!

Yeah, it's not so much a comfort thing than a confidence thing. The Ultimate geometry is already a little twitchy (responsive handling!) with its short wheelbase and low front end and I haven't the CX skills to truly feel comfortable handling it on anything other than pact dirt/gravel with anything <32mm.

msu2001la

Old thread bump - but I saw this news today that some pros are testing road bikes for the upcoming UCI Gravel World Championship race.
Mathieu van der Poel is racing a road bike at Gravel Worlds - CyclingTips



It's a Canyon Ultimate CFR with a Shimano Dura-Ace Di2 R9200 groupset and Dura-Ace SPD-SL road pedals. Dura-Ace C36 carbon tubeless wheels shod with what appear to be Vittoria Terreno Dry tyres in an assumed 35 mm. Article suggests it's possible MVDP actually runs a narrower tyre and more-aero wheel setup for race day.

Specialized has also apparently stated that it's riders will be running 32 mm-wide semi-slick (Pathfinder) tyres on aero wheels, possibly also on road bikes.

This might be an indictment on the difficulty of the course (or lack thereof), but it seems interesting to me that the fastest racers in the world seeking the fastest gravel setups possible are concluding that race-focused road bikes are the answer. Now that race road bikes fit 30-35mm tires, it begs the question - what is a gravel race bike anyways? These guys don't need super low gearing, or mounts for a bunch of gear, or more relaxed/stable geometry.

Also worth nothing (from the CT comments): MVDP has historically setup his CX and road bikes to have matching fit dimensions. This might be tough to achieve on Canyon's Grizl gravel bike, which has much more laid back geometry.

LittleMS2

Anyone tried this with the newer scultra? I’m debating between the scultra and the reacto and it’s down to the tire clearance right now.

vinuneuro

This is an example of what can be done with an Ultimate on pretty technical gravel. It's also a well done video in general. I don't have skills to even dream of riding a course like this on a road race bike.

Eric F 

Tyler (The Vegan Cyclist) went with a Grizl for this year's edition of that event.

From what he said last year, and from what I've heard from my friends who have also done it, at some point on the course, you will be on the completely wrong bike for the conditions, regardless of what kind of bike you decide to ride.