Hi

I want to build a short wheelbase recumbent with a 20-26" wheel configuration. My heros are the following, in no particular order: Lightning Phantom/Stealth/P-38, Vision R-40, Rans V-Rex/thunderbolt....

It seems like most of these production bikes have wheelbases ranging from 40" to 45". My design has a wheelbase of about 35", which as far as I can tell, would produce a "comfortable" configuration, as far as where pedals, seat, and wheels can be.

I suppose I wouldn't be too opposed to moving the rear wheel back a little and rotating the rider's position back a little more, for the lowracer effect (and hopefully more comfortable riding position).

I'm requesting that people pick apart the design [flaws] and give me suggestions! :D

Thanks a lot,
Kevin

P.S.
What kind of welding should I use? I'm not good at TIG, and was thinking welding this stuff would be VERY hard with ARC (stick). I'm not a big fan of brazing, so that leaves me with gas welding and MIG. Suggestions?


Notes:

I have a few junk bikes that I'll be using parts off of. I have a few junk bikes for frames and components.
I'm currently favoring Over Seat Steering because it seems easier to build and get good results.
The trail of the configuration in the picture is about 3.5".
dimensions are in inches
actual frame design is not shown (or known at this time)
I tried to take some measurements of myself (such as leg length, arm length, ect.) and impliment them in the drawing.
seat and crank are at about the same level. I hope this is a good (easy to use, and comfortable) configuration.
Seat height is somewhere in the 20-22" range. I would like lower if possible, but I couldn't figure out how to lower it, and still have my legs clear the 20" front wheel AND maintain reasonable seat<->bottom bracket angle.

Well, to braze a frame would require lugs, no? I'd TIG weld it, if I had the skills to do so, or take the frame components to a welding shop and pay a pro to do the job. What sort of tube are you building the bike out of? Welding frames differ in technique depending on materials used. As for the design, your dimensions should dictate the component dimensions. Main concerns with a recumbent bike centre around centre of gravity, height above the ground, length of wheelbase, etc.

Yeah, TIG is definitely the best option here, but if you don't feel confident in your skills and don't want to pay someone else, then go with the MIG.

Do you have access to a TIG machine? You could always get some practice...

OK, here goes:
1. The seat is at front-wheel height. Where will the frame go? Be sure to leave room for the fork crown, headset, head tube, brakes, etc.
2. The chain will be alongside the fork. Is this OK with you?
3. Adjustable boom design? because I only see room for about a 40 or 41 inch x-seam. That's OK if you are 5'4"; but if you are, I'd consider a smaller rear wheel. Remember that max leg extension goes to the outside of the pedal circle at 3:00 and max leg bent occurs at about 10:30, also on the circumference of the crank circle - and that you legs are 5-6 inches thick, not non-dimensional sticks.
4. 66 degree steerer is pretty lax. Not sure how that'll handle, but it'll put the riser/stem closer to your crotch.
5. A 35" wheelbase will be more unstable than a longer one. It'll tend to lift the rear wheel in heavy braking and lift the front wheel on steep climbs. That's why SWB wheelbases run in the 40-50 inch range.
6. A 35" wheelbase will give a much rougher ride. Again, that's WHY you don't see any 35" wheelbases on new production models.
7. If you put a mesh seat on it, remember that the mesh will flex several inches down and toward the rear tire. Allow space, including space for mounting hardware. Even if you put a hardshell on it, the hardshell will have more dimensions than that line.

Not trying to discourage you, but I think you need to think it out more.

Also, why do you want to build one? Is it so that you'll have something that no-one else does (bragging rights)? Do you just want to save a few bucks? Or do you want to go into production with these things?

EDIT: Oh, and exactly how many 'bents have you ridden so far?

Hi all, thanks so much for the responses! I appreciate it very much because there is a lot of stuff I need to think about! This is going to be kind of long, but it is because I found each point valuable, and needing a response (to have the ideas written down rather than floating around my head, if for no other reason).


Bianchiriderlon

"Well, to braze a frame would require lugs, no?"

I suppose, although I've seen some lug-less brazes holding bikes together on some homebuilt sites. They usually look nasty when done, and I wouldn't be sure they were strong, so in effect, brazing is a no-go for me.

It looks like this guy used mostly brazing: http://www.outsideconnection.com/gallant/hpv/sprung/

I'm in my first year of college, and am planning on visiting my high school welding teacher sometime to see if I can weld it up there. They have a TIG welder there, but I never got to experiment with thin steel on it...just a few bad experiences welding (puddle-ifying) aluminum! I will use MIG if gas welding is out of the possibilities. Otherwise, I might be able to find someone that will let me use their MIG welder.

"What sort of tube are you building the bike out of?"

I was planning on cutting up some old frames made of Hi-Tensile steel, or something like that. If those can't satisfy my project, I've heard of people using conduit. It is heavy and not very strong, so if I decided against that, I could either get some steel tubing of the right specs, or just do the exhaust boom thing. No matter which way I go, it will be steel. ...or maybe I should just do fiberglass and forget welding.

"As for the design, your dimensions should dictate the component dimensions."

I don't understand this statement. Are you saying that I should figure out my size and how I want my position to be, then build a bike around me?

"Main concerns with a recumbent bike centre around centre of gravity, height above the ground, length of wheelbase, etc."

I saw some formulas for calculating trail and rake, etc. Do you know of any guidelines I should follow? I'll have to keep looking for the formulas, as I have forgotten where they were.

jeff-o
I have 0 experience with TIG on steel of any thickness. I would love to learn to TIG, so maybe I will see if I can.

"Also, why do you want to build one? Is it so that you'll have something that no-one else does (bragging rights)? Do you just want to save a few bucks? Or do you want to go into production with these things?"

