Reflections on fat-tire road bike geometry

velotel · #1 12-06-2017, 04:16 PM

I have zero expertise in frame design. I’m just an old guy who’s ridden a good bit and has had the marvelous good fortune to ride on some astonishingly beautiful mountain roads, paved and dirt. But I enjoy thinking about what a fat-tire road bike is. I also asked a few builders what they think a fat-tire road bike is. You’ll find those at the end so if you want some expertise on the subject, jump on down.

My bike is a road bike wearing fat tires with a subtly tweaked geometry to bring its performance on single-tracks and dirt roads up to what I expect of a road bike on pavement, but without compromising its performance on blacktop.

Single-tracks here means relatively smooth trails, the kind trail runners enjoy, and dirt roads means tame enough that four-wheel drive and high clearance aren’t absolutely required. Grades are generally moderate enough so neither super-low gearing for going up nor dropping the seat post for going down is required.

Two biggies to clarify before looking at fat-tire road bikes: what kind of terrain you intend to play on and how fat a tire you’re willing to ride. Plus the future, what is ridden will evolve. Going from a trail or road like that doesn’t interest me to, well damn, I bet I can ride that. That happens, even at age 72!

Fat-tire road bike geometry has to deal with surfaces squirming around under the tires, with rocks, holes, roots, etc trying to knock the front or rear wheel caddywhumpus, with grades steep enough to climb out of the saddle only the surface is loose so you can’t, etc., etc. Always with impeccable handling on pavement where probably the majority of time will be passed.

Note a special bike for gravel/dirt roads and trails isn’t mandatory. Fatter tires aren’t even needed. I used to ride dirt and rock roads with 23 and 25mm tires and never gave it a thought. But having gone to the dark side so to speak, I now say anyone contemplating spending serious time riding dirt with a road bike and who can afford one would be nuts to not get a fat-tire road bike. They’re just way, way more enjoyable to ride off-pavement.

Chainstay length does not define a fat-tire road bike. Most gravel bikes have longer chainstays but it’s not some huge difference. My Stoner Bike’s stays are 16mm longer than my pure road bike’s. 16mm is nothing. Longer chainstays provide clearance for fatter tires, produce a stabler, smoother ride due to the resulting longer wheelbase, add to the rear end’s stiffness by enabling the use of larger diameter tubes with minimal tube bending for tire clearance.

Forget the back end. It’s only along for the ride, the bass setting the beat. The front end is where the action is, the lead guitar soaring in the stars. On blacktop there isn’t much for the front end to do because technical challenges are lacking. Trails and jeep roads are where a bike’s front end dances, following the rider’s eyes, flicking around rocks and roots, chasing the groove in a wonderful ballet of quickness and stability.

This is where frame builders can differ in their solutions. Steep or shallow head angle, more or less offset/trail, long or short stem, long or short top tube, stiff or soft front end, etc., etc. Which means you need to understand your riding style and be able to communicate that to a builder or someone helping you choose a bike. For my fat-tire road bike, Kent knew my riding style from way back and designed it with that in mind. In technical passages I’m in the drops, pressing down on the front end, the connection with the axle so intimate that the wheel seems to dart before I’ve even seen what it’s darting around.

You need weight on the front end to weave lines through rocks and trees, otherwise the steering’s vague. Which means pulling the handlebar closer in. But too much and the front end will be zigging and zagging every time you blink. A lot of fat-tire road bikes have pushed out front centers via a softer head angle, like mine, but again too much and keeping weight on the front end for steerage control in a steep climb can be tricky. Too short a front center means too much toe overlap plus dropping down steep descents can feel like you’re in the front car in a roller coaster going over the top into a monster plunge. A steep head angle with too much rake will make the front end so reactive you’ll forget what a straight line is. Same head angle with too little rake will be super stable at speed but reluctant to dart around obstacles. A shallow head tube angle with not much rake will be forever flopping into turns with the slightest lean of the bike. Etc., etc.

