From: HPV-request@Sonoma.EDU Date: Thu, 30 Jul 92 22:31:47 +0100 Subject: suspension in 'bents Recently I've come across an interesting evaluation of suspended front forks for MTB. It made me think whether such subsystems might be equally needed, indeed required, on recumbent bikes that differ from diamond frames also in weight distribution (article appended last) Taken on face value the LWBs with their relatively unweighted front and heavy butt ends stand to benefit from rear shock absorbtion more than anything. The few suspended 'bents in existence that I know of (LWBs: Dutch M-5, German Peer Gynt; SWB: US Presto) are all suspended in the rear. Still, my own limited experiences of mounting kerbs under speed while on a (stiff) LWB Roulandt were far from promising... I remember moments of instant fear whether the front rim would take it or if I bought it this time.... nothing I'd recommend while crossing a potentially car-infested road. Perhaps the small-diam wheel front wheels of LWBs contribute to making the them less sensitive to shock- induced deflection on account of greater "stiffness factor" (calculated by multiplying the rim's official designation by the difference in the US and Euro values of the constant pi ;-)) Strangely enough the Presto claims a weight distribution of 54% front/ 46% rear, yet offers rear dampening rather than one for the front end. --Ian encl. # