Idea for adjustable rake fork
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Idea for adjustable rake fork
I've been considering the idea of tinkering with changing fork rake. The problem is that existing "adjustable" forks only have a limited range of rake because they adjust at the dropout. I wondered if it might be possible to adjust at the crown instead.
Instead of a fixed crown, I was thinking weld or braze a round, horizontal crossbar to the bottom of the steerer. At the top end of each fork blade, a tube with a matching inner diameter surrounded by a couple of seat lugs. The blades would be able to pivot forward and back to a wide range of positions without affecting clearance between the bottom of the "crown" and the tire, although extreme positions would affect crown distance to the ground. Essentially, each fork blade would be attached like a heavy duty version of a mountain bike bar-end. My only worry is whether there would be enough friction and clamping force to keep the fork from slipping and rotating around the crown. One advantage would be that it would be possible to install a disc brake on this since the front axle position is fixed relative to the disc tab.
Would this work? Has it ever been done before?
Instead of a fixed crown, I was thinking weld or braze a round, horizontal crossbar to the bottom of the steerer. At the top end of each fork blade, a tube with a matching inner diameter surrounded by a couple of seat lugs. The blades would be able to pivot forward and back to a wide range of positions without affecting clearance between the bottom of the "crown" and the tire, although extreme positions would affect crown distance to the ground. Essentially, each fork blade would be attached like a heavy duty version of a mountain bike bar-end. My only worry is whether there would be enough friction and clamping force to keep the fork from slipping and rotating around the crown. One advantage would be that it would be possible to install a disc brake on this since the front axle position is fixed relative to the disc tab.
Would this work? Has it ever been done before?
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is this just for experimenting with adjustable rake, or for general use? I have considered doing this, and truth be told there is no good reason not to use a fork with, say, 4 dropout positions. It's probably the safest way to go and it would be easy to make it safe. There simply is no advantage to being able to have a fully adjustable rake.
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Well, among other things, I am interested in recumbents as well. The problem there is that recumbent geometry isn't as pat as on diamond frames. There's a lot more variation, not just the same old 71-73 degree head angle with stock rake. So to properly dial in steering with any given head angle if you're not riding a production recumbent, you'd need either a few different forks or one adjustable fork. Ideally, adjustable head angle would also be on the table, but that's very difficult. Even Angleset only gives you a couple of degrees variation, whereas recumbents can go as slack as 45 degrees (and require several inches of rake), a big difference from the 70s. Maybe if there was some kind of pivot on the head tube like on a swing arm. I suppose the front end is no more highly stressed than the rear end of a full suspension mountain bike, but roll stiffness would be much more critical at the head tube joint.
As for your multiple dropout positions, that would require readjustment of v-brakes every time you changed position, otherwise you risk cutting the tire sidewalls or pads falling into the spokes. Disc brake readjustment would be impossible, unless you made the disc tab also movable to any of four positions.
As for your multiple dropout positions, that would require readjustment of v-brakes every time you changed position, otherwise you risk cutting the tire sidewalls or pads falling into the spokes. Disc brake readjustment would be impossible, unless you made the disc tab also movable to any of four positions.
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If it's a prototype recumbent it can be as ugly as you like - I remember several prototypes made up out of sheet MDF which allowed the head tube to be bolted on at any angle, BB position to be varied, etc. for forks, you can have a standard fork but with a long flat dropout with several slots.
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is this just for experimenting with adjustable rake, or for general use? I have considered doing this, and truth be told there is no good reason not to use a fork with, say, 4 dropout positions. It's probably the safest way to go and it would be easy to make it safe. There simply is no advantage to being able to have a fully adjustable rake.
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My only worry is whether there would be enough friction and clamping force to keep the fork from slipping and rotating around the crown. One advantage would be that it would be possible to install a disc brake on this since the front axle position is fixed relative to the disc tab.
There are some people making their own lefty forks. You might be able to make a motorcycle style fork that is adjustable.
Any way you do this it's going to require careful design. Brakes are an issue with multiple dropouts, no doubt. One approach for disks would be to make an insert like the rear adjustable brakes that would move with the chosen dropout. For example, This one
A unicrown fork costs $50 plus whatever the dropout costs. An adjustable crown will cost hundreds in machining. You can build many forks for that. I don't doubt there have been some weird recumbent designs in the past, but I am also familiar with a number that use fairly standard front ends, i.e. road forks. I'm not sure why you would go all experimental when that seems to work fairly well
Last edited by unterhausen; 04-02-13 at 05:54 AM.
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The dropout with the 4 positions doesn't need to be flat- it can have the radius that follows the brake pad location. That way the pads would only need tilt adjustment rather than adjustment of radial position. If they require adjustment at all.
