LWB streamliner proto

Finally, the FWD model that I rode from Aix en Provence to La Rochelle and back.  Here she is in three different flavours; naked, touring, and tail-faired for more speed!

Initiation

My initiation into the obsessive world of recumbent building began with this wooden wonder.  Not (yet) being versed in the art of welding I realised this would pose a significant handicap to creating my own version of some of the home built masterpieces I had seen on the internet.  But being reasonably competent at working with wood, and having lots of odd bits of wood and stuff lying around, I acquired a thrown-away child's bike which I hacked up for parts and key frame elements - notably the front wheel, fork, and down tube - and the result was this.

A keen eye might notice that  the chain is on the LHS of the frame.  This was simply because  I did not yet possess a tool for removing the crankset from the bottom bracket, so I had to make do with it being on the wrong side when I exploited the down tube as my front boom.  The bike didn't yet have any gears shifters, so the problems that a LHS chain line would create at the rear sprocket for changing gears didn't arise.

Note also that I am already experimenting with an intermediate sprocket cluster as part of a future gearing mechanism.  I thought this was quite ingenious, and no doubt original.  I was soon to discover - as with almost every original, cunning, and unique new idea that I had - that someone else had got there first quite some time ago!

This first incarnation was enough for me to overcome the weird sensation of having my bum on the ground and my feet in the air, and was sufficient to convince me that it was indeed possible to ride this strange thing and that the project was worth pursuing.  The next step was clearly to add the ability to change gears, and this required moving the chainline to the RHS.  I still didn't possess a crank remover tool, so I cut the front boom in half, rotated the BB by 180 degrees, and used a seat clamp to join the two halves of the boom together.

This model was - surprisingly enough - able to be ridden with ease, and I spent some time acclimatising to the novel experience of the recumbent position by riding around an outdoor basketball court.  I was soon confident enough to do U-turns and figures of eight.  An early highpoint was when two young lads, who happened to be present during one of my test rides, asked me "where did you buy that mister?".

The main objective of this wooden wonder was to see what fork angle and trail I should use to make a good handling bike.  This was easy to adjust by simply loosening the four bolts on the wooden head tube clamp.  I spent a lot of time playing with these parameters, and - despite knowing from bike magazine journalists how just a degree or two difference can radically transform a MTB from a nimble handling XC bike into a rock stable Downhiller - I began to get the impression that quite large variations didn't actually seem to make any difference.  

Perhaps this empirical discovery was what gave birth to my cynicism and mistrust of ever believing anything of any technical nature about bikes gleaned from bike journalists or self-proclaimed experts on the internet.

Perhaps not the most elegant of contraptions but, as you can see, I'm quite chuffed with my new toy!

Full Metal Jacket

Flushed with the success of the wooden wonder, I nevertheless realised the limitations of prototyping bikes built from wood.  So I purchased a cheap electric arc welding kit and launched myself naively and blindly (sic) into the new world of welding metal.  I very rapidly discovered a number of things: (1) attempting to weld thin-walled bike frame tubes with an arc welder is not a sensible thing to do!  The result is always messy, and invariable burns more holes in the tubing than applying useful weld to join them together; (2) using a cheap welding mask with dark filters renders totally invisible the join that you want to weld together as soon as the mask is flipped into place.  This makes welding - for me - literally a blind art!

Unfortunately I don't have a garage or workshop (or any suitable indoor space) with good lighting to perform my welding, so my solution is to weld outdoors in bright sunlight.  This means dropping everything and taking a forced break whenever a cloud obscures the sun, or when it passes behind one of the several tall trees in my garden.  Welding for me is therefore a highly stressful activity, in which I rush to get my tubes welded before I lose the essential lighting conditions. My lack of training, coupled with the problems above, invariably leads to my welds being somewhat less than perfect.  Some might even describe them (not entirely unfairly) as a complete and total botch!  I try not to let this disturb me, and I have been totally amazed as how my prototype frames have held together with such seemingly inadequate welds, despite hurtling downhill at over 70km/h on the most irregular of french road surfaces.

