How to build a HPV bike

This is a description of how to make a short wheel base (SWB) recumbent cycle with under seat steering (USS).

I have become increasingly interested in HPV’s for a while now and wanted to give them a go and the cost has always been too prohibitive (Approx. $3000 + in Australia).
The HPV mailing list responses have been a great help as well as a few web pages from other home builders. I don’t come into the league of Don Boose for his quality of engineering but still have made something that is quite functional and cheap. Everyone seems to have their way of doing things and varying levels of confidence/skill and coordination and so I suppose mine are that I like to do things cheaply out of available material on hand and don’t like to see things go to waste if it can be recycled. I’m not shy on trying out new and unique things and trying to become more accurate in whatever I do. I’m finding more and more that a rushed work doesn’t show the real capability of how ones work will perform so the old saying “if it’s worth doing it’s worth doing right” is becoming more real to me now.
I hope this project is of some use to you in building your own. It’s an exciting challenge.

INTRO

I chose to MIG weld as I have recently had access to one and am not sure if it’s possible to ARC weld material this thin and retain the strength needed. See here for some interesting welding info.
I tried to see how cheap I could make the bike and so used old bike parts and scrap material as much as possible. This meant wrecking 2 bikes, back frame (27”) and another 20” for pedals as my (20”) forks, cogs, etc. The main tube is from a clothes line but car exhaust tubing is fine. Also refer to this choosing materials page.
I scored a 27” geared bike from a 2nd hand store for $25 and this gave me the majority of parts along with a 20” one I had available. After downloading many pictures of HPV’s from the net (and looking for articles of other home builders) I settled on this type of lay out.
I intended to try out as electric assist hybrid but because of bad planning will probably not go ahead with it now as it would have had to drive the pedals and main chain as well as the back wheel which in not the best way to set it up.
Apart from the welding and paint supplies it cost me $25 for 27” frame and $15 for derailleur. It took me about 2 weeks to build and a week for sanding and painting.
Sadly I had to modify a few things after final assembly so a bit of the paint work was damaged by heat from welding so do try to completely finish and test ride a lot if you can before the final painting.
I was surprised how much my confidence grew progressively as I built it. I have very little experience in this sort of work and have never seen a HPV in real life yet. (Apart from a hand powered trike I caught a glimpse of going the opposite way on a open highway). It may be handy to also refer to the Recumbent Cycle Component Design & Construction Web page.

Construction

Draw a chalk line on the concrete to represent the ground. Lay back frame with rear wheel in place on concrete and position it to be right height about ground as well as raising the front to make the existing pedal bracket sit where you want it. Mark the axle spot and bottom bracket with an X onto the concrete
Measure the wheel base onto the concrete and position the front forks (removed from the frame) with wheel installed near the right spot on concrete. It can be helpful to hold an old 20” bike in same position using the ground as a guide and the frame at the appropriate height to see the right angle of the fork tube. I have no experience in how the fork angle affects the handling but do suspect it does make some difference. My bike tends to steer too easily and at first I thought it was me learning the new recumbent style but now consider it is to do with not enough fork rake. Mine is set considerably less than a normal bike because of a mistake I made.
Draw on concrete using a suitable straight edge where the main pipe position will go linking the back frame up through the front head and then heading straight out parallel to ground marking. I used a ladies frame so the extra tubing available can be used to support the main boom joint to the frame. The frame can now be cut leaving an excess on the support tube. I'm sorry I have no close up pictures of this but refer to this picture of my trike with the same setup to see what it looks like.
File the boom to fit the pedal shell( bottom bracket) then cut the support tube to fit as this will hold the angle right when welding This can take some time to get right but it is worth being accurate. Also see this tube mitering page
To make the bend I filled the pipe with sand and packed it down hard to try to minimise kinking. Heat the part of metal that’s going to stretch and lever it into the right angle checking it against the markings chalked on the concrete.
Weld main pipe to frame being careful not to damage the crank threads and this is needed later on. Also be careful the frame is straight. It’s best to tack weld and check a few time.

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Steering

Place the frame back in position on the concrete and mark on the pipe the fork hole angle. I used a small drill bit making several smaller holes around the inside of the hole. Carefully chisel out the piece and finish with a rat tail file. For the fork tube head you could use one scavenged from a bike or better still find a piece of the same diameter inside as it won’t have the old frame holes in it.
It takes quite a bit of time to fit this tube into the main pipe and I didn’t find it easy to get the angle right but this will affect performance and handling later so it’s worth the time. I would suggest tack welding it just strong enough to ride and trying it out before completing the welding.
After welding the steering, assemble all what’s done together along with the back wheel then stand back and look at it.

Seat

The length of the pedal boom is totally dependent on the seat position and size so this is done next using an old table chair as the basic frame. The front of the seat frame is mounted onto the boom using a U bracket and the rear welded onto an existing bike seat tube so all of it can be removed and adjusted later.(a little). You may find you need to bend the seat tubes and this can easily be done by first heating the stretched side of the tube. 5 ply was used for the seat base with mattress foam and vinyl. Be sure to varnish or paint the wood that is bare after stapling the upholstery to protect it from road spray. You may like to make a mesh seat.