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I've been considering the idea of tinkering with changing fork rake. The problem is that existing "adjustable" forks only have a limited range of rake because they adjust at the dropout. I wondered if it might be possible to adjust at the crown instead.
Instead of a fixed crown, I was thinking weld or braze a round, horizontal crossbar to the bottom of the steerer. At the top end of each fork blade, a tube with a matching inner diameter surrounded by a couple of seat lugs. The blades would be able to pivot forward and back to a wide range of positions without affecting clearance between the bottom of the "crown" and the tire, although extreme positions would affect crown distance to the ground. Essentially, each fork blade would be attached like a heavy duty version of a mountain bike bar-end. My only worry is whether there would be enough friction and clamping force to keep the fork from slipping and rotating around the crown. One advantage would be that it would be possible to install a disc brake on this since the front axle position is fixed relative to the disc tab.
Would this work? Has it ever been done before?
Instead of a fixed crown, I was thinking weld or braze a round, horizontal crossbar to the bottom of the steerer. At the top end of each fork blade, a tube with a matching inner diameter surrounded by a couple of seat lugs. The blades would be able to pivot forward and back to a wide range of positions without affecting clearance between the bottom of the "crown" and the tire, although extreme positions would affect crown distance to the ground. Essentially, each fork blade would be attached like a heavy duty version of a mountain bike bar-end. My only worry is whether there would be enough friction and clamping force to keep the fork from slipping and rotating around the crown. One advantage would be that it would be possible to install a disc brake on this since the front axle position is fixed relative to the disc tab.
Would this work? Has it ever been done before?
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Okay, I guess I'll figure out how to add some kind of slotted dropout to a cheap used fork, with no front brake during testing. Once I'm sure of the optimum rake, I'll get triple tree clamps CNC machined with the correct forward offset then install untapered 1" 4130 tubing as straight fork blades, welded to Wright-style dropouts. Those tubes should be plenty strong to resist disk brake forces, and easy to align since there's no bending involved to create rake.
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How much adjustment range are you looking for?
I built an adjustable rake fork too many years ago using some long slot Campy 1010A rear dropouts in place of the normal front drops. I forget the range but it was over an inch as I recall.
Could you build a fork with slotted fork drops and get the range you desire?
dave
I built an adjustable rake fork too many years ago using some long slot Campy 1010A rear dropouts in place of the normal front drops. I forget the range but it was over an inch as I recall.
Could you build a fork with slotted fork drops and get the range you desire?
dave
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How much adjustment range are you looking for?
I built an adjustable rake fork too many years ago using some long slot Campy 1010A rear dropouts in place of the normal front drops. I forget the range but it was over an inch as I recall.
Could you build a fork with slotted fork drops and get the range you desire?
dave
I built an adjustable rake fork too many years ago using some long slot Campy 1010A rear dropouts in place of the normal front drops. I forget the range but it was over an inch as I recall.
Could you build a fork with slotted fork drops and get the range you desire?
dave
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The problem is that you're all thinking along the lines of conventional bikes. Steering geometry is well known with those and fork rake is in a very narrow range. With long wheelbase recumbents, it's very different. There are only a handful of such bikes with large front wheels. Do a Google image search for "dual 26 recumbent" and see the long wheelbase models with direct steering, not remote steering. Look at the enormous amount of rake their forks need because of the slack head angle needed to keep tiller effect manageable. We're talking four to five inches of rake, not a couple of inches like a standard fork. I'm sure if you have the resources of a bike company, it's easy to build as many prototype forks as you need. Those companies don't disclose their frame geometry, so it will be just guesswork based on pictures for those who want to try to roll their own. Don't judge if you're not familiar with this field.
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Having owned what might be considered the grand daddy of the current recumbent era (an Avatar 2000) and serviced (then ridden) dozens more i feel i have a respect and understanding of the recumbent steering situation. I agree that there are a wide range of geometries. Having many packaging restrictions (and resulting tire sizes and steerer angles) and handle bar configurations (meaning the steering inputs and leverages are all over the place) it's no wonder that recumbent are so varied. Add the different use of the rider's body to help control balance and recumbent can be quite a handful at some speeds.
Your first post did focus on the rake as the variable, not steerer angle or wheel diameter. So please excuse us if we also focus on rake. And, yes, in most of our experience an 1.5" rake range is all that's going to be played with (35mm-72mm of rake).
It seems to me that some of your research might be with steerer angle or wheel diameter too so a bolted together fork might prove most adjustable. This way you could change just one part to change the fork's spec and continue to use the other parts. Along these lines you might consider having a bolt on drop out. This way you could just cut out/machine a different axle slot location and still use the rest of the fork. Andy.
