When it comes to bikes and handling, after bottom bracket drop, the next most important dimension that will determine just how a bike handles is trail. What’s trail you ask? Fair question; I’m amazed at the number of bike companies that don’t include this figure in their geometry charts.
As I spelled out in my previous post on Trail, which you can read here, trail is easily the most complicated dimension in a bike’s design. It measures the difference between the steering axis and the front wheel’s contact patch, and it is affected by tire size. Three variables, summed up in one number. And no matter what any bike company or magazine reports about HTA, it doesn’t tell you much about how a bicycle steers.
This will help illustrate the point: Suppose you have a road bike with a 73-degree head tube angle and a mountain bike with a 66-degree HTA; all things being equal, the bike with the 73-degree HTA will steer more quickly than the bike with the 66-degree HTA. But trail is the bottom line and it isn’t set until we know the fork rake. Let’s pretend that pretend someone at Giant pranked Colnago and put a fork with 5mm of rake and it shipped to a bike shop with a 25mm tire. That bike would have 118mm of trail. More than any bike I’ve ever ridden in my life. Trying to turn that bike would be like trying to steer a battleship.
Now, let’s pretend some crazy intern at Fox thinks his downhill bike doesn’t turn fast enough and he machines a new crown with 100mm of rake and he runs 2.1-inch tires. That bike would have 54mm of trail, putting it in the realm of of a road racing bike. So it’s possible to make a bike where head tube angle tells you nothing of how the bike handles. Similarly, if you take a road bike with a 73.5-degree HTA and install a fork with no rake (0mm), on 25mm tires that bike will have 101mm of trail, putting it in the realm of a cross-country mountain bike.
That said, most bikes within a given category—downhill, trail, gravel, touring, etc.—will fall within a fairly narrow range because bike makers have tried enough different combinations to learn that combinations outside a certain range handle poorly, by either being much too reactive or by being too difficult to turn.
To be fair, exceptions do exist. Experimenting is fun, right? Arguably the most unusual design to come down the bike since I’ve been a cyclist is Evil Bikes’ gravel bike, the Chamois Hagar. If built with a 40mm-wide tire, its 66.67-degree HTA and 57mm-rake result in 91mm of trail. I’ve never been on a drop-bar bike with so much trail; making a bike with that much trail turn without the aid of at least a 760mm-wide handlebar must be interesting.
The table below begins with a matrix of head tube angles (the column at the left) and fork rakes along the top row. The first set are HTAs and fork rakes with a 2.3-inch tire as you would be run on a mountain bike. The second set of numbers are for gravel bikes with 40mm tires and the bottom set are for road bikes running 25mm tires.
Rather than base the table on theoretical fork rakes, I’ve included only those rakes that I actually see forks produced in. It’s certainly possible that there might be a fork with 46 or 49mm of rake, but what’s below will cover most everything out there. What I haven’t accounted for is the incredible range of head tube angles in use. More on that in my next installment.
Rake of some aftermarket forks (in mm):
32 Stepcast 44, 51 (29-inch wheel)
34: 44, 51 (29)
36: 37, 44, 51 (29)
38: 37 (27.5); 44, 51 (29)
40: 48 (27.5); 52 (29)
Sid: 44 (29)
Zeb: 38,44 (27.5); 44, 51 (29)
Lyrik: 37, 44, 46 (27.5); 42, 44, 51 (29)
Pike: 37, 44 (27.5); 44 (29)
Boxer: 36, 36 (27.5); 46, 56 (29)
Judy: 40 (26); 42, 51 (27.5); 42, 46, 51 (29)
Reba: 42 (27.5); 51 (29)
Road Fork for rim brakes: 40, 43, 45, 50mm (700C)
AR Disc Fork: 47mm (700C)
Gravel Fork: 47, 50mm (700C)
Adventure Fork: 49, 55.5 (700C)
Some examples of popular productions bikes and where they fall in the matrix
1Trek Madone (56cm): 73.5, 40mm
2Giant TCR Advanced (M/L): 73, 45mm
3Specialized Diverge (56cm): 71.75, 55mm (approximate)
4Surley Long Haul Trucker (56cm): 71, 45mm
5Ibis Ripley (L): 66.5, 44mm
6Specialized S-Works Stumpjumper EVO: 64.5, 44mm
7Evil Chamois Hagar: 66.67, 57mm, 50mm tires
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