Pedal Assembly

Cut pedal crank bracket from the surplus bike taking note that if a derailleur is used up front an upright tube is left on for the mount. Be sure the upright tube is the correct diameter. (I've made this mistake before)
The next part needs a helper. Sit on the bike and have the assistant hold the pedal crank assembly up at the approx. position. This give you the pedal bracket length and if it is to be permanently mounted cut the boom, file to fit and weld it on. This can be made adjustable by making the boom a little shorter and finding a pipe that either fits neatly inside or outside it. A car exhaust clamp works well to hold it in position or make a permanent boom clamp.

Chain

The chain will need to have some sort of guide to route it over the front wheel. First up I used a derailleur cog on the tension side of the chain but this didn’t work so great. Skateboard or roller skate wheels work better. The skate axles and nylock nuts can all be used with the axles, either welded onto the boom or using 2 parts of a car exhaust clamp. Welding does tend to weaken the axles and I have found over time they do need to be re-bent straight. If the chain comes off the skate wheel refer to this diagram. Alternatively try this option using 2 car exhaust clamps or a tube clamp. It does seem necessary to use some sort of chain tensioning device so I used a derailleur for this being mounted underneath the seat. Because the thick type of chain was used for the primary drive you can't use joiner links and the chain needs to be riveted. It is quite easy to do by centre-punching a link mostly out and re-riveting it back at the correct length. Just using the lower portion of the derailleur would work better if at all possible. I've not needed guides to keep the chain located onto the wheels as long as they are of not too soft a rubber. Also refer to the lower chain guide page and Plastic Chain Guides pages.

Handle bars

The USS (under Seat Steering) was made as in Diagram 1 with a bolt welded to the boom or put through a car exhaust clamp, passing through a tube welded into the handlebars to act as the bearing surface. The nut can be done up tight as the bolt needs to have a thread a little less than it's diameter. The whole arrangement needs to be made (and can be spaced a little with washers so that with the nut up tight the handlebars have free movement) with little or no slop. A push bike steering head may be able to be used instead of passing the bolt through the handlebars with the stem cut down short and a washer welded to the end of the stem to give the bars support.

For other Under Seat Steering handlebar mounting designs have a look at this page.

I sourced the steering ball joints from a throttle linkage on a Vauxhaul car. They can be bought new from bearing supply shops for about $9 each. See picture 1 and picture 2. The track arm was cut from scrap and welded onto the side of the front fork. Closed Loop steering could also be used.

Intermediate shaft Assembly

I realise now it would have been better to have the steering rod assembly on the right and the primary chain route on the left. This would make for a much better chain intermediate shaft and give a wider range of gearing. If it isn’t suitable to have different cog sizes it will mean having to make a 2 cog cassette which I found very hard to do, keeping it all square and centered being the biggest problem. Of course one continuous chain can be used but I chose against this way thinking it was too hard, tension and several other guide rollers need to be used because of the frame design. I have gone through a heap of changes in the gearing since first making the bike. The setup in the photo's above was geared way too high for me and you may notice in the pictures below the front cog is quite a lot smaller. In fact it is a standard coaster rear wheel cog welded to a pedal arm. The gearing here is a little low now but is functional for me due to my health. Also refer to this intermediate drive page.

Cog on both sides

To have a cog on both sides of the intermediate shaft it would be best to use a 3 piece crank with square spline instead of the cotter pin type. It’s simply a matter of cutting off the pedal arm of the crank. One side of axle is longer than the other and there can be a problem with the cog scraping the rear fork. A work-around is to use a spider crank, mounting the cog on the outside of the spider.

Cog on one side

A 2 or 3 piece cluster can be used if this gearing isn’t a problem. I had very little ground clearance so had to make the 2 cogs small. This was done by cutting the cog and arm from a crank and welding on 2 rear wheel cogs with a small spacer in between.

Bike Specifications

Parts

Boom (Exhaust tubing) approx. 47mm Dia x 1230mm long.
Derailleur
Table chair for seat,18mm dia
Skate board or skates
2 chains
steering ball joints

27” geared bike
- back part of frame
-back wheel, gears & tyre
-chain
-brakes, levers
-mudguards
-carrier
-pedals and crank
20” bike
-front wheel, tyre etc.
-pedal bracket and assembly
-back cog
-chain
-front forks and assembly
-seat pipe

Things to consider

1. Consider the chain route and intermediate shaft cog ground clearance before committing to the frame design. The tension side needs particular attention so there is minimal friction by having as few guides as possible.
2. Choose a frame that had derailleurs if using them because of the lugs and mounts needed.
3. Filling pipes with sand before bending can minimise kinking. Exhaust shops bend for very minimal cost, usually around $2.50.
4. Knobbed tyre on the front will give much more rolling resistance than a carefully chosen slick tyre.
5. The main boom needs to be large enough diameter to accommodate fork tube.
6. The old style of chain is wider than the newer derailleur type but still can be used as long as the joiner links are riveted.
7. A rubber seat stopper can be used to cover the bare tube ends as on the front derailleur support.
8. There is really no place to carry anything (not even on you lap) so consider a carrier.
9. For Mounting extras on frame e.g lights
   

Update

The bike has been converted to ASS (Above Seat Steering) and handles much better. This may not mean the USS is harder to use but could have been because of the slight slop in the steering ball joints. Some have told me a perfectly smooth steering action is needed on USS bikes and even the slightest drag or slop in either the steering head or ball joints will make it hard or possible to ride. The gearing has been raised considerably with the front cogs now with 28/38/48 teeth and the bike really moves (latest 40k/hr) with no handling problems. So here is the latest specs.

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Last updated Thursday, January 29, 2009