I never really "get" the fork rake thing, so here's but another question. If I have a 43 mm rake fork, and I replace it with a 50 mm, what happens? Does the bike get more stable and more compliant (all else equal)? Is there a negative? Thanks.
fork rake
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No, you got it backwards
If you put a 50mm rake fork on the bike, it'll get really squirrely at any speed over, oh, maybe 25mph. The negative is that you're gonna crash and bust your @$$.
BTW, 43mm to 50mm is a huge difference.
If you put a 50mm rake fork on the bike, it'll get really squirrely at any speed over, oh, maybe 25mph. The negative is that you're gonna crash and bust your @$$.
BTW, 43mm to 50mm is a huge difference.
Rake
Doug, I've never heard of a 50mm rake, but I can tell you that increasing rake will decrease trail and let the bike turn quicker (at least at higher speeds) and may cause it to feel less stable, even though the wheelbase will be longer. Wheel base does not effect how the front of the bike turns. Going from 43 to 50 would be a very big change. I changed the fork on my primary bike from 43 to 40 (both Ouzo Pro) and I could definitely feel a difference, and in this case both forks have advantages. The 40 results in more trail and enhances the bikes ability to return to or maintain a straight path.
Edit: (I did not see Dave's post before writing mine)
~Al
Doug, I've never heard of a 50mm rake, but I can tell you that increasing rake will decrease trail and let the bike turn quicker (at least at higher speeds) and may cause it to feel less stable, even though the wheelbase will be longer. Wheel base does not effect how the front of the bike turns. Going from 43 to 50 would be a very big change. I changed the fork on my primary bike from 43 to 40 (both Ouzo Pro) and I could definitely feel a difference, and in this case both forks have advantages. The 40 results in more trail and enhances the bikes ability to return to or maintain a straight path.
Edit: (I did not see Dave's post before writing mine)
~Al
maybe not...
50mm rake forks are not that uncommon. A lot of the traditional Italian frames with steel forks had 50mm of rake with appropriate HTA to create a reasonabe amount of trail.
The formula for trail is pretty simple:
Trail = (R/tanH) - (rake/sinH)
Changing from a 43 to a 50mm rake would increase the tail by bit more than 7mm.
For example, a Colango with a 71.5HTA and 50mm of rake would have a trail of 59.7mm and a bike with a 72 degree HTA would still have a trail of 56.6mm, which is not excessively small.
If you have a bike with a 73 degree HTA, then the trail would be a very twitchy 50.4mm.
I made my own excel program for calculating trail if anyone wants it.
50mm rake forks are not that uncommon. A lot of the traditional Italian frames with steel forks had 50mm of rake with appropriate HTA to create a reasonabe amount of trail.
The formula for trail is pretty simple:
Trail = (R/tanH) - (rake/sinH)
Changing from a 43 to a 50mm rake would increase the tail by bit more than 7mm.
For example, a Colango with a 71.5HTA and 50mm of rake would have a trail of 59.7mm and a bike with a 72 degree HTA would still have a trail of 56.6mm, which is not excessively small.
If you have a bike with a 73 degree HTA, then the trail would be a very twitchy 50.4mm.
I made my own excel program for calculating trail if anyone wants it.
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In years past...
50 mm of fork rake used to be common. Of course, the steering head angles were shallower back then as well.
Some common modern combinations include:
74 degree head angle w/40 mm fork rake
73 degree head angle w/43-45 fork rake
72 degree head angle w/50 mm fork rake
I've used a 50 mm rake fork on two different frames that have used a head angles of 73 degrees and have found them to have medium-fast steering with lots of bump compliance (key point here).
As a side note, Richard Sachs builds most (all?) of his frames using 50+ mm of fork rake. What does that tell you?
Ed
50 mm of fork rake used to be common. Of course, the steering head angles were shallower back then as well.
Some common modern combinations include:
74 degree head angle w/40 mm fork rake
73 degree head angle w/43-45 fork rake
72 degree head angle w/50 mm fork rake
I've used a 50 mm rake fork on two different frames that have used a head angles of 73 degrees and have found them to have medium-fast steering with lots of bump compliance (key point here).