The front end also needs to be designed for when you’re out of the saddle. A grade on dirt will be harder than the same grade on pavement which means climbing out of the saddle more often. But traction will be thin so you’ll need more weight on the back wheel and of coursre standing automatically shifts weight forward. If it’s too steep to climb seated you’ll have to drop into a low crouch over the top bar with the butt floating over the nose of the saddle, the hands in the drops pulling back and that means the frame needs to have the space for you to do that.

Some people like a low bottom bracket, relative to the line between the front and rear wheel hubs. They say low is more stable. Others like it higher. I think mine is relatively high, or it’s higher than my pure road bike’s. I don’t feel any lack of stability and it gives me more pedal clearance which I really like but to be honest, I’ve never given it any thought one way or the other. As usual, no rule, just personal preference.

Same with the handlebar height. Typical pure road bikes have what I consider crazy low handlebars relative to the saddle height. The flat on my road bike’s bar isn’t far off level with the saddle. The Stoner Bike’s is even closer to level. Again personal preference but I think for off-pavement and on technically demanding trails and dirt roads, the higher position enables shifting weight front and back and keeping your eyes on the trail ahead easier. Also makes riding in the drops super comfortable and ideally the default position. That’s the power position for driving the bike through rough sections.

Lots of parameters to play with to achieve a fat-tire road bike that works for you. Which is why in my opinion they were made for custom builders. Production frames are designed around common denominators someone decided will work for most people. If you fit that criteria, excellent. But riding rocky roads and trails has technical skill requirements that paved riding doesn’t even know exists plus the whole genre of gravel bikes has so many nuances that factory sizing won’t necessarily answer for everyone. Happily there are some production companies providing a wide choice in sizing and design, Moots for one. It was a long week of riding a Moots Routt in Colorado that convinced me a by-design fat-tire road bike totally outperforms a standard road bike on dirt and trails.

Given all this performance criteria, a custom build can be super attractive with the ability to meet any and all needs. Like forks. Apparently off-the-shelf forks for fat-tire road bikes, from Enve for example, are available with only one fork rake, so a front end has to be built around that fork’s rake. A builder who does his own forks could build a frame with a custom fork to provide exactly the handling characteristics desired. I don’t think too many builders do this. Building forks is apparently a pain and expensive. Forks used to be as much the domain of the builder as the frame itself but carbon forks pretty much killed that. Not that people are apparently complaining. My Stoner Bike’s got an Enve CX carbon fork and I’m definitely not complaining.

Which brings up the front end stiffness argument. Some claim the old bikes with their thin steel tubing and steel fork with curved blades serve up a superior smooth ride in rough conditions. Others go the opposite route with an oversized head tube and fat fork blades with a fat, tapered steerer tube, resulting in a stiff front end that refuses to be knocked about by ruts and holes and rocks. Personal preference again. I went with a stiff front end and I’ve never ridden a smoother riding bike in the rough. Bottom line is the suspension is generated by the tires, not the tubing. I also have to admit to not having ridden an old style steel frame in longer than I can remember.

Then there’s this evolution to way fat tires, like the Moots Baxter. My son rides one and loves it. Apparently more and more gravel riders are going fatter and fatter. Gravel bikes or drop bar mountain bikes, the distinction becomes blurred. Which is again why I refer to my bike as a fat-tire road bike because that’s what it is, a road bike wearing fatter tires.

As an aside, I don’t think putting drop bars on an old style rigid mountain bike is going to produce the equivalent of a fat-tire road bike. I rode some mountain bikes back in the 80’s with the WTB flared drop bar and while I loved the drop bar position, they never felt quite right. I think because the frame’s sweet spot for the rider’s mass over the bike was designed for a flat bar and that sweet spot isn’t the same for drop bars.

You also might want to keep in mind that whatever gravel bike you buy today could be considered archaic in a couple of years. I’m sure people are working on suspension forks for gravel bikes and that will be followed by full suspension. I have no idea what those will be called. They also don’t interest me.

I love my Stoner Bike’s simplicity, no shocks, no disc brakes, no electric shifting. Just a straight up road bike tweaked to kick ass off-pavement. Super reactive when needed, super stable at the same time, modestly fat but light tires but skinny enough to keep me knowing I’m riding a road bike, goes everywhere I want to go with more grace than me. Places where I’d need fatter tires and lower gearing I ignore. I’m a dinosaur.

