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Fork rake.....(12 posts)

Fork rake.....DannyBoy
Mar 26, 2002 12:35 AM
You've been giving me some advice on new forks. I think I'm going to go for an Ouzo Pro (I like the straight slim Look fork, but at 175lbs think it's too skinny for me).

Somebody posted this in reply to my question;

"It comes in a 40, 43, and 45 mm rake in both 1" and 1-1/8"

How does rake affect steering response/ride? in this example how would a 40 be different from a 45? Ta.
depending on the original fork rake:Alexx
Mar 26, 2002 4:19 AM
A 40 could either be perfect, or make it sluggish on steerring. A 45 could be perfect, a bit sluggish, or unstable at high speed. What is the rake of the existing fork? That's the only question you need to ask here.
if there is no original fork???C-40
Mar 26, 2002 10:15 AM
Different brands will have different fork rakes (with the same head tube angle) according to how the manufacturer wants the bike to behave. If you buy a bare(new)frame, you have to start somewhere.

Colnago's for example, have slack head tube angles (around 72 degrees)combined with a 43mm rake to produce a relatively large amount of trail (65mm+)for stable handling. Many brands have considerably less trail (55-60mm). Neither is right or wrong, but the amount of trail makes the bike more suitable for a particular use. Crit bikes would generally be set up with less trail for quicker steering than long-distance road racing bikes.

Trail is also changed with frame size. A large frame with it's longer wheelbase, will often have less trail to speed up the steering repsonse. Small frames with short wheelbases have more trail to slow the steering response.

The real thing that you want to know is the amount of trail that is produced with a specific fork rake. To calculate trail you need the tire radius, the head tube angle and the fork rake. The formula is (tire radius/tanH) - (rake x sinH), where H is the head tube angle.

As an example, with an H of 73 and a rake of 40, you get 64.5mm of trail, assuming a 336mm tire radius.
Another fork rake questionDY
Mar 26, 2002 9:56 PM
Wouldn't increased fork rake increase trail? Maybe I don't have a good understanding of trail.

What I am confused about is that some mention that more fork rake makes a bike unstable or fast handling and less rake more stable.

I guess it depends on the bike in question. I have a bike with 700C tires, 43mm of rake and 73.2 degree head tube. If I wanted to make the frame more stable what would I do?
Another fork rake questionChen2
Mar 27, 2002 7:04 AM
Increasing rake decreases trail because the tire's contact with the ground is behind the projection of the axis of the head tube. Increasing rake moves that contact forward toward the axis of the head tube. If you decrease rake and increase trail the bike will have a greater tendency to continue in a straight line. Increasing rake and decreasing trail will cause it to turn quicker.
Look at the formula...C-40
Mar 27, 2002 9:32 AM
The first half of the equation (tire radius/tanH) is the trail with no rake. The second half of the equation -(rake x sinH), reduces the trail. Therefore, all rakes reduce the trail. The more rake the greater the reduction in trail.

Smaller trails (larger rakes) increase the steering speed, larger trails (smaller rakes) slow it down.
re: Fork rake.....davet
Mar 26, 2002 6:54 AM
Dan: If you are still looking at a Serotta, they specify a 43mm rake. If the rake is changed it will adversly affect the handling of the bike. Longer rake, 45, will quicken the steering. Shorter, 40, will make the steering more sluggish. It is best to go with bike manufacturers specifications when changing forks.
It is indeed with the Serotta in mind I ask this qDannyBoy
Mar 26, 2002 9:38 PM
I'll take note.

Whoa, this is all technical stuff and no mistake!
big frame=less rake and vice versaBarnyard
Mar 26, 2002 5:12 PM
not necessarily...C-40
Mar 27, 2002 9:38 AM
Many manufacturers use the same rake on all frame sizes (trek for example). Trek increases the head tube angle on their larger frames to speed up the steering.
I did the mathDY
Mar 27, 2002 1:23 PM
Actually I drew a diagram in CAD and found out that with the 672 tire dia. and my 73.2 head tube and 43mm fork rake, I get about 57 mm of trail. It didn't make sense to me so that is why I had to draw it out, but yes the small rake (40mm) fork would give me about 60 mm of trail. Do you think that would be a big enough increase to feel?

Now all I need to do is find a 40 mm rake fork to try this out before I shell out $300 for the expensive ones. The only forks I've seen so far that come in 40mm, 43mm and 45mm options are the Reynolds and Time.

Thanks for the explanation.
won't make a big difference...C-40
Mar 27, 2002 3:25 PM
An additional 3mm of trail will barely be noticeable, IMO. If you want the bike to respond a little more slowly, the 40mm rake should do the trick. FWIW, I set up a 55cm Litespeed which has a 73 degree head tube angle, with a 40mm LOOK HSC2 fork and it steered just fine; very stable at all speeds. My 55cm Colnago has significantly more trail (about 68mm, if the 72 degree HTA and 43mm rake advertised by the US distributor is correct). I've never thought that the steering was sluggish.