Can someone please explain "Fork Rake" to me?
 

My CAAD 9 fork has a "45" rake. I'm looking at a 3T model that come in either 43, 49, or 53 rake. Nothing in the same rake as mine has now.

So, what will this do if I get the 43, or the 49? I've heard it makes steering either more sensitive (twitchy), or more stable, depending on which way you go.

I just don't understand. If I get the 43, which is closest to what I have now, what difference will that make?


Sorry for the ignorance, but I can't find a chart, graph, or anything else to explain it to me.


:D
RD

thanks Namenda,



So looking at that picture, if I go from a 45 to a 43, which would move the rake back towards the bike a little, then the steering becomes more twitchy? How much difference would a 49 make in stability if I got that from the current 45??

Fork rake, as shown above, is the amount of offset from the steering axis to the dropouts. In combination with head tube angle and wheel size, it determines the amount of geometric trail. A larger trail makes the bike more stable but harder to turn. A smaller trail makes the bike more responsive but "twitchier." Most modern bicycles use a head angle in the 72-74 degree range and a rake of 40-50mm, giving trail of 60-65mm.

http://davesbikeblog.blogspot.com/20...le-bit-of.html

The bike will become less twitchy due to a larger trail. You would not want to buy a fork with more offset, since the trail is already on the small side, to start with.

The formula for trail is R/tanH - (offset/sinH), where R is the tire radius and H is the head tube angle. All fork offset reduces trail, so the more offset the smaller the trail and the quicker the steering.

Get the 43 not the 49. You'll develop wee better skill set and your bike won't feel like an old piece of crap.

If you can't get the exact same rake, go with smaller rake, not larger.
You don't want to learn the hard way that the larger rake was enough to put you into highspeed shimmy.
This is most likely on tall frames where the *(&$#@# mfgrs use steeper headtube angles to keep the wheelbase short.
I have a tall custom Lightspeed for which I had to order a custom-built fork with 38mm rake, because forks with 43-45mm rake would result in shimmy.

I don't think you understand that less rake results in *more* stability, not less.
But your advice to go with less rake is good.

EXCELLENT link.
When I was diagnosing my shimmy problems, I discovered Moulton's articles, which were best discussions anywhere on steering geometry.

Typically we don't ever hear complaints from people that their bike is too stable and is too unresponsive in turning. Most of the time it's complaints about the steering being to twitchy and the bike being too unstable. In which case, going with less rake (for more trail) will result in a more stable bike. Go for the fork with 43mm rake.

Twitchy is a term used by people who do not like quick steering, those of us who do like quick steering just call it quick steering.

^^^^ i call it responsive, which the CAAD9 is with the stock fork (rake). it was the most obvious difference i noticed from my Giant TCR. later.

My cyclocross frame has a 72 degree headtube angle. I'm currently using a fork with a 44 mm offset. I think the handling of the bike is perfect and am trying to duplicate the geometry on my next bike.

The frame I'm considering buying also has a 72 degree headtube angle. The wheel size will be the same, as is the fork length.

The new bike uses a 47 mm offset standard from the factory. However, consider that the headtube angle and tire size is the same as the old frame, I'm going to use my existing 44 mm fork.

I'm assuming that I'm duplicating the geometry of my existing bike as long as the headtube angle, tire size and fork length is unchanged.

Am I correct?

Only correct if the frame is built for a fork with the same axle to crown length. Not all forks have the same axle to crown length. A longer fork would reduce the HTA and increase the trail. A shorter one would reduce the trail. It takes about a 9mm length diffence to change the HTA by .5 degree.