For myself fat-tire road bikes are the coolest thing to come along in cycling since mountain bikes. The world of road cycling’s been beautifully expanded and in my case passion for riding thoroughly rejuvenated. Plus, and this is kind of easy to forget at times, they’re awesome just as pure ol’ road bikes, or at least mine is. That was something I insisted on in my conversations with Kent for the bike. The 35mm tires running relatively soft soak up the asphalt’s bumps and ridges, cling like crazy in turns, and just feel friggin fast. And when the blacktop stops and the road doesn’t, neither does my bike.

I can also guarantee that the currently perceived limits of where these bikes can go are in the process of being blown away. I saw that happening with my son and his riding buddies and even more so in videos of riders on gravel bikes riding insanely well in totally insane locations. Which means that everything I’ve said about these bikes is coming from someone who is already off the back in what’s happening in the world of fat-tired road bikes. As in, you’re just going to have to go out and discover all this for yourself. But believe me, you’ll love it.

velotel · #2 12-06-2017, 04:17 PM

From David Kirk in his usual clear style

Hank shared his take of what a gravel bike is, and isn't, and nicely asked that I do the same. After thinking about it for a day or two I'm not sure I have a solid answer. I'll let you be the judge.

There was a time years ago when bikes of different genres had clear lines of differentiation and today the lines are wide and blurry. When I was a young man road bikes and mountain bikes shared little to nothing aside from two wheels, a drive train and brakes. One had skinny tires and the other wide, one had narrow range gearing and the other very wide, and one had drop bars and the other flat bars. It was simple. Now bikes are designed and built to use a mix of components and they don't fall neatly into a given genre but instead they fall somewhere on a continuum. So where does a "gravel bike" fall on that continuum? I'm honestly not sure and feel that the wants of the rider and the component choices made to best suit the terrain to be ridden will answer that question. It depends.

For decades I've ridden my road bike on roads...paved roads, chip seal roads, dirt roads, and gravel roads. It was a road bike and when the road changed from being a paved road to being an unpaved road I kept going. For the first few decades it was on a pure road race bike with the 23 mm tires that were the norm at the time. Later I switched to the Vittoria Utmost tubulars (best tire I ever used - 27 mm wide and very light, supple and fast). They fit under a standard road race brake and if you didn't have a silly bike that was designed to limit tire room they fit well. I, like countless others, had no idea that we were doing a specific type of riding. We got on our road bikes and rode them on all types of roads. The bikes were not in any way optimized for gravel riding but they worked well and still do.

That said if the aim is to make the best bike one can for use on gravel - optimized for gravel if you will - how will it differ from a classic Euro stage race road bike design? I think the changes fall into two areas - component choice and fit/handling numbers.

Components - of course the one thing that shapes all the other choices is tire size. The current trend is toward wider and wider tires and it seems inevitable that the pendulum will swing toward really wide and then correct itself some and go a bit less wide. If the roads are really bad then the widest tires could be a good thing...if they are smooth dirt then all that tire can start to take away from the quickness, versatility and nimble feel of the bike. In other words one can go too big for a given use of the bike – bigger is not always better. Once the tire size is chosen then the next choice is the brake and I think that if the bike is ridden in the dry mostly and that we are talking flat or rolling terrain then a rim brake can be more than enough. Riding in the high mountains or in the rain and a disc is a very good thing...and certainly if you pick a really wide tire then discs will be the only choice.

I like to run a slightly wider bar to give better leverage and with the generally lower speeds the aero drag of a wider bar is not so much an issue. I also like a slightly shorter stem and if need be compensate with a longer top tube to help stretch the wheelbase out some...putting that front wheel out in front of you on a gravel descent is a very nice.