Getting the frame alignment right requires a little ingenuity.  Here I discover that, however well the tubes are aligned initially, the heat applied while welding invariably causes them to be out of true when the frame is finally removed from the "jig".  Fortunately - just like steering angle - I discovered that perfect wheel alignment is surprisingly not a prerequisite for a well handling bike.

Don't look too closely at my welding as it's not that pretty.  Just try to look past it to the bike as a whole!  This first incursion into a fully metal frame was to serve as the basis for a number of different prototype bikes, experimenting with different chain drives (both rear wheel and front wheel drive) as well as different methods of managing the chain line and gear selection.

The wooden seat is made from two 5mm thick plywood side panels (to give vertical rigidity) cut with a profile to match the curve of my back.  A number of cross-members hold the two side panels together, both of which are slightly curved in the horizontal plane and bulge outwards in the middle, to add tension and rigidity horizontally. The seat panel is 3mm plywood, soaked in water to render it more pliable, and carefully formed over the side pane profile and screwed down into the cross-members.  This adds further rigidity in the horizontal plane.  

The seat unit is fixed on the frame by front and rear bolts passed through the side panels, so the weight of the rider is supported by these panels which have perfectly adequate rigidity in the vertical plane for this task. Finally, a sheet of closed-cell foam is cut to shape and taped down on top.  

The result is a fairly lightweight (less than 1kg, depending on how many cross-members I add), rigid and remarkably comfortable seat.  It is also very versatile, and - by repositioning its fore and aft mounting bolts on the bike frame - can be readily adapted to allow different rider positions on the bike.  The seat back can be more or less inclined, the seat height raised or lowered, and the whole unit moved forward or back.  I shall use this seat design for all my further incarnations.

Even more chuffed with my second - all metal framed - prototype, and first truly fully functional and road-going bike.  This one's even got brakes front and rear, as well as a fully working set of 24 gears!  (Yes, I'm still using an old 8 speed rear cassette.)

Just producing a bike that is rideable seems to me a major triumph and gives me a real kick!  I am already experiencing the heady thrill of riding a home-built recumbent that will lead into a near-obsessional relationship with recumbent engineering that no doubt all home builders are familiar with.  I have not yet considered or researched the implications for efficiency and power transfer of elements such as chain line management and idler quality.  I am currently happy to make do with a derailleur cog on a bolt as the drive chain idler (!) and another on its sprung derailleur cage to guide the return chain line over the front wheel.  All this will change in the near future!  

SWB LR1

Recumbent laboratory test bed

My early experience road testing my first prototype demonstrates a number of things.  Pedaling in a recumbent position is not at all the same as pedaling on an upright bike; it seems to use different muscles.  I'm also aware that my prototype doesn't seem to climb very well.  I put aside my tools and arc welder for a while to devote some time to research on the internet.  A couple of themes rapidly become apparent as important and contentious issues; body position and chain line management are two of them.

Body position for a recumbent rider is generally defined by two parameters; seat back angle, and the difference between seat and bottom bracket height.  However, one or two more astute internet contributors (in my opinion) consider that a more significant parameter is the "body-hip angle", or the angle that the torso makes with the upper thigh when pedaling.  It is noticeable that the body angle is considerably more "closed" (i.e. smaller body-hip angle) on an upright bike than on a recumbent.  Could this be a clue as to why my bike doesn't climb well?  (I also discover that I am not alone; apart from a few contenders, most recumbent riders accept that they do not climb as rapidly as upright bikes.)

Chain line management is all to do with solving the practical problems of guiding the chain so that it doesn't interfere with the front wheel or the seat.  This generally involves (at least for low seated bikes) guiding the chain over the front wheel and under the seat.  This is usually achieved by the use of idlers (pulleys or cogs), but the addition of any intermediate idler between the cranks and the rear wheel - as well as any deviation of the chain from a straight line - is a potential source of efficiency loss.  Much debate seems to accompany this subject, with various views on how significant this efficiency loss may be.

I decide that both these issues merit serious scientific study, and that a "laboratory environment" is most suited to do this.  I set out to construct a "laboratory efficiency test bed".

Here, then, in all its splendid glory is the Fitzhugh recumbent laboratory test bed. The seat inclination can be varied to give a more closed or open body-hip angle, and the intermediate idler can be repositioned to give more or less chain deviation in the drive chain line that passes under the seat.  Pedaling resistance is provided by my hydraulically damped home trainer.

My optimism for the advancement of objective scientific study and analysis of the variables that govern recumbent pedaling efficiency and performance is rapidly diminished by the realisation that I am missing any means to measure key factors such as power input by the rider or resistance overcome at the rear wheel.  This technical deficiency renders any analysis of changing the parameters of my laboratory test bed no more than a subjective impression.  For objective measures I would require at least a power meter at the rear wheel and, preferably, a second on the crankset to be able to study the power in/power out ratios.

I briefly consider adding a dynamo to the home trainer with an electronic multi-meter to measure power output as a means of giving some objective value to the power at the rear wheel.  However, I recognise that the resistance of the home trainer is non-linear so the speed of the rear wheel - and hence power output of the dynamo - will not be easily or reliably related to the true power developed to overcome the home trainer resistance.

I reluctantly admit that these pose serious problems to the objective value of my laboratory test bed.  I finally accept that it should perhaps best R.I.P. and I shall have to be content with using laborious and less reliable road tests to investigate the performance of different configurations of critical variables.  Oh well....

Body angle revisited

Seat fairly laid back; BB 7cm above seat (bum area).

This gives a fairly open body-hip angle, as can be seen from the angle between the upper thigh and the torso which is around 110 degrees (when the pedal is in the recumbent zero degree position, where the thigh is raised its maximum).

Not so clearly shown in this photo because (a) the raised thigh is hidden by the extended leg and (b) the pedal is not in the zero degree position, but you can see that the raised torso makes for a more closed body-hip angle.

Here the seat angle is intermediate between the previous two configurations, the BB boom has been shortened, and the seat is slightly lowered in relation to the BB.  This gives a body-hip angle of around 80 degrees.  Note that body-hip angle is tricky to define and to measure; I try to use a line running down the centre of the torso, and a line joining the centre of the hip to the centre of the knee (when viewed from the side).  The hip-knee line is more readily distinguishable in this photo thanks to the seam running down the upper thigh of my thermal leggings.

Home made idlers

High or Mid Racer?

SWB LR2

BB boom rigidity

Foray into FWD

So here is my faithful, all-purpose frame being used to prototype a FWD version.  One of the first things to strike the eye is how the FWD removes all that lengthy chain and its hard-to-manage chain line running the whole length of the bike.  I personally find it both functionally and aesthetically appealing (despite what you may think of the rest of the bike!).

The use of FWD does introduce its own challenges, however.  Firstly it requires an intermediate idler to divert the chain line from the near horizontal to the near vertical.  To avoid pedal-induced steering torque, the drive chain line from the idler must lie parallel with, and as close as possible to the steering axis.  Initially I used a development of my early home-built RWD idlers, using a large cassette cog cut out of a cassette block and bolted onto a roller skate wheel body, which was mounted with a 6mm bolt to a box section welded at the BB boom/steering head junction.  (The roller skate wheel has a very nice quality, low friction bearing for smooth rolling; see the post on Idlers for further discussion.)  Very quickly one discovers that a potential issue with FWD idlers is their tendency to suck in the hairs on your leg as your upper thigh passes by as you pedal.  This is not a desirable feature!  I cut out a plastic disc and fixed it to the outside of the idler to prevent this.  It worked OK.

However, the limits of my roller skate-bodied idler soon became apparent when my enthusiastic pedaling ripped the cassette cog from the skate wheel body.  Something beefier was needed, so I turned to the next thing that came to hand in my box of bits... a second bottom bracket.  I welded a BB housing cut out of another bike in place of the box section unit at above the BB boom, and fitted a BB into it with a 34 tooth single ring chainset taken from a child's bike with the crank ground off.  This is not only a bigger diameter - and hence more efficient (see later discussion) - but it comes with its own plastic guard.  Brilliant!

