Recumbent - Please critique my design

It's project time! :D

I have an old CrMo '80s road bike that's in good shape but way too small for me, a cheap alu framed mountain bike with a good groupset (Deore triple), a desire to try a 'bent, a very good quality 3-phase pulsed MIG welder and the skill to use it. All I need now is a design, this is what I've come up with so far....

The rear triangle (seat tube, seatstays, chainstays) is fron the old road bike so the rear wheel is 700c size, the front is 20". That rear triangle is braced to the main horizontal frame with some 1" square CrMo tube, which will also support the seat. The rest of the frame is 1.5" square setion CrMo.

I don't think I'll be making the boom adjustable as I'll be the only one riding it. I have no idea what I'll be doing for a seat yet, maybe do something in fiberglass or CF along with some sort of aerodynamic rear fairing.

I'm inviting you to rip my design to bits, point out everything that is or could be wrong, anything that could or should be improved or just different. All feedback is welcome. Remember that the design brief is to keep it simple to build at home as a one-off, using what I already have to keep cost under control.

I think you've got the general idea down, assuming you have the scale right. It'll produce a nice quasi-lowracer with minimal chainline problems - only one idler needed on the power side, plus a small (100mm or so) length of chain tube to help guide the return side of the chain. You may want to cut the old seat post down to about half what's shown, then bend the seat stays down to match. Otherwise the tube may end up hitting you in the head/helmet when riding. You could go with any number of commercial seats if you merely chose ahead of time and then used the appropriate size tubing for the frame. Or, you could make your own hardshell by covering plywood with fiberglass/carbon.

I think you've got the general idea down, assuming you have the scale right. It'll produce a nice quasi-lowracer with minimal chainline problems - only one idler needed on the power side, plus a small (100mm or so) length of chain tube to help guide the return side of the chain. You may want to cut the old seat post down to about half what's shown, then bend the seat stays down to match. Otherwise the tube may end up hitting you in the head/helmet when riding. You could go with any number of commercial seats if you merely chose ahead of time and then used the appropriate size tubing for the frame. Or, you could make your own hardshell by covering plywood with fiberglass/carbon.

Thanks BP. Yes, that drawing is to scale with my body dimensions represented.

I see what you mean about the top of the triangle but I was going to use it to mount a headrest and to support a rear fairing. I also want to leave the rear brake caliper where it is so I'm wary of modifying that area. The chainline as you describe it is exactly what I had in mind during the design, seems to be the least complicated to build. If I can get a commercial seat at some sort of reasonable price here I'll go with that.

Do you think 1.5" X 1/16" thick square CrMo tube will be strong enough for the frame without being too heavy?

It'll certainly suffice, but a bit expensive at over $12 a foot. IMHO, 2" x .035 4130 round tube would be a much better choice - and a lot lighter. Or you could make do with 2" x .062 mild steel exhaust tubing, and have a mufler shop create "Z" bends to eliminate time consuming angle joints. Also, you might want to swing by this site before you start cutting:
http://bikesmithdesign.com/Design/12Steps.html

Thaks for the link, that's a fantastic site! Just the information I needed to put some numbers on the steering geometry. :)

I have been leaning towards square tube just because it's easy to get everything lined up, but you're right, round tubing in thinner wall would be a much better choice. Replacing those two welded joints with bends cancels out much of the argument for square tube. I also just happen to have a hydraulic tube bender at home. ;) ...makes you wander why I didn't go with round in the first place, doesn't it :o

My local supplier doesn't have 2" in .035, the smallest they have is .049 (1.2mm) wall thickness. A bit heavier but not enough to worry about I think.

OK, here's the updated version using 2" round tube bent to shape. Steering geometry not finalised yet.

I think I quite like it, it looks 'right' if you know what I mean. :o

How you gonna bend the tubes?

Here's a design for a seat that you might be interested in:

http://www.eng.uwaterloo.ca/~bkwillia/contour.html

Yeah, bending chormoly ain't no joke. You'd do better to cut and weld at the correct angles. square tubes would make it MUCH easier.

Your points about bending are valid ones. I have a ram type bender, it has quite good dies but whether it will bend the material in question is a matter of experimentation. Even then I'm also concerned about the two bends being perfectly aligned on the same plane and not ending up with a pretzel!

I've decided to forego the bending experiments for this one, instead I'm going to simplify it by using a straight tube instead. It satisfies the initial requirements of keeping it simple, inexpensive, and to build it 100% at home.

It has morphed into more of a highracer rather than the lowracer I originally envisioned, but it should still be fast enough I'm sure. At least it will be much safer as a commuter.

What do you think?

Your points about bending are valid ones. I have a ram type bender, it has quite good dies but whether it will bend the material in question is a matter of experimentation. Even then I'm also concerned about the two bends being perfectly aligned on the same plane and not ending up with a pretzel! As Amy pointed out, bending thin wall 4130 is problematic. Without a internal mandrel, wrinkling is nearly impossible to avoid. I use JD Square's very precision dies and followers, and haven't successfully bent 4130 any larger than 1.25" (thinner than .049). The smaller diameter tubes are not a problem if plenty of lube is used on the follower and it's bent in one smooth motion. And don't worry too much about bend alignment if you later decide to tackle the curves. Just run a carpenter's level across the forming die first, then make your first bend. Reinsert and position the tube for the second bend, then place the level across the previously bend at right angle to the die before you start the second bend. Piece'o cake. BTW, what wire are you burning?... ER70S6, or S2?
And, here's another valuable link I think you'll appreciate:
http://www.kreuzotter.de/english/elenk.htm

Are there any machinist's shops where you could take your tube to be bent for you on pro-level machines? They have some awesome ones that are totally computerized, you just stick a straight tube in one end and get a curved one out the other.

