Friday, June 08, 2012

Equipment - Road vs Track geometry

Over the past few years the "fixie" trend has permeated the cycling industry. Although it involves all aspects of life, the focus on the fixie is riding a fixed gear bike, usually on the road. Until now fixed gear bikes belonged in the velodrome ("track bikes") or under fearless bike messengers (although, truth be told, many messengers simply used converted bikes, i.e. not a "track bike").

Since road bikes have slightly different needs from track bikes the cycling industry set about to fill this new gap in the product line - the road going fixed gear bike.

True track bikes have a few features that prevent them from being ridden safely on the road. Two stand out over the rest, other than the fact that a track bike has no brakes.

1. On a real track bike the fork blades were round. Since there are no potholes on a track, the frames were not designed to resist impacts very well.
2. On a real track bike the fork is so slickly packaged there isn't even room to drill a hole for a brake. Most track forks are not drilled for a brake.

If you're looking for a real track bike it's tough to find those tidbits when looking at a spec sheet. There isn't a column that says "round or oval fork blades", and in many pictures it's tough to tell if the fork is drilled for a brake (unless there's a brake mounted on the bike already).

There are other ways you can check a potential candidate frame - you can compare geometry specs. Focus on headtube angle, fork rake ("offset"), trail, chainstay length, and BB drop.

I've found two bikes, from the same line, as an example.


For a 52 cm track vs a 52 cm road bike check out the differences:

HT angle: 75 deg vs 72 deg
Rake: 35 mm vs 45 mm
Trail: 61 mm vs 63 mm
Chainstay length: 380 mm vs 410 mm
BB drop: 45 mm vs 69 mm

The steeper head tube angle allows for quicker steering, meaning to actually turn the bike right or left. On a road bike it might be much, even if outfitted with a proper fork. The Spago team in the late 80s used such steep angled head tube Rossins that I thought Scott McKinley (on a long solo break) had crashed his bike and partially collapsed his frame.

In order to make the bike stable with a steeper head tube angle you need LESS rake (to give you about the same amount of trail; however, most track bikes have less trail). In the case of the two geometries above the track bike is only slightly less stable than the road bike, with just 2 mm less trail.

(The idea of less rake for more trail is counterintuitive, but think of a shopping cart - it has a really steep "head tube angle" of 90 degrees. When you push the thing forward the wheel drops back behind the pivot - that is what trail does, sort of auto-aligns the wheel for stability. Well a shopping cart wheel has something like negative 50 mm rake because the "fork" that holds the wheel actually points back, not to the front.)

Shorter chainstays means you have a bike that's more maneuverable, especially out of the saddle. Once you're seated it's not as noticeable. It also allows you to put more weight on the bars without having the rear wheel get loose under you.

A higher BB (meaning less BB drop from the axle height) means more clearance, necessary when riding slower on a steeply banked track. You can see the difference is substantial here - about an inch. This means your whole bike sort of moves up the same amount, since sizing is taken off the BB. Due to the higher BB height, everything else is taller. My 50 cm track frame looks like a 53 cm because both the head tube and seat tube are so much longer than what I'm used to.

Any "track" or "fixed gear" frame that has normal looking numbers is meant for road use.

Track bikes are different creatures.


mattio said...

While "traditional track geometry" (dual 74-75deg angles) is still pretty popular, there's definitely been a rise in more moderate geometry even for high-performance track bikes. Some of this is just trends moving back and forth but I also think there are more bikes being made for dual purpose mass start & pursuit use (like the Felt TK1), hence more 73deg angles - much closer to road stuff.

Alex Ellis said...

What would you say about the Langster 2013 geometry? I have read a review saying it isn't even road geometry.

Aki said...

I looked briefly but couldn't find something with the 2013 Langster geometry. On the Specialized site the Langster specs seem close to a track frame, albeit with the kind of geometry that mattio talks about - a bit more dual purpose. I did notice that it has a high BB, short stays, and a shorter front end. Do you have a link on the 2013 Langster?