The intermediate idler takes care of the drive side chain line, but we also need to manage the return side chain line. Unlike the drive side chain, which acts to transfer the significant pedal forces to the front wheel hub, the return line is under essentially no tension (only that from the sprung derailleur arm). I chose to use a derailleur cog as the return idler because it is small, light, and well-suited to the purpose. The issue with the return chain line is that undergoes significant lateral movement as the front wheel is steered left or right, and this can lead to interference with the front tyre.















Note that this chain interference is not symmetrical.  That is, when the fork is steered to the left it is the upper part of the tyre behind the fork that will come into contact with the drive chain line running up the back of the right fork.  Conversely, when steering to the right it is the upper part of the tyre in front of the fork that will contact the return chain line.  In general, if the drive chain line is well placed in relation to the fork leg (more on that in the discussion section), it is the steering arc to the right that is most limited by chain interference.

A keen eye will have noticed a key difference in the return idler mounting between the photo above and this photo here.  Above, the idler is mounted to the BB boom, while here it is attached to the fork leg.  In both cases it is beneficial to mount the return idler as close to the fork leg as is practically possible and above the tyre, since this allows a greater degree of steering arc before the tyre contacts the return chain.  But, significantly, when the return idler is attached to the fork leg it also moves laterally with the fork when steering; this means that the fork can be steered even further to the right before the tyre contacts the chain.

While this fork-mounted idler does wonders for the steering arc, it introduces a new issue of its own; torque steer.  Essentially, although the return chain is under significantly less tension than the drive side, the light tension in the chain between the return idler and the front chain ring (due to the derailleur spring) is sufficient to "tug" on the right fork leg and thereby introduce a small steering torque to the left.  In practice this is not a huge problem, and for anybody seeking a tighter turning circle for their bike it might be a good solution.  However, it does mean that straight line hands-off riding is compromised, assuming that the bike's geometry is such that this would otherwise be possible.

A further issue with a FWD version of a small front-wheeled Low Racer is that of gear ratios; or rather - as the late, great Sheldon Brown explains it best - "gain ratios".  (See Gain Ratios).  Essentially, what this means is that the smaller driving  (front) wheel cannot propel you at the same speed as a larger RWD driving wheel when pedaling in the same gear selection at the same cadence (and, to be really precise, with the same length cranks).  A common solution to this issue is to purchase an especially large chain ring - typically 60, 62, or even 65 teeth - to replace the standard 52 or 53 tooth large chain ring.

However, this is a BIG ring, both functionally and aesthetically displeasing (at least to me).  An alternative is to effectively "step up" the gear ratio at the intermediate drive idler.  This requires two elements.

Firstly, the drive idler is modified to comprise an "input" and an "output" cog.  The ratio of teeth on these cogs determines the step-up of the overall gear ratio between the crank and the cassette.

Second, a chain tensioner needs to be installed in the return line between the drive idler and the chainset.  Here, I've adapted my trusty derailleur cog unit to add the tension at the chain ring by simply hanging it by an elastic cord.  This was the simplest and quickest method I found to hand in order to test out the step-up concept, but in practice this tensioner device is not appropriate; it has too much vertical and horizontal play, and being so close to the chainset it creates problems when changing rings.

  

The total step-up system looks like this.

Once again I had the same reliability problems with the fixing of the cogs to the skate wheel body, as mentioned above, so when I decided to weld a bottom bracket unit to the frame to serve as the idler mount I bolted the secondary "output cog" to the idler pulley as below.

This step-up idler gear was more reliable, but both systems suffered the same conceptual problem; that of their width.  The issue of "hair suck" when using a single idler gear is essentially due to the "Q factor" of the idler unit; a narrower unit with a lower Q factor presents less chance of contact with the thigh when pedaling.  By filing the taper on the end of the BB square axle I was able to place the 32 tooth chainring idler as close as possible to the frame tubing at the steering head/BB boom mounting.  This gave a sufficiently low Q factor that hair suck or "thigh rub" was not a problem.  However, by adding a secondary gear to this unit, the width is increased noticeably and this forces an unnatural pedaling action with the knees parted in a bow-legged manner to avoid contact between the idler and the thighs.  For this reason I finally decided to abandon further exploration of the step-up drive idler concept.