Not production, at least not now! Recumbents are interesting to me, and it would be FUN for me to not only ride one, but also to build it! I don't really care if anyone has the same thing, as long as it works well. I am also trying not to spend a fortune while building it. This is a recreational project, so I'm not concerned about weight and expensive components and the like.

"EDIT: Oh, and exactly how many 'bents have you ridden so far?"

A few years ago, I rode an EZ-touring-whatever long wheelbase recumbent. This past summer, a fellow let me ride a homebuilt (not by him though) short wheelbase under seat steering recumbent. I like SWB bikes because they are small, hopefully lighter, and posses a sportier characteristic.

BlazingPedals

Thanks for giving me specific things to re-examine. I just drew up another model that I'll attach (hopefully) using your suggestions.

"1. The seat is at front-wheel height. Where will the frame go? Be sure to leave room for the fork crown, headset, head tube, brakes, etc."

Good point, that is something [i]I didn't think about[i] and will draw in the future. However, that is all stuff that will go between my legs, so it shouldn't really matter a whole lot, should it? That is, as long as I can build a frame that makes it feasable...?

"2. The chain will be alongside the fork. Is this OK with you?"

NO! Thanks for pointing that out! I want to avoid interference with the front wheel, and want to keep a straight path for the drive-side of the chain. On my next drawing, I put the chain close to the seat, and plan to use a chain-tube to enclose it and keep it/me clean.

"3. Adjustable boom design? because I only see room for about a 40 or 41 inch x-seam. That's OK if you are 5'4"; but if you are, I'd consider a smaller rear wheel. Remember that max leg extension goes to the outside of the pedal circle at 3:00 and max leg bent occurs at about 10:30, also on the circumference of the crank circle - and that you legs are 5-6 inches thick, not non-dimensional sticks."

No, I was planning on having everything fixed, except for maybe the seat, made to fit me. I'm almost 6'. I tried to measure my x-seam (kinda hard to do) and came up with 45". I used that on this next drawing, with my hip pivot being a few inches foreward from that. As you can see, I've been doing the bent leg thing at 9:00, but in the future, I'll check between 9 and 12:00. I tried to account for 3-dimensional legs on this next drawing...best to get as much worked out on paper before noticing the real life "oops"s!

"4. 66 degree steerer is pretty lax. Not sure how that'll handle, but it'll put the riser/stem closer to your crotch."

66 degrees is what I measured on a BMX-style bike. I straightened this next one to 71". What is a good number? What is typical? Also, are you saying that you would basically move the whole front wheel back? I try to put the front wheel where it "fits" best, because I haven't thought of a better way. What are you suggesting? How far back can the front wheel be? Behind knees? I wouldn't think recumbent-endos are much fun. :)

"5. A 35" wheelbase will be more unstable than a longer one. It'll tend to lift the rear wheel in heavy braking and lift the front wheel on steep climbs. That's why SWB wheelbases run in the 40-50 inch range."

That makes sense. This next drawing came out to ~45.6, which is what "fit" based on your other suggestions.

"6. A 35" wheelbase will give a much rougher ride. Again, that's WHY you don't see any 35" wheelbases on new production models."

I was considering adding some mild suspension to this one, but I think I'll have to pass. "maybe next time..."

"7. If you put a mesh seat on it, remember that the mesh will flex several inches down and toward the rear tire. Allow space, including space for mounting hardware. Even if you put a hardshell on it, the hardshell will have more dimensions than that line."

Good point. I was telling a friend about my project, and he suggested using an old computer chair, cutting and whatever to make it fit.

"Not trying to discourage you, but I think you need to think it out more."

That was exactly what I wanted. Much better than thinking "I should have thought of that BEFORE!"

-------

Thanks again! I await your comments on version .02!

Kevin

p.s. next time I'll try to color some of the things, so it isn't such a big mass of lines

Actually, you don't need lugs to braze. For many years, "Direct Brazing System" bikes were made which used no lugs, just closely fitted frame tubes.

Brazing is more forgiving than electrical welding.
Brazing doesn't change the temper of the metal as much as welding can.
Brazing usually causes less distortion than welding.
Brazing needs less equipment; just a brazing torch, similar to a propane-air torch, and a can of brazing fuel. Both are available at any hardware store.

Oh, almost forgot:
Brazing is usually as strong or stronger than welding, depending on the materials.
Brazing is more forgiving on thin materials, such as "hi-ten" bike tubing.

As far as Tig welding, you need the electrical welder and a cylinder of gas which is available only at welding suppliers. Sort of the most expensive of both worlds.

Also, why do you want to build one?

Jeff-O, lots of people build bikes to build bikes. They enjoy the process. That's what I think.

I think version 2 looks like something that'll make you happier. Longer wheelbase, especially for a 6-footer, will make for a better ride. The chain is coming *really* close to the seat, so a chain tube will almost be a necessity. If you can, you might want to use an idler to help route the 'return' side of the chain over the fork - say, on the 'downtube' area. I measured a 406 fork that I have. From axle to headset race is a hair over 12.5 inches. Add a half inch for the lower headset bearings and an inch or two for the head tube poking below the main tube, and the bottom of the main tube can start at around 15 inches, meaning you can lower the frame at that point by about 2 inches from what's shown? Aim for having the bottom bracket at the same height to leave the chainline the same because dropping the bb will also increase potential chain interference at the fork.

BTW, here are a few pics of my homebuilt. My design specs called for dual-26 wheels. Also built out of junk frames, as you can see. This was brazed - no lugs, one gusset. Wheelbase is only 41 inches, which was determined by the primary donor frame; my preference would have been a little bit longer.
http://www.biketcba.org/TRICORR/junk2.jpg http://www.biketcba.org/TRICORR/junk3.jpg