Your first post did focus on the rake as the variable, not steerer angle or wheel diameter. So please excuse us if we also focus on rake. And, yes, in most of our experience an 1.5" rake range is all that's going to be played with (35mm-72mm of rake).
It seems to me that some of your research might be with steerer angle or wheel diameter too so a bolted together fork might prove most adjustable. This way you could change just one part to change the fork's spec and continue to use the other parts. Along these lines you might consider having a bolt on drop out. This way you could just cut out/machine a different axle slot location and still use the rest of the fork. Andy.
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The adjustable rake project was geared toward better understanding speed wobble and how trail might affect it.
As for the recumbent deal.........I don't know squat about them. I just wondered how much adjustment range the OP might need to get the answers he's looking for. It's clear to me now any long slot adjustable will not have enough range to get the job done.
The issue with having a fork with blades that pivot fore/aft at the crown to adjust rake (aside from safety which would be a big concern IMO) is that as you add rake the span (length) if the fork would change and with it the head angle.
I don't think you are going to find a quick and easy way to make an adjustable rake with a wide range........short range sure..........long range you get into span issues.
That said I'd try something like this - I'd get an old suspension fork that has slip in blades that get pinched in place with side bolts on the crown. Pull the suspension blades out and toss them and make/have made new blades that you can slip into the MTB crown that have differing rakes to test. this way you can slip one pair out, slide another pair in, put the front wheel in place and tighten to align the blades and then tighten them in the crown. The advantage of this is that you can control span for a given rake and keep the variables down to one.
You could make a few different blade sets with some adjustment at the dropout end so maybe you could have just 2 or 3 sets and a huge range with small increments.
Does this make sense?
Dave
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I'm thinking in terms of bikes where if your fork collapses, you get injured, which is all bikes.
I got a short education about 'bents, but I'm still pretty sure you would be better off with an adjustable dropout. I probably would decide where the middle of my design range was, and put the fork blade itself near there. The blade would bisect a bar-shaped dropout that would accept an insert with disk brake mounts machined into it. This dropout could be 6" long. If you slotted the blade, the slots wouldn't have to be continuous. Rake isn't all that sensitive as long as you are close. Triangulate the front and back of the dropout to the fork blade with 1/2" tubing. I think it would work pretty well and be a lot safer than trying to clamp at the crown
I have considered building a fork like this for an upright. A little overbuilt, but I don't see that as an issue.
I got a short education about 'bents, but I'm still pretty sure you would be better off with an adjustable dropout. I probably would decide where the middle of my design range was, and put the fork blade itself near there. The blade would bisect a bar-shaped dropout that would accept an insert with disk brake mounts machined into it. This dropout could be 6" long. If you slotted the blade, the slots wouldn't have to be continuous. Rake isn't all that sensitive as long as you are close. Triangulate the front and back of the dropout to the fork blade with 1/2" tubing. I think it would work pretty well and be a lot safer than trying to clamp at the crown
I have considered building a fork like this for an upright. A little overbuilt, but I don't see that as an issue.
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Just to make my posts clear--- I do not recommend that forks are used where the parts can pivot or collapse. i was suggesting that a fork with clamping between the steerer, crown blade and drop out would offer the most flexibility to swap out parts to change the geometry with the least cost. Kind of like the millions of suspension crowns with pinch bolts or the Switchblade forks of the pre suspension era.
Dave- You hit my question on it's head but without any answer. What did you find out with an adjustable rake WRT stability/speed wobble?
While i have an interest in recumbents and respect to what they do for certain riders, i build traditional "wedgies". It is this geometry that i was asking Dave about. My comments to duopar were to let him know that not all on this forum are narrow minded. Andy.
Dave- You hit my question on it's head but without any answer. What did you find out with an adjustable rake WRT stability/speed wobble?
While i have an interest in recumbents and respect to what they do for certain riders, i build traditional "wedgies". It is this geometry that i was asking Dave about. My comments to duopar were to let him know that not all on this forum are narrow minded. Andy.
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I saw no 'bent disrespect, so I have no idea where that notion is coming from.
As explained to me after I asked a 'bent riding engineer what the OP is looking for, the tiller effect is having too long of an moment arm over the fork. This results in slow steering and makes control impossible. So for a long wheelbase 'bent, you can end up with a very slack head tube angle to make the moment arm shorter.
I think that if the OP actually drew up a design, even fuzzing over the crown area, he would find that pivoting the crown doesn't really get him what he wants. As Dave Kirk notes above, you are trading rake for axle to crown distance. And with a slack head tube angle, the moment on the crown is increased over road-like head angles.