It was noted above that my quick fix tensioning idler on an elastic cord was, at best, adequate for testing the concept using a single front chainring, but could not follow the chain as it was shifted across different chainrings.  But that's not to say that a better designed idler could not work, but it would probably need to act on the return line of the "input" chain as it leaves the step-up gear idler. 

It's worth mentioning that an alternative - and conceptually rather elegant - step-up gear solution exists, where the "input" chain is moved to the LHS of the BB boom.  In this method a single chainring at the pedals drives the input gear of the step-up drive gear unit, with the chain loop tensioned as for a single speed or "fixie" road bike.  The entire input drive system - chain ring, input step-up gear, and chain loop - is swapped to the LHS of the BB boom, and the single output gear on the step-up drive is replaced by a two or three ring chainring set on the RHS.  A front derailleur unit is added on the RHS at this chainset to shift the chain across the output gear rings, and - together with the cassette on the front hub - the output drive chain loop acts precisely as the chain drive on any upright bike to provide the full range of gears.

To best visualise this, in the photo here, imagine simply that the crankset+upper chain+small drive idler gear are swapped as a unit to the other side of the BB boom, while the single large drive idler is replaced by multiple chainrings+front derailleur.

Perhaps it is the symmetry of having two chain lines on either side of the BB boom that appeals to me, but this system is also not without its own issues.  Firstly, inverting the crankset means that the pedaling action will cause its BB to unscrew from its frame mount.  This is easily resolved by simply flipping the BB mount when it's welded to the boom.  The second issue is once again that of Q factor, and here there is not only a 2 or 3 ring drive gear unit between the frame and the right thigh, but the front derailleur as well.  The potential for "thigh rub" and "leg hair suck" is greatly increased and can render the pedaling action unacceptably painful.  A very low seat and fairly low crankset, combined with a high drive gear unit, may allow the knees to pass below the drive unit as so avoid this problem, but this imposes significant constraints on the rider's position and frame design which might not be acceptable.  Nevertheless, I know of at least a couple of riders who employ this system on their (very) Low Racer FWDs.

Seat suspension

I'd noticed that quite a lot of commercially built recumbents had rear suspension.  Some even had suspension front and rear.  Seemed time to see if some kind of suspension could improve the ride comfort and remove some of the brutal shocks transmitted directly to the rider's back from big bumps, potholes, and speed bumps.  However, I was (and still am) reluctant to consider adding suspension to the frame primarily because of the power-sapping pogo effect when pedaling hard that all but the best and latest MTB suspension bikes have experienced.  Bearing in mind that climbing ability is a key requirement of my ideal bent bike, this factor is significant.

The ideal solution seems to me to keep a rigid frame for optimum power delivery, but to add suspension to the seat - and particularly the rear of the seat - to reduce the road shock transfer into the upper back.  The front of the seat is mounted to the frame by a lateral bolt through its side panels to allow the seat to pivot as the seat back moves up and down under the action of the suspension system.

Inspired by Alex Moulton's venerable but effective elastomer suspension (See Moulton Suspension) I initially experimented with my own variant, replacing his specially tuned rubber elastomer with a more readily available tennis ball, acquired by searching in the long grass around my local tennis club.  Despite the unsophisticated and very Heath Robinson implementation of this concept it actually worked pretty well.  The tennis ball compresses and squashes under the weight of the rider, but still seems to retain enough vertical compliance to absorb bumps and vibration.  As a super lightweight form of seat suspension it may still have future merit as it combines both spring and damper functions in a single element.  However, my initial and very quick experimental implementation lacked the necessary lateral stability required of a rear seat mounting.

Further development of the tennis ball concept was abruptly brought to a halt by the recovery from the local rubbish dump of a full suspension MTB that someone had thoughtfully thrown away.  Despite being very cheap and cheerful it did provide a number of useful components, as well as more frame tubes to add to my ever-growing pile of raw materials.  Here the tennis ball is replaced by the spring and damper unit taken from the MTB's rear suspension.  (I soon discovered that the "damper unit" - no doubt made in China - contained absolutely no internal damping element whatsoever, but served only to look the part!)

