Grave Bike Geometry - why the big differences?

[Clancy]

The recent thread by LegendRider on Gravel Bike Decision got me to thinking about gravel bike geometry and how it ranges all over the place. Below are the trail figures for some of the bikes listed. All are based on frames in the 54 size or close to it. From the Thesis to the Rodeo Lab there’s 20mm difference which I find pretty staggering.

Salsa Warbird 71
Thesis OB1 60
Giant Revolt 74
Santa Cruz Stigmata 70
Rodeo Lab Flaanimal 80

Based on the trail, it seems these bikes are going to handle very differently. Where it seems for road geometry the consensus is a trail in the mid-high 50’s is considered the sweet spot, with gravel it appears to fall all over the place.

I also never see this mentioned in bike reviews. Never a statement on how the bike steers in relationships the geometry but usually just broad statements on bike handling and always positive. I’ve ridden gravel bikes with trail in the 70’s and I can state that the steering is certainly different than one with the trail in the lower 60’s.

Yet it seems as this isn’t really mentioned.

The amount of trail has been pretty much standardized with road bikes as well as cross and touring bikes yet is all over the place with gravel bikes.

So why the big differences and why isn’t it more emphasized?

And what would be the ideal range of trail for gravel bikes?

Or am I missing something else in this picture?

[OtayBW]

Originally Posted by Clancy

The amount of trail has been pretty much standardized with road bikes as well as cross and touring bikes yet is all over the place with gravel bikes.

So why the big differences and why isn’t it more emphasized?

And what would be the ideal range of trail for gravel bikes?

Or am I missing something else in this picture?

Three of my road bikes have trails in the mid-upper 50's but an older DeRosa that I have has trail closer to mid 60's. I LOVE the ride of the high trail bike, but that is at speed. I'm not a gravel kinda guy (yet?), but I'd guess that the last thing you'd want is something kind of 'less certain' or 'noodly' on a gravel bike at lower speed on variable terrain. I dunno....

[charliedid]

The trail is not the bike, you need to consider the complete geometry.

All of them designed around same size wheels and tires? BB drop? TT length? How many sizes does the frame come in? Adventure or racing bike? (will it be loaded with gear etc.)

Don't get hung up on trail.

Not sure if this helps

[Jaybee]

I also think that there is large variability in "gravel" terrain and so different manufacturers have different ideas about what should perform best for a set of conditions they have as a design starting point. Compare this to paved roads and there is almost no variability in surface conditions. For example, the Rodeo Labs guys use their bikes as essentially drop bar XC mountain bikes, so the Flanimal has an XC hardtail type trail - though the bike is capable over a variety of surfaces, it really shines on singletrack.

Also, the modern version of the genre is in relative infancy so there hasn't been decades of optimization like a road racing bike.

[Mark McM]

I'm not at all surprised to see a wider range of geometries (including trail) on "gravel" bikes than road bikes, because the types of unpaved roads come in a wider variety than for paved roads. If you compare road and gravel bikes to MTBs, you'll see that MTBs have an even wider range of geometries and trails than even gravel bikes do, because they are used an an even wider variety of conditions - where a "cross-country" MTB might have a trail in the 70s or 80s (in millimeters), a "downhill" MTB might have a trail in the 120s or 130s.

Since gravel road will never become as standardized and uniform as paved roads, I'd expect that gravel bikes will always have a wider range of geometries and trails than do road bikes.

[pbarry]

It's true, the numbers are all over the place with these bikes. Some are more adventure bikes, with the intent to be on rockier terrain with bike packing gear. Others are tuned more towards paved and smooth gravel with little to no load. A bunch are trying to be one bike for any use, and there are certainly compromises with doing that. Lynskey's gravel geometry has me confused: Slack HTA with 77.75 BB drop. Surely it holds a line well, but pedal clearance is hampered.

The Ritchey Outback has one of the best thought out gravel geometries, imo, but definitely more road-bike-like. Fork length is only 383mm, versus 395 for most others, which leads to a lower center of gravity. Still takes a max 40c tire.

A block from my house, on the maiden voyage with the gravel bike, I had a low speed spill doing a u-turn: 40 years of racing bike handling did not prepare me for the higher trail, (65), low speed handling. Better on the washboard for sure, but it took a little getting used to.

[prototoast]

Originally Posted by pbarry

It's true, the numbers are all over the place with these bikes. Some are more adventure bikes, with the intent to be on rockier terrain with bike packing gear. Others are tuned more towards paved and smooth gravel with little to no load. A bunch are trying to be one bike for any use, and there are certainly compromises with doing that. Lynskey's gravel geometry has me confused: Slack HTA with 77.75 BB drop. Surely it holds a line well, but pedal clearance is hampered.

I think pedal clearance is overrated, unless you're doing particularly rough terrain. Go back 10 years and most road bikes were designed around 23mm tires and 70mm bb drop--and even that was usually overkill outside of crit racing. Go up to a 40mm tire and you're still riding 10mm higher on the Lynskey than you would be on the older road bike. I believe Richard Sachs's cross bikes all use an 80mm bb drop. I don't know how the scale of pedal clearance got so deep into people's heads, but unless you're taking a gravel bike on "mountain bike" trails, I doubt you'll ever have to worry about pedal clearance.

[Jaybee]

True story: I seem to have more pedal strikes on my FS MTB riding trails than I do on my gravel bike. Differences in terrain and riding style. Plus the BB on my MTB is really low - better for flow trails and drops than rock gardens.

[Aang]

Originally Posted by Clancy

Thesis OB1 60

• Santa Cruz Stigmata 70
• Salsa Warbird 71
• Giant Revolt 74

Rodeo Lab Flaanimal 80

Your small, five bike sample has only a 4mm difference in the middle of the curve, which is probably below most useful perceptual thresholds for trail.

More data is needed before we can interpret the significance of the two outliers. Are they part of a meaningful range, or just oddballs?

Right now we can look at your measurements and say, “Most gravel bikes have settled on a trail figure in the low to mid 70s,” which is the opposite of the conclusion you've drawn.

In short: We need mo' numbahs!

[HTupolev]

Originally Posted by Clancy

So why the big differences

The big differences are because different people have different concepts about what a gravel bike should be. This is driven both by personal preference, and by variations in the gravel riding in different regions.

Some are fat-tired road bikes, some are drop-bar mountains bikes, some are something different.

All are based on frames in the 54 size or close to it. From the Thesis to the Rodeo Lab there’s 20mm difference which I find pretty staggering.

Salsa Warbird 71
Thesis OB1 60
Giant Revolt 74
Santa Cruz Stigmata 70
Rodeo Lab Flaanimal 80

They vary by a lot more than that.

Gravel bikes of the "fat tire road bike" variety often have steering geometry similar to road bikes. Some people even like the trail of the bikes to be a bit lower than on road bikes, to compensate for the effects of high tire width; with its intended 650x47 tire, a Black Mountain Cycles Road+ has about 48mm of trail.

And then there are low-trail designs, like some "randonneuring" bikes, with figures around 40 or lower. An example of this sort would be the Masi Speciale Randonneur. The reason for these designs is usually to reduce the wheel flop, so that adding a front load in front of the steering axis doesn't make the steering annoyingly floppy when rocking the bike out of the saddle.

I also never see this mentioned in bike reviews. Never a statement on how the bike steers in relationships the geometry but usually just broad statements on bike handling and always positive.

Professional bike reviews are largely just marketing releases. While they're designed to look like sources of useful information, conveying information is not their purpose.

[false_Aest]

product manager's idea + Engineer's intuition + HTA + Reach + Offset + Front Center + ISO 4210 requirements = geometry


or


a lot of brands have figured that a mass market gravel bike is about 1/2 way between a road bike and a mtb.



also. listen to the Paul Sadoff interview on The Pull podcast. He's got a great explanation about why he builds his CX bikes the way he does and it translates to gravel.

[nmrt]

Trek Checkpoint has similar BB drop and slack HT angle. I own one and nary any pedal strikes.

BTW, Specialized Diverge has 86 mm of BB drop!!

Originally Posted by pbarry

It's true, the numbers are all over the place with these bikes. Some are more adventure bikes, with the intent to be on rockier terrain with bike packing gear. Others are tuned more towards paved and smooth gravel with little to no load. A bunch are trying to be one bike for any use, and there are certainly compromises with doing that. Lynskey's gravel geometry has me confused: Slack HTA with 77.75 BB drop. Surely it holds a line well, but pedal clearance is hampered.

The Ritchey Outback has one of the best thought out gravel geometries, imo, but definitely more road-bike-like. Fork length is only 383mm, versus 395 for most others, which leads to a lower center of gravity. Still takes a max 40c tire.

A block from my house, on the maiden voyage with the gravel bike, I had a low speed spill doing a u-turn: 40 years of racing bike handling did not prepare me for the higher trail, (65), low speed handling. Better on the washboard for sure, but it took a little getting used to.