BTW, what wire are you burning?... ER70S6, or S2?
And, here's another valuable link I think you'll appreciate:
http://www.kreuzotter.de/english/elenk.htm

I have plenty of ER70S6 and S4, but I was thinking of getting a minispool of ER80S-B2 for this one, just to try it out. I'm sure what I already have will suffice....

Thanks for that link, it's a beauty!

Are there any machinist's shops where you could take your tube to be bent for you on pro-level machines? They have some awesome ones that are totally computerized, you just stick a straight tube in one end and get a curved one out the other.

I haven't checked but I'm sure there are. If there is another 'bent project after this one (highly likely :rolleyes: ) it will be a lowracer built nice and light for speed ;) , and I'll most probably use the services of a professional to bend the tube for me as you suggest.

I haven't checked but I'm sure there are. If there is another 'bent project after this one (highly likely :rolleyes: ) it will be a lowracer built nice and light for speed ;) , and I'll most probably use the services of a professional to bend the tube for me as you suggest.

Be sure to use Titanium (http://titaniumjoe.com/tubing.htm) on your next project, too! You'll need a TIG welder, though.

File this under "miscellaneous comments." Consider the chainline from the idler back to the cassette. If you're on the 11T cog, you will want the bottom side of the idler wheel placed high enough that the chain won't be dragging across the chain stay. Since you want to avoid the use of chain tubes wherever possible, this is one spot where you can design to avoid a problem. I see two possibilities. 1) you can lower the main tube (where the seat mounts) so it's lower than the rear axle, which would mean the chain stays angle UP to the dropouts. or 2) you can raise the idler so that the lower surface is an inch or two above the frame. My preference would be #1, because it would't mean reengineering anything in your design, except to add a bit to the 'down tube' and reposition the rear triangle. Also, lowering the seat is a good thing. Option #2 would mean adding a bracket for the pulley, which might then get in the way of your legs while pedaling. And you would always have some problem with chain slap, no matter how high you raised the pulley - it happens anytime the chain is running parallel to the frame. If you keep the pulley beneath the seat with option #1, then you could even gang two pulleys on the same shaft and use one for each direction.

File this under "miscellaneous comments." Consider the chainline from the idler back to the cassette. If you're on the 11T cog, you will want the bottom side of the idler wheel placed high enough that the chain won't be dragging across the chain stay. Since you want to avoid the use of chain tubes wherever possible, this is one spot where you can design to avoid a problem. I see two possibilities. 1) you can lower the main tube (where the seat mounts) so it's lower than the rear axle, which would mean the chain stays angle UP to the dropouts. or 2) you can raise the idler so that the lower surface is an inch or two above the frame. My preference would be #1, because it would't mean reengineering anything in your design, except to add a bit to the 'down tube' and reposition the rear triangle. Also, lowering the seat is a good thing. Option #2 would mean adding a bracket for the pulley, which might then get in the way of your legs while pedaling. And you would always have some problem with chain slap, no matter how high you raised the pulley - it happens anytime the chain is running parallel to the frame. If you keep the pulley beneath the seat with option #1, then you could even gang two pulleys on the same shaft and use one for each direction.


Sorry to go against your preference on this one BP but I'm going to try to keep the current frame geometry and position the pulleys accordingly. I was thinking of making up a bracket that clamps on to the frame to alow a bit of experimenting before commiting to a permanently welded mounting point.

LOL - no need to apologize! I was merely pointing out a potential complication. If you want to keep the frame as drawn, option #2 will work, as will a few others I can think of. It's not a huge problem and I'm sure you'll figure out a solution if it becomes necessary.

The great news is that I found a trike & 'bent manufacturer here in Australia that is a real enthusiast and has been in the business for 10 years, and he still has that enthusiast's enthusiasm, if you know what I mean. I had a chat with him and he's happy to supply me with bottom brackets, chain pulleys, head tubes, custom made forks, etc. all the 'bent specific bits and at very reasonable rates. The company is www.trisled.com.au if you want to check them out.

Thanks all for your input, it's been extremely valuable. If you see anything else I should be worried about please hit me with it. The chain routing thing was a good one that I hadn't considered in detail yet! It's all good :)

I'll post some pictures as I go and a rundown on her performance when I'm done, if y'all are interested. :D

I'm sure interested. I like the looks of the general design, and I think you'll like the finished product once you get it polished. Good luck and keep us posted.

Hey Cyclaholic,

What CAD program are you using?


---Matt

Hey Cyclaholic,

What CAD program are you using?


---Matt

Hiya Matt,

I'm using Turbocad version 1 :o It was the very first piece of software I ever bought (after Windoze 3.1 :rolleyes: ) It now lives in my 128Mb memory stick, along with all my projects, and I throw it on whatever PC is handy when I want to use it.

Hiya Matt,

I'm using Turbocad version 1 :o It was the very first piece of software I ever bought (after Windoze 3.1 :rolleyes: ) It now lives in my 128Mb memory stick, along with all my projects, and I throw it on whatever PC is handy when I want to use it.

You feel like emailing me a copy?

I like the Bicycle Forest BikeCAD, but I'm designing a long wheelbase bike. BikeCAD does not support LWB designs.

While we're at it, do you know any good carbon fiber fabricators out there?


--Matt