As a side note, Richard Sachs builds most (all?) of his frames using 50+ mm of fork rake. What does that tell you?
Ed
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591 Posts
rake
Trek makes the 520 with a fork offset of 52mm and the Hilo with an offst of 38.Al1943 said:
Rake
Here is a calculator for Rake, HTA, and Trail. Best to set all units to metric.
http://www.kreuzotter.de/english/elenk.htm
~Al
Here is a calculator for Rake, HTA, and Trail. Best to set all units to metric.
http://www.kreuzotter.de/english/elenk.htm
~Al
More rake = less trail
The more trail, the greater the stability. The greater the rake, the less trail.
Think about those choppers you see on the roads. They have very slack head tube angles and therefore have a lot of trail. They are very stable, but the payback is that wheel flop becomes a serious issue.
With a steep headtube (like one of those silly scooters), increased rake decreases the trail. Once trail vanishes altogether, the wheel will want to turn around like a caster on a grocery cart. Very unstable.
If you were to increase rake on your road bike to the point where trail is zero, the wheel will naturally want to pivot all the way around.
Roadbike designers have found that trail of 56-59 mm is optimal for stability and control. Everytime I look at one of those scooters with a hint of trail, I think about how easy it would be for the front wheel to turn in a real hurry. No telling how many kids are getting hurt on those things.
-Plus Vite...
Doug, let me add a little something.DougSloan said:
The more trail, the greater the stability. The greater the rake, the less trail.
Think about those choppers you see on the roads. They have very slack head tube angles and therefore have a lot of trail. They are very stable, but the payback is that wheel flop becomes a serious issue.
With a steep headtube (like one of those silly scooters), increased rake decreases the trail. Once trail vanishes altogether, the wheel will want to turn around like a caster on a grocery cart. Very unstable.
If you were to increase rake on your road bike to the point where trail is zero, the wheel will naturally want to pivot all the way around.
Roadbike designers have found that trail of 56-59 mm is optimal for stability and control. Everytime I look at one of those scooters with a hint of trail, I think about how easy it would be for the front wheel to turn in a real hurry. No telling how many kids are getting hurt on those things.
-Plus Vite...
The wheel on a grocery cart turns around BECAUSE of the trail. Push or pull a cart and the Trail moves the wheel in the opposite direction of the movement of the cart - just like a bike when we push it forward. They both have the tire contact patch behind the steering axis.plus_vite said:
Disagreement here?
Let me revise and extend my comments:
Trail, in and of itself, can't do anything. It's just a definition of the physical relationship of a wheel, its steering axis, and direction of travel. Bikes always move forward, so that's a given.
Doug asked what would happen if you increased rake. The obvious answer is this: trail decreases and the result is less stability. When you get to the magic level of zero trail, then a wheel will want to spin into a position in which it has trail, i.e., the caster effect. You could fight and hold it with some kind of steering mechanism, but when left to its own, the wheel will naturally spin 180 degrees into a position where it has trail.
Technically speaking, when you move a grocery cart forward and back, the switching of wheel directions occurs because trail disappears in relation to the direction of motion just as you describe it above.
-PV
I am not sure we are disagreeing. My goal is to make the idea of rake/trail as understandable and as clear as possible.Mike T. said:
Let me revise and extend my comments:
Trail, in and of itself, can't do anything. It's just a definition of the physical relationship of a wheel, its steering axis, and direction of travel. Bikes always move forward, so that's a given.
Doug asked what would happen if you increased rake. The obvious answer is this: trail decreases and the result is less stability. When you get to the magic level of zero trail, then a wheel will want to spin into a position in which it has trail, i.e., the caster effect. You could fight and hold it with some kind of steering mechanism, but when left to its own, the wheel will naturally spin 180 degrees into a position where it has trail.
Technically speaking, when you move a grocery cart forward and back, the switching of wheel directions occurs because trail disappears in relation to the direction of motion just as you describe it above.
-PV
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