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Old 08-14-09, 11:21 AM   #1
intx13
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Confusion about no-rake forks and handling

Hi folks.

I'm learning about bike design - I'm way too new (and poor!) to be designing / building my own bike right now, but it's fun to pretend.

Anyway, I'm a little confused about forks with no rake (as in, 0 mm rake; not straight forks that have the rake set at the crown). People on BF and elsewhere online claim that no rake yields a twitchy bike or a bike with "poor" (by some standard) handling.

My reference is here, which is linked from Sheldon Brown's website (so it can't be all wrong!). He says,

Quote:
For a given steering angle, offsetting the hub forward reduces trail, while offestting the hub backward increases trail. This may seem counterintuitive, since very stable cruiser bikes usually have more fork rake than twitchy track bikes. But the other factor at work is the angle of the steerer -- cruiser bikes have very slack head tubes, so they have more trail despite their fork rake, not because of it.
By that logic, a bike with no rake is more stable than a bike with rake, all other things being equal (which they never are). So are the BFers and other Internet denizens wrong? Or am I misinterpreting this?

Any information is appreciated!
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Old 08-14-09, 12:41 PM   #2
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Zero fork rake (offset) results in longer trail and sluggish handling.

Framebuilder Dave Moulton says in one of his blogs on rake and trail that for a road bike with a 73 degree head angle the optimum trail seems to be around 2 to 2 ½ inches (5 to 6.3cm.).

From the diagram below, you can see that zero rake on a bike with a 73 degree head tube angle and 700C x 25C tires, the trail would be a whopping 103.3 mm (10.33 cm).

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Old 08-14-09, 01:45 PM   #3
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Thanks for the info Scooper. So a bike with no rake will have a strong centering force which makes it harder to turn (but easier to balance). That sounds like it would be good for a learner bike for kids, where the turns are going to be gentle and the main challenge is balance.
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Old 08-14-09, 03:10 PM   #4
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I think a lot of the incorrect "conventional wisdom" about rake got started when we used to use steeper angles and short rake on racing bikes. This thread reminded me I want to build an adjustable rake fork for experimenting with this.
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Old 08-15-09, 12:36 PM   #5
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Quote:
Originally Posted by Scooper View Post
Zero fork rake (offset) results in longer trail and sluggish handling.

Framebuilder Dave Moulton says in one of his blogs on rake and trail that for a road bike with a 73 degree head angle the optimum trail seems to be around 2 to 2 ½ inches (5 to 6.3cm.).
The huge majority of forks these days fall in the 40 - 45 mm range and only custom made frames tend to use anything beyond 47 mm. My personal frames have 72.5 degree head angle and a 45 mm fork. I've never seen a frame with a zero rake fork although I'm sure they are out there.
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Old 08-15-09, 02:55 PM   #6
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It may seem that for a beginner, the slower the response, the better, but too slow is not safe, either. It's best to stay within the range of normal practice. Bikes today seem to have trail ranging from perhaps 65 mm (Mrs. Road Fan's hybrid bike) to around 20 mm (a builder here on BF used this for his own touring bike). It really doesn't take a big change in trail to make a significant difference in a bike's response.

You could make a bike with a negative rake, and get a LOT more trail. But what would it be good for?
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Old 08-15-09, 03:34 PM   #7
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Just as an aside...

I have had a couple of hipster fixie riding guys ask me to build them a fixed gear frame and fork, fork with zero rake and straight blades. My response was "no". I assume they wanted the fork for aesthetics and trend factor, and had no idea what that would mean in terms of handling.
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Old 08-15-09, 04:40 PM   #8
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There is a builder in France that makes frames with 0 rake. Can't find the link sorry. The head angle is super steep (like 78*) to acheive normal trails. But as a result the stem is super short because of the necessary long TT. Was it 700c or 650c can't remember.

Edit: here it is... doesn't look so bad.

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Old 08-15-09, 07:53 PM   #9
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traditional reason for zero rake forks is for doing tricks. I forget what the type of competition is called, but fixies with zero rake forks are required. Last video of this type of riding was actually a synchronized pair.

Video example

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