Road Geometry Versus Gravel Geo

[fignon's barber]

Over the last few years, the geometry of gravel bikes has drifted away from that of a road bike. Longer top tubes, super short stems. It's getting more difficult to match your road fit to a gravel bike. For example, my road fit is a top tube of 555mm, with reach between 385 or 390 with a 110 or 120 stem based on reach. That puts me squarely in a size medium for most brands.
I was just looking at the geometry chart of a popular gravel bike, and a medium size came with a 411mm reach and a short 70mm stem.
Is there a formula for matching road/gravel? Can you simply add reach and stem length to compare the two, or am I missing something?

[thermalattorney]

Originally Posted by fignon's barber

Can you simply add reach and stem length to compare the two, or am I missing something?

If you're just looking to quickly compare that math with get you 90% of the way there. Reach doesn't take into account STA, so maybe do that math with effective TT + stem.

If you're serious about buying a new frame, I'd recommend using a geometry comparison tool that lets you input spacers and stem length/angle to see a more precise A/B. The slacker HTA of most gravel frames means the spacers reduce reach more than on a road frame.

[Mark McM]

I don't think there can be a simple formula to compare fit/geometry between road and gravel bikes. Gravel bikes span a wide range, from "Adventure" bikes that are closer to MTB fit and geometry, and gravel racing bikes which are closer to road fit and geometry. So it's really going to depend a lot on the type of bike and usage that is intended.

[bigbill]

My gravel bike is a Coconino Disc Dirt Road Racer. Steve Garro, the builder, based the fit on my Serotta Fierte Ti which was my main ride at the time. 58cm TT and 120mm stem. After about six months, I replaced the stem with a 110 to compensate for the wider bars. I wanted a double crankset so the rear triangle will only clear a 40mm tire. I look at the new gravel bikes, almost all are 1X with room for 45mm or larger tires. The line between gravel and MTB is blurred.

In the mid to late 80s, MTBs that weren't klunkers were glorified touring bikes with 26" wheels. We were using road freewheels and making our own microdrives from touring cranksets by replacing the big ring with a bash guard. When Suntour came out with their cranksets that were called Microdrive I got one. The introduction of suspension forks changed the geometry to adjust for sag and to slacken the head tube for handling off road.

Everything started with a road bike, even when the roads were mostly dirt, and evolved from there.

[.RJ]

Originally Posted by fignon's barber

or am I missing something?

Yeah, there's a whole range of gravel bikes - from what are basically fat tired road bikes (Diverge, Aspero) to much more adventure bikes (Stigmata) built for chunky fire roads, bikepacking and singletrack - with everything in between.

So they're not all long top tube/short stem/dropper post bikes.

Gotta figure out whats the best fit and hone in from there, but in general expect that you're not going to have as much stem length or handlebar drop.

[benb]

I've found the following two pages quite helpful with this kind of comparison:

This one has a DB of bike geometries, but you don't put your fit in:

https://bikeinsights.com/

This one you have to enter the geo but it allows you to put in two bikes, including all the fit elements, and then compare the results:

https://www.bikegeocalc.com/

[benb]

I really think you want to be looking at bikes that can take a 50c tire at least.

That is where things seem to be going, if not already beyond that. I was just watching a GCN video comparing tire sizes and they were mentioning they didn't have any bikes that took a 55c, yet they knew the top pros are using bigger than 50c.

They keep seeming to decide there is almost no rolling resistance penalty to the bigger tires and they have so many advantages.

[.RJ]

Originally Posted by benb

They keep seeming to decide there is almost no rolling resistance penalty to the bigger tires and they have so many advantages.

Have they done any testing to see what, if any penalty there is for road/smooth gravel use though? I would think 50-55mm is overkill for that, but who knows? My bike with 55mm tires, even the RH 55mm slicks, does feel a touch slower than my Crux on 38mm tires.

[Turkle]

I specifically chose the Cervelo Aspero because its geometry was comprehensible to me.

It turns out that the top tube is basically 1cm longer for any given size than a road bike, so I just worked out the fit with 1cm shorter stem and it fits and steers great. Really happy with it.

[benb]

Originally Posted by .RJ

Have they done any testing to see what, if any penalty there is for road/smooth gravel use though? I would think 50-55mm is overkill for that, but who knows? My bike with 55mm tires, even the RH 55mm slicks, does feel a touch slower than my Crux on 38mm tires.

I think that comes more down to amount of tread. Also the bigger tires can be worse on aero and are definitely heavier. I think the claims they don't lose anything are just rolling resistance.

It is such an interesting balance. Some places you're probably going to find the range of surfaces fits in a defined range and other places you're going to find stuff all over the place or polarized...

Here you are basically most likely to find pavement mixed with stuff that borders on MTB terrain.

You can always use the smaller tires.. as long as the bike isn't overly optimized for the bigger tires, which might be the case.

[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.

[.RJ]

Originally Posted by benb

I think that comes more down to amount of tread

The rolling resistance has more to do with the casing, than the tread. Its why we see the very light mtb tires beat out a lot of gravel tires in rolling resistance.

The weight and aero penalties are real, but then again unbound being raced at 20+ mph over 200 miles with all the top pros on ~50-55mm tires, maybe that penalty isnt as big as we think.

[Alistair]

Originally Posted by .RJ

Have they done any testing to see what, if any penalty there is for road/smooth gravel use though? I would think 50-55mm is overkill for that, but who knows? My bike with 55mm tires, even the RH 55mm slicks, does feel a touch slower than my Crux on 38mm tires.

Yes. Dylan and a few others have done a decent amount of testing.

The tread patterns has less impact on the rolling resistance than the rubber compound and carcass construction. And because many 2"+ tires use a lighter, more supple casing, they end up rolling better.

The flip side is aerodynamics. Dylan's Unbound bike was more aero with the widest tires, but that's likely down to his unique combo of wheel/rim, fork, and actual tire. But, it's quite possible most people will be more aero on a more typical 40mm tire than a 2"+. In Dylan's case, it all came down to how the air off the tire interacted with the fork legs.

[Alistair]

Originally Posted by fignon's barber

Can you simply add reach and stem length to compare the two, or am I missing something?

Pretty much. Check the bar shape as well, as the reach and drop can differ quite a bit between a wide flared gravel bar vs a more typical road bar.

[robt57]

Don't overlook the longer WB and Front center. The position on the bike is likely more rearward regarding fore/aft CG rolling. And why you need more trail you see on these machines.

That said, my main 2 bikes for 6-7 years are Race shop Domanes (long low). Pretty gravel geom VS RR geom. sans lower front end. And notibly low trail in the 51mm range, plus for my druthers.

If you are like me and dislike the chopper feel that huge trail gives, watch for that mainly in your choices IMO. I see huge trail spec on most 'gravel' geoms. Or did, not as up on new new offerings.