Road Geometry Versus Gravel Geo

[fourflys]

Originally Posted by Likes2ridefar

I recently went to a fast 45mm tire from a fast 38mm tire and while my average speed is mostly the same the bike handling changed for the worse feeling slower and less responsive and can feel the increased weight in the front wheel.

this is an interesting observation.. my only true gravel bike has been a 2022 Giant Revolt rolling on Maxxis 40mm tires.. it never felt particularly quick or spry, despite the Revolt being known as more of a roadish gravel bike to my knowledge.. previous to this, my non-MTB dirt road travels had been on true cross bikes that always felt quick and nimble.. of course, they had narrower tires, higher bb, steeper HT, probably shorter chainstays.. I typically had a blast on those bikes on the local flowy trails in San Diego.. think I was running 35mm Conti Speed tires? Hmm.. maybe I just need to get another cross bike..

[Buzz Killington]

I just had a custom gravel bike made for me this year. I've had the same custom road bike for the last 15 (?!) years I've been happy with but went to a local experienced shop for a fitting before I finalized the gravel bike geo. I felt that the shop was doing more of a "road fit" than a "gravel fit." I guess like everything, it depends on where/how you are going to ride your bike. My best advice is ride/demo as many bikes as you can and see what you like/don't like.
My gravel bike is kind of a pig on road, but feels great off road.

[vespasianus]

I also think the type of riding a person does, impacts the geometry. If you are riding really technical trails, with lots of steep downhills, a very slack geometry and long wheelbase would be preferred. If you are riding mainly flats trails, with road to connect sections, road bike geometry with bigger tires can work great.

There is no one geometry that works for all gravel.

[benb]

It probably depends on what else you ride as well. If I ride my fat bike 1 day a week it's probably impossible for any type of gravel bike to feel like a pig for me. It's probably going to be true if you have any mountain bike you regularly ride, maybe with the exception of a high end race focused XC bike.

[AngryScientist]

Interesting conversation.

It's all lost on me.

Way back in the stone ages, when "gravel bikes" were not a thing, people were riding the dirt roads of new england and elsewhere on cobbled together touring bikes, mountain bikes and cyclocross bikes.

When I got my first real gravel bike, I went to a custom builder and ordered, basically a road bike with clearance for wide tires. That was one of the real reasons to go custom back then, to get a gravel bike with road manners.

I don't see why I would want anything other than a copy of my road fit for gravel, allowing wider tires, but I'm far from enlightened.

[Alistair]

Originally Posted by AngryScientist

I don't see why I would want anything other than a copy of my road fit for gravel, allowing wider tires, but I'm far from enlightened.

If your gravel is mostly maintained dirt roads, you're absolutely correct. A bike with "endurance road" geometry plus clearance for 40-50mm tires is great.

If your gravel is unmaintained, chunky forest access roads, then something a little more monstercross-y might be warranted. This the longer TT/reach, slacker HTA and longer WB seen in some of the newer bikes.

Which isn't to say you can't rife those chunky roads on the more road-focused bike, it just will be a bit sketchy.

And then you get into oddities like Leadville - it used to be a mountain bike race. But, if it was created today, it would probably be a gravel race (maybe). And the top guys are riding XC mountain bikes converted back into gravel bikes (drop bars, big chainrings, etc).

[Jaybee]

I think the other thing that gets lost in the "Pro Gravelers are all on 2.2" Tires" discussion is that the lead pack at say, Big Sugar or Steamboat are doing 24-25mph in a group, it's dusty, they can't really see where they are going and nailing a rock wrong and ruining a tire or going down means no podium. Bigger tires mitigate that somewhat. There is also very little pavement on most courses in the US gravel scene, so you won't pay too badly for your bigger knobbier tires. I would evaluate whether the tradeoffs the pros are making for their specific use case are right for your own non-race riding.

[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..

[.RJ]

Originally Posted by benb

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.

Also take into a account that wider bars are going to require a shorter reach, too.

[Tandem Rider]

I have a road bike, a winter training bike, and a modern gravel bike. The contact points are all the same. 28s on the road bike, 35s on my fendered winter bike, and 45s on my gravel bike. The fatter tires are definitely more sluggish and seem slower, but I'm not sure about rolling resistance. The times for my morning rides seem to follow the line of sluggishness. I'm not sure if it's due to cold weather or the tires, maybe a topic for another discussion.

[lorenbike]

Originally Posted by Alistair

If your gravel is mostly maintained dirt roads, you're absolutely correct. A bike with "endurance road" geometry plus clearance for 40-50mm tires is great.

If your gravel is unmaintained, chunky forest access roads, then something a little more monstercross-y might be warranted. This the longer TT/reach, slacker HTA and longer WB seen in some of the newer bikes.

Which isn't to say you can't rife those chunky roads on the more road-focused bike, it just will be a bit sketchy.

And then you get into oddities like Leadville - it used to be a mountain bike race. But, if it was created today, it would probably be a gravel race (maybe). And the top guys are riding XC mountain bikes converted back into gravel bikes (drop bars, big chainrings, etc).

That’s what hardtail or rigid 29ers with flat bars are for.

[lorenbike]

Originally Posted by Jaybee

I think the other thing that gets lost in the "Pro Gravelers are all on 2.2" Tires" discussion is that the lead pack at say, Big Sugar or Steamboat are doing 24-25mph in a group, it's dusty, they can't really see where they are going and nailing a rock wrong and ruining a tire or going down means no podium. Bigger tires mitigate that somewhat. There is also very little pavement on most courses in the US gravel scene, so you won't pay too badly for your bigger knobbier tires. I would evaluate whether the tradeoffs the pros are making for their specific use case are right for your own non-race riding.

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.

[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.

[.RJ]

Originally Posted by lorenbike

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

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.

[osbk67]

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.

There are tradeoffs with every added millimetre of tyre width, both in aerodynamics and weight. With the record average speed almost 48kph there’s a reason van der Poel starts on 28mm tyres and changes to 30mm for the cobbled secteurs.

With the advent of modern road disc frames I doubt there’s much to stop many teams from testing at least 34-35mm tyres, other than the availability of specific tyres from their sponsors. However, even with 30 and 32mm tyres available the adoption of the wider option seems about 50/50 across all men’s World Tour teams at P-R.

Repeated accelerations and sustained attacks to and beyond 55-60kph remain decisive in road races, especially at World Tour level. The effect of tyre aerodynamics and wheel system weight in these situations isn’t picked up by the slower steady state and roll down tests sometimes used to peddle ever wider tyres, nor by the rest of us bumbling along at 30-40kph and below.

Every competitive team is looking for any edge, and employ multiple people to find and test them. In this context slightly wider tyres are very low-hanging fruit in plain sight that none are choosing to pick, at least for Paris-Roubaix.