As explained to me after I asked a 'bent riding engineer what the OP is looking for, the tiller effect is having too long of an moment arm over the fork. This results in slow steering and makes control impossible. So for a long wheelbase 'bent, you can end up with a very slack head tube angle to make the moment arm shorter.
I think that if the OP actually drew up a design, even fuzzing over the crown area, he would find that pivoting the crown doesn't really get him what he wants. As Dave Kirk notes above, you are trading rake for axle to crown distance. And with a slack head tube angle, the moment on the crown is increased over road-like head angles.
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Just to make my posts clear--- I do not recommend that forks are used where the parts can pivot or collapse. i was suggesting that a fork with clamping between the steerer, crown blade and drop out would offer the most flexibility to swap out parts to change the geometry with the least cost. Kind of like the millions of suspension crowns with pinch bolts or the Switchblade forks of the pre suspension era.
Dave- You hit my question on it's head but without any answer. What did you find out with an adjustable rake WRT stability/speed wobble?
While i have an interest in recumbents and respect to what they do for certain riders, i build traditional "wedgies". It is this geometry that i was asking Dave about. My comments to duopar were to let him know that not all on this forum are narrow minded. Andy.
Dave- You hit my question on it's head but without any answer. What did you find out with an adjustable rake WRT stability/speed wobble?
While i have an interest in recumbents and respect to what they do for certain riders, i build traditional "wedgies". It is this geometry that i was asking Dave about. My comments to duopar were to let him know that not all on this forum are narrow minded. Andy.
I didn't go there as I didn't want to get into too much thread drift...............so with that said I can say that when changing trail we were able to change the frequency of the vibration but not eliminate the oscillation itself. So it was an interesting experiment that taught us something but did not answer the bigger question.
Back to the tandem fork -
Dave
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As explained to me after I asked a 'bent riding engineer what the OP is looking for, the tiller effect is having too long of an moment arm over the fork. This results in slow steering and makes control impossible. So for a long wheelbase 'bent, you can end up with a very slack head tube angle to make the moment arm shorter.
f = b sin ∂ cos ∂ *
Where:
f = "wheel flop factor," the distance that the center of the front wheel axle is lowered when the handlebars are rotated from the straight ahead position to a position 90 degrees away from straight ahead
b = trail
∂ = head angle
Where:
f = "wheel flop factor," the distance that the center of the front wheel axle is lowered when the handlebars are rotated from the straight ahead position to a position 90 degrees away from straight ahead
b = trail
∂ = head angle
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That said I'd try something like this - I'd get an old suspension fork that has slip in blades that get pinched in place with side bolts on the crown. Pull the suspension blades out and toss them and make/have made new blades that you can slip into the MTB crown that have differing rakes to test. this way you can slip one pair out, slide another pair in, put the front wheel in place and tighten to align the blades and then tighten them in the crown. The advantage of this is that you can control span for a given rake and keep the variables down to one.
As logn as you are expirimenting with fork adjustability, the RS fork would also allow you to easily alter the axel-to-crown measurement, no lip at top of the crown pinch hole for the legs.
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apparently they go with very low trail to improve steering response. I was going to mention that a lot can be learned from bike design software. If you pick a head angle from the desired tiller distance, then you have reduced the adjustment needed in the fork by quite a bit. Minimizing wheel flop is a problem, and there are only a few trail values that make sense
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technical term is "lawn chair with wheels" or LCWW
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I think of mine as being an oversized potato chip with wheels, but OPCWW is too clumsy.
The OP should read this: https://bikesmithdesign.com/Bent_Design.html
Most specifically this: https://bikesmithdesign.com/Design/12Steps.html
My thoughts are to:
Set head tube angle to set the desired amount of tiller.
Set rake to yield the desired amount of trail (not much on a LWB LCWW)
Fork axle to crown comes out in the wash.
A long, horizontal slotted end on a fork would have the very beneficial aspect of not messing with the head tube angle as you adjusted the rake (only).
The OP should read this: https://bikesmithdesign.com/Bent_Design.html
Most specifically this: https://bikesmithdesign.com/Design/12Steps.html
My thoughts are to:
Set head tube angle to set the desired amount of tiller.
Set rake to yield the desired amount of trail (not much on a LWB LCWW)
Fork axle to crown comes out in the wash.
A long, horizontal slotted end on a fork would have the very beneficial aspect of not messing with the head tube angle as you adjusted the rake (only).
Last edited by Steamer; 04-08-13 at 11:47 AM. Reason: spelling
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