Road Geometry Versus Gravel Geo

[.RJ]

Originally Posted by osbk67

There are tradeoffs with every added millimetre of tyre width, both in aerodynamics and weight.

That makes a fundamental assumption that wider tires are less aero - but are they? One of the US gravel racers (Dylan Johnson) found that his 29x2.2 race kings were more aero than narrower tires. Could be unique to his setup, but maybe there's something to be learned there. And then there's zipp's new gravel wheels with a 30mm ID.

So who knows, maybe in 10 years the wheels, frames and other tech will shift what those tradeoffs are... PR at nearly 30mph shows there's already a hell of a lot gained with the new stuff.

[Jaybee]

Originally Posted by lorenbike

And yet they still race with 28-32mm tires at Paris Roubaix.

It would be cool to see if we ever see tires there at 35 or 38mm, besides the sprint track finish I don’t think you’d be giving anything up with wider tires.

Originally Posted by .RJ

I wonder if silca, zipp, jumbo or anyone has done testing to see if a 40+ mm tire would be faster, or if there's any tradeoffs to that.

How often is the final selection at PR determined by a puncture or equipment failure? I have no doubt* that 35+ is the right tire for that course, but pro road is pretty tradition-bound.

*Maybe I should have some doubt- I've never attacked out of a group going 28mph on cobbles and have no idea what that feels like.

[vespasianus]

Originally Posted by Dave

Look at stack and reach only. Adding TT length provides no additional information. My Cervelo Rouvida can be set up as gravel or road. It has flip chips to length the fork and chain stays, but the fit difference is insignificant. Cervelo made the STA slightly more slack, but not enough to justify a zero setback seat post. I have the same stack and reach as my previous road bikes.

An e-bike is a very different thing to me when comparing things. You can get away with things when you have a motor...

[osbk67]

Originally Posted by .RJ

That makes a fundamental assumption that wider tires are less aero - but are they? One of the US gravel racers (Dylan Johnson) found that his 29x2.2 race kings were more aero than narrower tires. Could be unique to his setup, but maybe there's something to be learned there. And then there's zipp's new gravel wheels with a 30mm ID.

So who knows, maybe in 10 years the wheels, frames and other tech will shift what those tradeoffs are... PR at nearly 30mph shows there's already a hell of a lot gained with the new stuff.

Fair point. Product development will continue to improve the aerodynamics of wider tyres through better rims, better tread design, and better tyre/rim integration, although I suspect Dylan Johnson’s testing was against his alternative i. e. 700x45/48 or whatever rather than against 700x30/32 on 55/60mm deep carbon rims…

My main point is that is essentially that Paris-Roubaix is a well-understood and extensively researched and tested event on a defined course. There has been little to stop teams from choosing 700x35s for it for several seasons now, and the reason they’re not, at least to date, isn’t because they haven’t tested for it, much less didn’t think of the idea.

[Dave]

Originally Posted by vespasianus

An e-bike is a very different thing to me when comparing things. You can get away with things when you have a motor...

Fit is fit. Ebike makes no difference. My bike fits the same as previous bikes.

[.RJ]

Originally Posted by osbk67

My main point is that is essentially that Paris-Roubaix is a well-understood and extensively researched and tested event on a defined course. There has been little to stop teams from choosing 700x35s for it for several seasons now, and the reason they’re not, at least to date, isn’t because they haven’t tested for it, much less didn’t think of the idea.

True - but it also did take several years for riders to come around to using wider carbon rims, too. Will be interesting for sure to see what happens over the next few years.

I wonder if Wout wished he'd had some 35's last year.

[Onno]

Originally Posted by benb

It's probably important to not conflate fit and geometry.

You can put your contact places in identical places on a MTB, Gravel Bike, and Road bike and yet they can have substantially different geometry. That might not be the perfect bike fit for all three but you can definitely do it and still have 3 different bikes.

All the other differences that don't affect your contact points still matter and make those bikes handle differently, handle different terrain differently, have different strengths and weaknesses, etc..

This seems to me the heart of the matter in thinking about how to select the geometry for a gravel bike, and is the hardest part for non-experts to understand--both in terms of what actual effects of gravel geometry might have on the ride, and what we might actually want those characteristics to be.

I've gone the route of using 650b wheels with what to me are fattish tires (38 on the front, 42 on the back--the limits of the frame/form) on my endurance road to ride some gravel roads. Those roads have been fairly smooth. I've also done one gravel race on another endurance bike (my Synapse) fitted with 34 mm cross tires. So my experience is very limited. The race on my Cannondale did feel VERY sketchy on some quite rocky gravel, and I wouldn't want to do that again. The fatter tires on my Pursuit with 650b wheels feels pretty good on most of the gravel roads I would prefer to ride. And those wider tires make the bike feel more stable than the regular 30mm tires I have on the 700 wheels. But I have to keep those wider tires at about 30psi or more, or I get oversteer (is that the same as wheel flop?), which I find quite disconcerting.

All of which is to say that like Angry Scientist, I feel quite at sea in thinking about what kind of gravel bike geometry I should be looking for. I feel pretty confident that I want a bike with similar fit, which because I'm long-legged, means higher stack and shorter reach than most standard endurance or gravel geometry.