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Recumbent What IS that thing?! Recumbents may be odd looking, but they have many advantages over a "wedgie" bicycle. Discuss the in's and out's recumbent lifestyle in the recumbent forum.

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Old 05-23-07, 10:07 AM   #1
Bikewer
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Looking for input

This is the homebuilt recumbent that I more-or-less completed last season:



It's built up along the lines of the "bolt together" plans that have been available on the web for some time.
This is my more-refined version.
Overall, it works quite well. Comes in at 38 pounds, which is not bad for a bolted-together item. The seating position is quite comfortable, I've got the gearing sorted out so it's useable throughout the range, and the brakes are....adequate. (The cobbled-together front is marginal, but the rear V-brake works well.)

I've refined the chainline since I last posted, the top run is nearly straight now, with that top idler only working to keep the chain from contacting the brake arm.

I only have one problem, high-speed instability. Cruising around on level ground is fine, it tracks straight and doesn't wiggle or pull. However, going down hill is "pucker time". It doesn't do anything really frightening like wobbling, but the steering gets highly sensitive and it does not feel secure at all.

As a result, I have to feather the brakes on even minor hills, and am denied the fun of higher-speed downhills.
As far as I can determine, everything is in-line and the bike has no tendency to pull.
I feel that it's the steering angle; the front fork angle is just too steep.

I don't see any easy way to change that; I'm considering working at the rear end. I removed the "shock absorber" from this cheap aluminum frame and replaced it with a strut. I'm considering shortening the strut a bit. Looks like I can lower the frame at least a few inches without any problem; this might lower the center of gravity slightly and result in a degree or three of increased fork rake.

Any ideas? (I don't weld...)
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Old 05-23-07, 10:38 AM   #2
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I suspect your front fork needs to be on a slightly greater angle, or you need to add more "tiller" to the handle bars. That is, add a short stem or something to bring them closer to you. This will decrease your turning radius slightly, but should smooth out some of the twitchiness.
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Old 05-23-07, 10:48 AM   #3
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Hello Bikewer,

Putting a straightedge on the image on my screen, I would judge you have no (or not enough) trail. There are various calculators out there and several discussions I think in the forums, here is one. If it measures out zero or close, then you need to bend the fork or get a new one.

http://www.bentrideronline.com/messa...ht=trail+steer
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Old 05-23-07, 11:55 AM   #4
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Quote:
Originally Posted by Wilbur Bud
Hello Bikewer,

Putting a straightedge on the image on my screen, I would judge you have no (or not enough) trail. There are various calculators out there and several discussions I think in the forums, here is one. If it measures out zero or close, then you need to bend the fork or get a new one....
He appears to be using a 20" wheel on the front of a 26" bike frame and fork. Definitely not enough trail, and too steep a head angle too.
~
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Old 05-23-07, 12:38 PM   #5
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Quote:
Originally Posted by Doug5150
He appears to be using a 20" wheel on the front of a 26" bike frame and fork. Definitely not enough trail, and too steep a head angle too.
~
Heh, so I guess if he converts to a highracer, then he'll fix the wobble AND the front brake problem all at once!
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Old 05-23-07, 01:31 PM   #6
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That is indeed a 20" wheel. I actually considered trying a 26" front, which would allow me to use the existing brake studs. Lowering the rear end and intalling a 26" front wheel might result in an interesting riding position....

A couple of guys around the university where I work have "high racers" and seem quite happy with them.
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Old 05-23-07, 07:33 PM   #7
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Quote:
Originally Posted by Bikewer
That is indeed a 20" wheel. I actually considered trying a 26" front, which would allow me to use the existing brake studs. Lowering the rear end and intalling a 26" front wheel might result in an interesting riding position....

A couple of guys around the university where I work have "high racers" and seem quite happy with them.
You could do this two ways and get the proper steering angles--either use two 26" wheels, or two 20's, though with the 20's you'd need brakes somehow.
~
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Old 05-23-07, 08:04 PM   #8
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I agree: not enough trail. Considering the pedal height now, putting a 26" on the front might make the pedals too high though. Putting a smaller rear wheel would do the trick, too. Dual 24" perhaps, using the same fork?
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Old 05-23-07, 09:12 PM   #9
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Quote:
Originally Posted by Wilbur Bud
then you need to bend the fork ...
When we were kids riding our mothers's mixte's, we would actually straighten the forks!
So clamp the fork crown in a vice and use a BIG steel pipe to bend the fork blades.
Crude, but effective.

and you didn't hear it from me!
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Old 05-24-07, 03:07 PM   #10
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Quote:
Originally Posted by Bikewer
This is the homebuilt recumbent that I more-or-less completed last season:



It's built up along the lines of the "bolt together" plans that have been available on the web for some time.
This is my more-refined version.
Overall, it works quite well. Comes in at 38 pounds, which is not bad for a bolted-together item. The seating position is quite comfortable, I've got the gearing sorted out so it's useable throughout the range, and the brakes are....adequate. (The cobbled-together front is marginal, but the rear V-brake works well.)

I've refined the chainline since I last posted, the top run is nearly straight now, with that top idler only working to keep the chain from contacting the brake arm.

I only have one problem, high-speed instability. Cruising around on level ground is fine, it tracks straight and doesn't wiggle or pull. However, going down hill is "pucker time". It doesn't do anything really frightening like wobbling, but the steering gets highly sensitive and it does not feel secure at all.

As a result, I have to feather the brakes on even minor hills, and am denied the fun of higher-speed downhills.
As far as I can determine, everything is in-line and the bike has no tendency to pull.
I feel that it's the steering angle; the front fork angle is just too steep.

I don't see any easy way to change that; I'm considering working at the rear end. I removed the "shock absorber" from this cheap aluminum frame and replaced it with a strut. I'm considering shortening the strut a bit. Looks like I can lower the frame at least a few inches without any problem; this might lower the center of gravity slightly and result in a degree or three of increased fork rake.

Any ideas? (I don't weld...)
You might look into rake and trail, how they interact, and how they affect handling. (Head tube angle is also a factor; but you can take a given head tube angle, by changing the rake and trail.) The latest edition of Bicycling Science goes into it fairly well.

You could also try out some other forks, with different rake -- ride them, test them out and see how handling is affected. (Or you could find a junker fork and experiment with it (carefully).)

If you have very little rake, you may have more trail than you should have. Two and five eighths inches of trail is fairly high but still normal -- if you have much more, though, you may be moving outside the parameters for good handling.
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Old 05-24-07, 03:11 PM   #11
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Another possibility: Check out some similar recumbent designs, with similar geometry, and see what the rake and trail are.
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Old 05-24-07, 05:51 PM   #12
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It's clearly an estimate, but I don't think trail is a major issue here - as seen in the photo. The handle bars are not visible so estimating tiller (if any) is impossible, but looks as though the grips might be forward of the steer axis. If true, then modify the stem to reposition the handle bar further aft and behind the steering axis - as drawn (in red). NOTE; You can reduce steering sensitivity by simply widening the handlebars (grips further apart), and/or moving the CoG further aft (moving the seat rearward and shortening the boom)


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