corwin1968

I know road bikes tend to have a trail measurement of about 60mm and MTB's often have trail measurements in the 70mm range. Pavement and singletrack are the extremes but what are your thoughts on trail measurements for gravel grinding and cyclo-cross? I'm assuming neither is done on terrain that would be considered "technical". Is road geometry too twitchy for gravel and hard packed dirt? Does MTB geometry provide too much stability for gravel and dirt? Is splitting the difference (ie, Cross Check's trails is 66mm) the answer?

I would be interested in hearing people's experience with a trail measurement that you liked on gravel/dirt and trail measurements you didn't like.

Kopsis

I've done SSCX races on a Motobecane FCXUno. That bike has very aggressive geo for CX, bordering on track geo. I haven't measured the trail, but rough calculations put it at under 55mm. On hardpack dirt and gravel it doesn't bother me at all. On sandy stuff (which is a staple in FL CX), it can be a handful. But the real problem isn't the terrain, it's the dismounts. As a CX noob, I'm not that fast/smooth on dismounts yet, and trying to keep a twitchy bike tracking straight as I'm braking and throwing my weight all over the place is "harrowing" to say the least.

corwin1968

Your experience on sandy soil supports my concerns. My only experience is that I live in a neighborhood with a lot of construction and I often go riding on the areas they are digging out for new roads. This can range from hard packed dirt to sand and mud and there are always clumps of dirt. The bike I normally ride has 74mm of trail and I never gave it a thought. My new bike has 61mm of trail using the same tires and wheels. I ventured into the dirt and saw a section of loose dirt clods so I picked a line to ride thru it and no sooner than I got into the clods I was off that line and headed where I would not choose to go. I had to immediately stop and dismount, almost falling over in the process. That experience has me a bit concerned about what trail measurment to look for in a bike that likely will see some off-pavement riding.

grolby

For the sake of clarification, track bikes end up with trail in the same ballpark as road bikes - 55 mm isn't bordering on track geometry, it's lower trail than most track bikes which typically hover around 60mm.

I hope this also makes it clear that looking at trail numbers can be deceptive. Track bikes don't have crazy-low trail because they have much less fork rake than a road bike - usually around 30 mm as opposed to 43-45. What track bikes have is a very steep head angle, usually 74 degrees or steeper. This makes them extremely sensitive to input, both from the rider and the surface. Which is usually not a problem on a track. The smaller amount of fork rake keeps them from moving as far from a certain amount of input.

The bottom line is that trail numbers are difficult to interpret. In general, lower trail bikes do have faster handling, but that doesn't mean two bikes with the same trail are going to handle the same way. Comparing a road bike and a track bike is a perfect example - similar trail, but very different in feel. If you want a bike that doesn't feel twitchy, go for one with a slacker head tube. I'm not sure I can make a recommendation, though, and honestly, I would say to go with a cyclocross or gravel bike and have good faith that the geometry is appropriated for the intended use. I happen to like the handling of my Cross-Check, but I have not raced another cyclocross bike and might prefer a different geometry were I to try it. I just don't know.

Kopsis

Interesting stuff. Got me curious so I actually went and ran the numbers. The 54cm FCXUno I had has 40 mm rake and 73.5 deg head angle. Puts it right around 60 mm trail (with a 32 tire). Not as low as I thought and consistent with the notion that the steep head angle more than the actual trail amount was the major factor in the twitchy handling I experienced. My other CX bike with a 72 deg head angle and 45 mm rake only has 5 mm more trail but feels dramatically more stable on the CX course.

corwin1968

Interesting observations that fit quite nicely with my own. I spent the better part of a year researching frame geometry and experimenting with my existing bike. I determined that I wanted to try a lower trail bike (the one I was riding then had 74+ mm of trail) and set my sights on a 73/73 seat/head angle frameset. I found it in the Handsome Cycles Devil and my initial impressions were that I loved the lower amount of trail (60mm)

As I've ridden the Devil I've continued to study bike geometry and play with BikeCad in anticipation of an eventual custom build. I thought about going with 26" wheels so I picked up a 1995 Trek 850 MTB for cheap, overhauled it and started riding it a bit. This thing has about 72mm of trail. After a couple of days on the MTB I got back on my Devil and the best way to describe the sensation was that the front wheel literally "flopped" from side to side as I turned it. It was, for lack of a better term, twitchy and not at all pleasant to ride. I know a lot of bike feel is just getting used to a particular bike and after riding it a bit it felt normal again.

However, a steep head angle has other drawbacks. It decreases wheelbase, increases the chances of toe clip overlap and can limit the size tires you run. Plus, with the fatter tires I prefer, it just looks funny. I also prefer a short top-tube and that just makes those issues worse.

I test rode a Surly Karate Monkey with a 71.5 degree head tube and a 40'ish mm trail fork. That thing was a stable as I can imagine a bike being. I wanted to find things to ride off and over, it was so confidence inspiring. The bikes I rode from 1995 to 2012 all had 71.5 degree head angles with rake being in the 40-45mm range.

After playing with BikeCad I've determined that a slacker head tube angle is the way I want to go. I'm thinking something between 70-72 degrees, depending on what a builder can do with fork rake. I'm also not sure about overall trail but I'm leaning toward higher numbers (65mm +) which is a complete reversal from where I was six months ago.

I think I'll put my old Multitrack back together and see how it rides. It has similar geometry to the Karate Monkey.

On a related note, I think I've pretty much nailed down my seat tube angle at 73 degrees. All the bikes I've ridden have had 73 degree seat tubes but I've been curious about slacker angles. The Trek 820 has a 72 degree seat tube and no matter how I position my saddle, my knee starts aching within a few minutes of riding it. I can immediately get on my Devil and the knee pain is gone until I get back on the 820.

The Cross Check was on my short list of framesets but I wanted to try a lower bottom bracket and longer chainstays, both of which my Devil have. I'm definately sold on the lower bottom bracket (Devil has 70mm drop compared to my old bike's 60mm) but the jury is still out on chainstay length. Right now I have the wheel all the way back in the dropouts giving me a CS length of 45.5 cm and a wheelbase of about 1045mm.....almost touring bike territory. I suspect I like shorter chainstays but haven't done enough experimenting with that particular variable to know for sure.