My 2 cents...

You'll pay a larger penalty if you run 35s. With 35s you'll have to choose between fast (low) tire pressure + pinch flats, or slow (high) tire pressure and no pinch flats. I haven't run liners before, but i guess there's a chance you could get away with fast pressures with liners installed.

But i would run something in the 40-43 range with good sidewall protection, no liners, and fairly low pressure.

Neither are aero unless you have insanely fat and deep rims, so go with the fatter tire.

Rene Herse just put out a long post on this very topic. Much of this is over my head simply because I don't really care that much. I'm not going to be winning any races and I prefer to optimize for comfort. I run Terravail Cannonball's in 650x47 @ 28 and 31 psi front/back. Normally I use the supple casing version but will be throwing on the durable casing for Unbound.

Thanks for everyone's input. The article was really helpful. I'll be going with the fatty tires.

I have no idea how rough the gravel is, but I would lean toward wider if the surface is not very smooth. It is a long day so the comfort and grip advantage are probably worthwhile.

I think clothing choice is likely to yield bigger aero savings as would some clip on bars. The other thing to consider is rolling resistance. I put the Challenge HTLR tires on my gravel bike and Tom A’s testing put the Crr savings at around 10+ watts at typical gravel speeds.

Ummm...yeah, he calls that article "Tire Test Results"...but, I think I'd call those more like "Test Non-Results". A closer look at the methodology reveals they aren't able to reliably distinguish between the Crr of the tires being tested. The number of repeats, even with low variation, doesn't change that fact. Even in the low speed tests, >50% of what they're measuring is the effects of aerodynamic drag. This has been true for all of their tire field "tests", I'm afraid to say...I know what they did took a lot of work.

To be clear, I'm not saying they don't make nice tires. They do. I've used them AND recommended them to others at times. It's just that their "testing" often leaves much to be desired, and incorrect conclusions often drawn from them (such as, a non-statistically signficiant difference means there's "no difference"...that really just means you can't reliably SAY there's a difference. There may actually be a difference, but the data taken doesn't support that...and that could be because your test isn't sensitive enough to the thing you're trying to measure)

A physical measurement analogy of the same relative magnitudes for what they're trying to accomplish is (this is the math for their 30pkh tests, where aero drag accounts for ~75% of the time measurement they took):

--It's like trying to measure the thickness differences between a .23" shim and a .25" thick shim, by placing each separately against a .75" thick block and then measuring across both the block and shim with a ruler that has 1/32" (.031") gradations...by eye. Many repetitions isn't going to get a more accurate measure of either shim.

Anyway, for the OP...I agree with the suggestions to go with the wider tires for that purpose :-)

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Jan Heine does have some good ideas... but they usually have nothing to do with controlled testing ;)

Besides the influence of wind (even wee little breaths of it) and position changes, coasting down a smooth ramp is missing the affect of suspension losses, and tire distortions due to torque or the subtle twisting that occurs when you are pedaling. Roller testing is actually closer to "real world"!

On the subject of this thread, I remember many years ago Schwalbe doing a test of MTB tires and optimal pressure (for resistance) for different surfaces. They found that on dirt or grass you can't go too low... though I think they only tried down to 20 psi. The more surprising result was that this was true for gravel as well. I'd favor going big with low psi...

Oh, and aero bars... heck even aero helmet, skinsuit, and whatever is aero that will work. Disc cover? That would be cool...

Love that front chainring !

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The wider tire no doubt is less aero.

However, DK is such gnarly gravel the largest tire you can run will almost certainly be faster because its less risk of flatting as stated above.

A 42 mm tire in the high 20s for pressure is great for rolling and flat reduction on a course with infinite opportunities for flat tires otherwise. Maybe mid 30s for pressure for the bigger riders :)

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Yeah, there's some studies from the Swiss MTB National Team (IIRC) that basically boil down to "On non-paved surfaces, the lower the pressure you can run, the faster"...which means in my mind, run the largest tire you can, at the lowest pressure. That said, if taken to the limit (as in "fat bikes") you can start having the other performance factors coming into play (i.e. mass, aerodynamics, etc.)...but for the OP's question...yeah, if the wider tires fit, go with those :-)

N=1 anecdote: The last year I did the Belgian Waffle Ride (2017) I showed up with my newly assembled Fuji Jari shod with Compass Snoqualmie Pass tires. They're listed as a 44C tire, but measured 42mm on Zipp 30 Course wheels. People looked at me like I was crazy running tires that wide, due to the course having quite a bit of pavement. That was my best finish relative to the rest of the field in 3 completions of BWR, despite bonking for the last 20 miles. The wide "slicks" (and lower pressures) didn't slow me down on the pavement relative to the other riders, but DID help a ton in the dirt sections, and ESPECIALLY in the sand.

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DarkSpeedWorks wrote:

Love that front chainring !

When it's your 1990 Down Hill World Championship Race bike (and your XC bike)...
.... you set it up to go fast!

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Yep, and regarding the other issues that happen when you go real fat, those foam inserts used with smaller tires and low pressures can mitigate some of that.

I wonder if any studies that suggest low psi is best are including a full MTB course... with mostly climbing (timewise) and a lot of technical riding? I can see low psi being best for strictly rolling resistance on rough or soft surfaces, but when you throw in climbing and handling it can get murkier. Low psi is certainly good for traction on a climb, but I'm thinking that the torque distortion could be an issue if traction is adequate. It would be cool to do some controlled tests of this.

This is giving me flashbacks of getting absolutely crushed in the offroad sections the one time I did BWR especially in the latter half of the race when I was tired.
I'm pondering whether to use my Panaracer GK SS this year. My road wheelset is 28C conti 5000's.

As I recall, the test loops were on a XC course...I'll have to see if I can find the link again.

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If pack riding is an option, I think pick the one that is most likely to keep you from getting dropped...

Interesting since the "plus size" fad has waned a bit... at the same time that "almost plus" (2.4-2.6") has gotten more popular.

I'll be going fat and low psi anyway... putting together a Kona Unit frame very soon that I hope will fit 29x3.0 Bontrager XR2s on i40 rims.

Interesting show regarding tires:

I recall that year I described above (2017) hitting the start of the "Sandy Bandy" section with a fairly big group just on my tail...a couple miles later, nobody to be seen as I ducked under the closed gate at the end of that section and got back onto pavement. Never saw those folks again :-)

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I did 9 years of bike racing and can handle myself well in a pack. Wondering if I split the difference and get some 32's.

It isn't a matter of "handling yourself in a pack". Rather where are you most likely to lose contact with the pack?

I'm not familiar with the BWR, but it's usually not hard to hang with a pack on flat or rolling pavement... so you don't need to optimize for that. If dirt/gravel sections are the most critical part, you'll be better off with big tires and low psi. If there are serious climbs on pavement, that might favor smaller tires.

And... if you end up solo, aero becomes pretty important.

HA!
A bit of everything.
I got dropped hard on narrow tires in the dirt sections.
There are some serious paved climbs.. not super long though.
I spent probably 30% of the race solo... mostly in the last 40 miles.

Yep, if you are getting dropped on the dirt, then that's where you need to optimize. Fat fast tires, low psi... just durable enough. Good fat tires will be only a small penalty on road climbs.

The longer "serious climbs" are on dirt...AKA "Black Canyon"...though ;-)

edit: Have you guys seen this yet? A good resource for BWR courses: https://www.gravelstoke.com/...fle-route-since-2012

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