This unit was also effective, and responded better to big bumps, but it was heavy (springs are like that!) and still lacked decent lateral stability to act as a good seat support to prevent the seat from rocking laterally when pedaling.

The next step was to refine the system by combining the spring/damper element with some form of laterally supportive seat stays.  My approach was to employ a pair seat stays to provide the support and lateral stability, but articulated so as to allow them to "flex" longitudinally in response to road-induced vertical forces.  The amount of flex (shock absorption) is governed by a preloaded spring unit with associated damper.  For this I took the spring from the suspended front fork of the cheap MTB and slid this over a hydraulic piston unit used to keep vertical kitchen cupboard doors from falling shut when you open them.  This  damper body conveniently fitted within the spring, and an aluminium tube was slipped over the unit to maintain the spring under preload (the degree of preload being determined by the effective length of the outer tube which was adjusted by adding a number of washers as shims). 

A pair of plastic V-brake arms served as the short links in the articulated seat stays and a lateral bolt acted as pivot and held the three elements - long links, short links, spring/damper unit - together.  In action, a road bump applies a vertical force to the rear wheel which would normally be transferred directly into the vertical movement of the seat back via the seat stays.  However, the articulated seat stays cause some of this force to be resolved into a horizontal component which acts on the pivot bolt.  Since this bolt is restrained from moving horizontally by the long seat stays, it effectively rotates forward and downward around a radius centred at the long stay pivots on the rear axle.  This action compresses the spring/damper unit which serves to resist this movement.

The overall action is fairly complex to visualise, but the amount of road shock transmitted to the seat back is determined by the combination of the spring rate and preload together with the relative geometry of the long and short links and spring unit pivot point on the frame.  By playing with these parameters, the amount of reaction at the seat can be varied from extremely rigid to excessively floppy; the former offers no suspension value, while the latter provides insufficient support to hold the weight of the rider when pushing back into the seat.  The video clip below illustrates the seat suspension action when these parameters are adjusted to give a moderately soft suspension.

The same kind of seat reaction is obtained whether the initiating force arises from a (vertical) bump in the road or a (horizontal) force from the rider pushing back against the seat when pedaling hard.  This means that a compromise has to be sought where the suspension is not so supple as to allow the seat to react when pedaling hard, but not so rigid that it adds no absorption of road shocks.  With the geometry of the link and spring elements used here, the whole unit acts to provide a falling spring rate.  Most - but not all - MTB and other vehicle suspensions are designed to give a rising spring rate.  What this means is that the resistance to further movement in the system increases non-linearly as a function of the amount of movement.  

A rising spring rate therefore gives an initially supple suspension, but hardens up as more and more compression is involved.  This is normally a desirable property of most  suspension systems, but not in this case.  The reason is simply because of the seat reaction to rider-induced forces from enthusiastic pedaling.  What is required is a seat that is initially hard to move (to avoid this rider-induced action) but which responds to greater and more rapid inputs from road-induced forces.  The falling rate system achieves this, and sacrifices some loss of small shock absorption as a compromise to avoid rider-induced inputs, while offering useful suspension to more significant road bumps and potholes.  (However, very large road bumps will cause the falling rate system to bottom out when the short links' further rotation is halted by contact with the seat side panels.  In practice, this is still better than having no suspension at all, and such mega bumps are fairly rare.)

With this design the system is stiffer and less responsive when the interior angle between the long and short seat stay links is very open (approaching 180 degrees in the extreme) and/or when the spring/damper unit is fairly vertical.  The system softer and more responsive when the links' interior angle is more closed (approaching 90 degrees in the extreme) and/or when the spring/damper unit is more horizontal.  The particular geometry of my system used here was determined by certain factors that I couldn't - or didn't want to - vary (e.g. the point of the spring/damper unit pivot mount on the frame).  However, with an hour or so of experimenting I arrived at a suitable compromise to give the kind of suspension response I wanted.  

This system was adopted on the bike that I took on a 1500km maiden touring trip across the SW of France (including the very steep and unrelenting climbs of the Massif Central) and it worked very effectively.