chas58

A long wheelbase will let the bike drift/slide through gravel turns better. A short wheelbase will give snappy acceleration (I mostly notice it when I am out of the saddle) and a tight turning radius (not necessarily what you want on gravel, but great on road or CX).

Gravel specific designs tend to be long/low/slack. Those are 3 different things that add up to a more relaxed bike. (wheelbase, BB drop, head tube angle respectively).

Garfield Cat

If I looked at the 2018 Dirty Kanza's results both men and women, say top 10, what would I find as the bike of preference? It might be that these riders are at a pro level and get their bikes as sponsored. Even then, these riders would have preferences. Would Cannondale, Giant, Specialized, still be there with the less relaxed geometry?

Then there are the mere mortals who may not have the natural abilities and less skills on that same Kanza, maybe it would be a different geometry to accomodate their strengths and weaknesses.

chas58

Well, Specialized Cross bike - the Crux - is all over the top 10 last I read. That doesn't mean its the bike for mere mortals. It is super light, climbs great and very fast. I love a bike like that when I want a playful bike, but it wouldn't be my choice for a cross state race.
For sure - a long wheelbase is good for touring or endurance events. Not great for more aggressive riding where agility is important. I can't get a long wheelbase bike to do tight turns in aggressive urban riding, but those graceful turns are great for sweeping gravel turns.

Spoonrobot 

Tight cornering and turning is based much more around trail, handlebar width and individual rider skill than wheelbase, IME/O.

I think this is partially mis-attribution due to a lack of understanding how small changes in the first two variables have more outsize effects on the feel of a bike much more so than wheelbase. Especially during high speed and aggressive movement of the bike/rider. Something like ~7mm of trail and corresponding ~2-3mm of flop can have a very large effect on how a bike feels as it begins to lean over. Conversely, when riding blind I've found most riders have a hard time differentiating less than 20-40mm of wheelbase change, especially if it's balanced on both the front and rear end.

Looking at the bikes that finish DK is interesting but most people do not understand geometry well enough to have had it be a defining factor in what bike they choose to purchase/ride in comparison to all the other variables involved. It takes quite a bit of $$$/experience to really suss out the small changes and their effects.

I will also say that the terms relaxed and aggressive are poor descriptors and do not describe how a bike rides. It would be better to describe bikes along a spectrum as:

Use steering to turn or maintain a straight course, leaning and body english produce much less reaction and are much less necessary. Also has less stability at the front end as the speed increases.

or

Use leaning the bike at an angle, counter-steering (push handlebars inside of the turn) and body english to turn or maintain a straight course, steering with the handlebars produces much less reaction and is much less necessary. Also has more stability at the front end as the speed increases.

Low Trail 35mm<---------------------------------------------Mid Trail 55mm--------------------------------------------->High Trail 70mm
Steering, less stable at speed-------------------------Balanced steer/lean, stable at speed---------------Leaning/countersteer, very stable at speed

Note: larger tires, more aggressive tread and lower pressure all increase trail for a given geometry. Wider handlebars provide more leverage and increase feedback on low trail, too wide creates a nervous descending bike that is overly sensitive to small corrections - oversteer condition. Narrower handlebars do not have enough leverage and decrease feedback for high trail, the bike is hard to maneuver during descending and resists cornering - understeer condition.

Personally I find high trail bikes extremely hard to corner with handlebars less than 52cm wide. Even then they require a significant amount of leaning and benefit from very aggressive side knobs to prevent sliding at lower speeds than lower trail bikes. Conversely I find low trail to be much too nervous descending - compared to mid/high trail, smaller rocks and road features tend to wrench the front end around requiring significant focus to make corrections to track the preferred line. Mid trail is my preferred front end geometry as it has a good balance of steering/leaning and although it lacks the stability of high trail on rough descents at very high speed it is still acceptable for the additional ease of cornering.

sweetspot

I think that the very interesting case is with new giant revolt 2019. It can take 45 mm 700c tire but only 47mm 650b according to what Giant has told me on their FB profile. It is a proof that not necessary all of 40c bikes can acommodate 47mm with smaller wheels.

Garfield Cat

A rider like Peter Sagan, able to ride anything, anywhere

Peter Sagan hits the singletrack in Austria - Video | Cyclingnews.com

unterhausen

Just from trying to design a frame with decent clearance, this seems blindingly obvious to me. Width is the big problem, so it's quite possible that clearance for a 650b tire is exactly the same as for a 700c tire. Getting more radial clearance is no problem, just add a half inch to the chainstays. I know people are loathe to do that nowadays, but for a bike that is going to be used on the road, the difference in ride from such a small difference in chainstay length can be boiled down to confirmation bias.

Metieval

Because in keeping a shorter chainstay, Giant (2019 Revolt) unlike........... Open cycle , Norco with the Search XR, 3T exploro, Salsa Warbird V4 (maybe more, brain isn't awake yet) Ibis Hakka MX, have all dropped the drive side chain stay to allow for the 650x2.1 (wider) tire size.

on the Flip side...... there are a large number of bikes that can take a 40c and also fit the 650x47..... but the 650 isn't by design. So take a bike with a lower BB drop for the 40c tire, and then by running a 650x47 the BB is another 12-13 mm lower to the ground. If my checkbook was larger, I'd try some of them bikes that have a lower BB with 160 crank arms and a 650x47 swap. yet a 650x47 on a longer chainstay, higher trail, Lower BB... doesn't seem like a very good combination for my spirited style of riding. Especially when it comes to riding the fatter tires Moto style and getting everything out of them they have to offer.

The the bike that was 'Designed' around 650x47 has a 65mm BB drop and shorter chain stay 425. The Midnight Special.

none the less, if WTB doesn't have your 700x40 Bike listed as fitting their 650x47 tire, a Longer chainstay might be a clue as to if it will fit or not.

sweetspot

Another question: Why is that fairlight secan or bombtrack Hook ext can acommodate even 57 mm tire without dropped chainstay and bike you mentioned can't? Is it the "carbon" issue because both bike i mentioned are made from steel. It is not the case of the length of chainstay because warbird v4 has 430 mm chainstay (the same as fairlight secan) and yet to acommodate larger tire they used dropped chainstay.

Metieval

the Bombtrack is obvious, they didn't use tubing but rather a flat piece of steel (drive side).
Fairlight pinched their tubing, from what I can see.

in my opinion there will always be compromise. What did Fairlight and Bombtrack give up, for that tire clearance?

steel allows for smaller tubing which helps with clearance. but how far can they go with smaller, or pinched tubes, or flat steel before they give up stiffness. How much are you willing to compromise for larger tires?

carbon is light, steel is heavier. but hey big tires are heavy too. heavy + heavy = heavier.

at a certain point, I just say screw it. I go all in and start looking at 27/29 +

chas58

I think I mentioned this above, but you can flatten or crimp the chain stay, design only for 1x (not 2x or 3x), drop the chain stay and/or widen the BB spindle. Obviously fat bikes have a pretty wide spindle.