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Frame Design: Fork Rake, Head Tube Angle, Trail
March 12, 2009, 11:31 am
Filed under: frame design, how-to, the cycling world

In the part of the cycling world that’s perpetuated on the internet – message boards and such – there’s a lot of hipster-bashing, centered around ubiquitous photos of very stylish bikes featuring some very nonsensical tradeoffs in build/component choices. A surprising amount of Bike Snob‘s blog is about these bikes, so I assume you can forgive me for not going into detail. However, for all the talk of hipsters who don’t understand how their bicycles work I see road bikers making plenty of silly decisions that seem to stem from an ignorance about the way that bikes work.

One of my pet peeves is misunderstandings about front end geometry: head tube angle and fork rake. Maybe it’s because in the fixed gear scene there’s a lot of neophyte fretting about “true track geometry” and an infectuous perpetuation of the terms “tight” and “twitchy” without real meaning or understanding behind them. Meanwhile I’ve heard plenty of road folks who greatly misunderstand the differences between front end design of time trial bikes and road bikes. While I don’t claim to be an expert I do think it’s important to share good information when it’s widely available. And so, some of my favorite resources:

Best introduction I’ve found is the An Introduction to Bicycle Geometry and Handling, by CHVNK. Dave Moulton‘s piece is also very well-explained. In particular he narrates the changes made to frame design (with regards to front end design: head tube angle, rake, and trail) in the 50s, from the long-wheelbase bikes of the 1940s and before to the more contemporary-looking bikes with tighter geometry that became the status quo in the 60s. From Urban Velo is a piece by Don Walker: The Truth About Track Geometry, which features a good bit on trail and a bike’s handling. And, if you want to run the calculations for your own bike, you can do so with this Excel spreadsheet from Anvil Bikeworks.

The boildown is that head tube angle and fork rake work against each other in order to reach an equilibrium, a sweet spot of trail measurement (60mm, according to Don Walker). Overcompensating one because of a lack of the other is counterproductive: putting a road fork on a track bike with a steep headtube angle will make the handling less stable – the higher-rake fork reduces the trail measurement. A “road” fork doesn’t make a bike’s handling more stable – it will make it less self-correcting. Putting a low-rake fork in order to tighten up the front end of a bike with a slack head tube, rather than bringing a bike’s handling closer to neutral, will push it toward very-high-rake, sluggish handling. Which is fine, if you want a road bike that handles like a cruiser. I learned this first hand, picking up a carbon fork for my every-day bike with a 72deg headtube. The fork turned out to have 28mm of rake: too low to neutralize the handling – it was sluggish.

Don’t make the mistake of making uninformed decisions about this stuff. You don’t have to know everything about how your bike works in order to ride it, but if you’re going to change things and make product decisions, be discerning about what information you file away as reliable in your head. Not everybody posting on Bikeforums, nor every shop employee, gets everything right all the time.

For me, soaking up information about this stuff is part of what makes cycling beautiful. When I lean my bike into a corner, I know that there’s a very elegant design that’s practically making the bike handle itself. Furthermore, knowing how and why your bike works the way it does just might make you a better rider when the going gets rough…

(closing image courtesy of Cycling Art Blog)
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