fork rake

[hypnos]

Looking for a carbon cyclocross style fork that is configured for cantilever brakes. These forks have 47-50mm of rake. Currently riding with a 43mm raked fork. How will a fork with greater rake differ with regard to handling. I've tried to educate myself on this subject, still confused.

[Bruce K]

My undrstanding is that the rake of a cross fork will make your bike feel a little more stable due to a slightly longer wheelbase.

The bike should also feel a little less "darty" in that a longer wheelbase should yield slower turning. This does not mean that the bike will be sluggish or unresponsive, just a little slower than "normal".

BK

[davep]

Counterintuitively, and all other things being equal, increasing the rake will decrease the trail. This will make the steering less stable, or more responsive, take you pick of descriptors.

For more details, see http://www.phred.org/~josh/bike/trail.html

[BigMac]

The longer rake will shorten trail which can make front end feel quicker. This can be advantageous at slow speeds making it easier to avoid obstacles but for most 'cross duty a short trail is not a good thing. You'll experience much more bump-steer problems and high speed stability on loose gravel fire roads/trails can get scary.

The biggest concern however should be fork crown - dropout dimension. Most cross forks have much greater clearence for dirt/mud, big fat knobbies, perhaps fenders, etc. This typically results in 20-25mm larger crown race-dropout distance than typical road fork (400mm vs 375mm). If your existing frame was designed for a road fork, the much taller cross fork will raise front end, slacken HT angle, ST angle, basically create a can of worms that's not really worth getting into. Steel forks are a better choice for true 'cross duty. If you are looking to convert a road racing frame to cross duty, it can be done with a custom steel fork, be warned you'll be limited to small tires (30C, max.). A better choice for cheap 'cross duty is to find a used touring frame (Serotta Rapid Tour is a fine candidate) which was intended for taller fork and lots of tire clearence.

Ride on!

[Bruce K]

Sorry about that....

In checking the geometry of my cross bike vs my road bike I flipped the fork info.

The cross bike DOES have a longer wheelbase, but the fork rake is less.

The guys are right, if you put a cross fork on a road frame the bike would be MORE squirrely. Lengthening the wheelbase on a cross bike offsets some of that.

BK

[hypnos]

This fork will be used with a new custom frame. Want to use cantilever brakes, larger tires and fenders. Hoping to stay away from steel, but, in the end, that may be the best option.

[Bruce K]

Ok guys, you got me thinking about this a bit more.

My 16 year-old son is designing a TT bike as a term project for his CAD class in high school.

He is using the book, Bicycle Design , by Mike Burrows as a reference.

I decided to do a bit of reading to see what Mr. Burrows had to say.

Inteerestingly, he says that given the range of head tube angles used in current design, the final angle is more selected for ergonomic feel than steering dynamics. It takes changes in excess of 10 degrees to create any real handling difference.

However, he does say that trail is key. And as previously stated the more trail, the more stable the ride (see recumbents).

Interesting stuff.....

BK

[BigMac]

Hypnos:

Identify the cf fork you intend to use -- I would also suggest you purchase the cf fork PRIOR to ordering custom frame. Many, if not all, cf forks have relatively highly variable rakes with similarly high tolerances. An AlphaQ for example that may have a 47mm rake spec, could vary as much as +/- 3mm...yes that will effect ride, feel and response of any frame. The builder will use measured specs to design new frame with appropriate trail -- somewhere in the 59-60mm range is desirable for 'cross bikes. In simplest terms, the longer raked cf cross fork will require a shallower ht angle. I would also suggest you shorten TT 1.5-2cm and lengthen stem to acheive proper reach. The result will be slightly slower steering, less bump-steer and better fore-aft balance when grinding up steep, loose terrain.

It's certainly your call but I'd demo a few bikes with steel forks in off-road conditions. While I prefer steel forks for road duty, the F1 fork is really the only cf fork I have ever found acceptable and yes I've tried basically everything on market, I would NEVER consider anything but steel in off-road conditions. A cf fork may save 1/2 lb, but honestly forks are the last place one should look to reduce weight, especially in demanding off-road conditions. A properly designed flat-crown steel fork will provide plenty of vibration damping for most conditions. Just my opinion.

Ride on!