Fit/geometry - I like to shorten the reach and lessen the bar drop to get the rider in a slightly more upright position. Again the lower speeds mean aero drag is less an issue for most riders. The chainstays will need to be longer to work with the tire width and this is fine as the added wheelbase in most cases is welcome. One needs to make sure the front center is also stretched to balance out the longer ass-end and give proper weight distribution. The tires are taller and the bikes generally aren't leaned very far over and pedaled so the BB can go down to give more stability. I like to give the bikes a more slack head angle and add fork rake to keep the trail number happy while at the same time making the wheelbase longer. Lastly I like a gravel bike to be stiff torsionally so it will track in rutted gravel more predictably. A bike that is soft in torsion will tend to wander and tramline where a torsionally stiffer frameset will hold its line better - especially at speed. Those big soft tires will give plenty of comfort and there's no need to make the frame overly soft to buy comfort.

So...what is a gravel bike and how it is defined? I'd say it depends. It will generally have a wider tire, a more upright position and a longer wheelbase but everything else depends on the terrain being ridden and the type of riding being done on it (racing, touring, day tripping...etc.). So, as I said all those words ago - it depends.

I hope that makes sense.

velotel · #3 12-06-2017, 04:19 PM

From Steve Potts, super concise, that's Steve

1 . Classic Road, caliper brake ( 47 mm reach) 700 x 25 mm max tire size ( 28 mm on some calipers )

2. Gravel, Disc or larger rim brake, 700 x 45mm max tire size, generally smaller front chain rings and new 1 x 11 drive trains.

3. Classic Mountain Bike, 26" 27.5 " and 29" , 2.35" max tire size.

4. Plus Mountain Bike, 26" , 27.5 " and 29" . 3" max tire size.

5. Mountain Snow, anything goes.....

Of course there are some geometry concerns with each design,,,,, back to work.

weisan · #4 12-06-2017, 04:30 PM

Quote:

Forget the back end. It’s only along for the ride, the bass setting the beat. The front end is where the action is, the lead guitar soaring in the stars

.

John H. · #5 12-06-2017, 04:37 PM

A buddy of mine got a Seven Evergreen with 45cm chainstays- At 1st he was worried that the bike would be sluggish or otherwise uninspiring.
He found that to be far from the truth- He loves it.
So much so that he sent it back to Seven and had fender mounts added to it. Now he even rides it in the rain with full fenders.

thwart · #6 12-06-2017, 07:01 PM

Liking this discussion.

joosttx · #7 12-06-2017, 07:19 PM

Quote:

Originally Posted by John H.

A buddy of mine got a Seven Evergreen with 45cm chainstays- At 1st he was worried that the bike would be sluggish or otherwise uninspiring.
He found that to be far from the truth- He loves it.
So much so that he sent it back to Seven and had fender mounts added to it. Now he even rides it in the rain with full fenders.

Your buddy is not sluggish on that Seven.....

Kontact · #8 12-06-2017, 07:34 PM

The geometry/handling discussion could have been lifted from a road racing bicycle discussion in the '80s. Road geometries have largely stabilized compared to the '80s where you had low trail Merckx bikes designed for cobbles, bikes with variable trail based on size, the school of neutral trail and the "steady descenders" with long trail. I'd doubt any of the new gravel bikes are really exceeding the steering geometries seen then.

I would also wonder if gravel bikes are really any different than race bikes in the long reach centerpull era or the medium reach sport/touring class of the '80s.

At the end of the day, people can learn to ride anything that's reasonable, so you can go custom to service a need to make the bike work like you think is ideal, or you can learn to ride what's available and probably be fine with that, too. Road handlebars offer hand positions that vary the effective stem leverage so much that I would be truly shocked if the difference between getting up a hill came down to a centimeter or two of loading over the front end.

Given the ability to vary fork rake to produce a target trail, I don't think it is a good idea to consider head tube angle as a lot more than a way to produce a front center number given a target reach.

John H. · #9 12-06-2017, 08:16 PM

No he is not- He is quite swift.

Likes that Seven so much that he has another one on the way- This time for an Enve road disc fork.

Quote:

Originally Posted by joosttx

Your buddy is not sluggish on that Seven.....

ColonelJLloyd · #10 12-06-2017, 08:39 PM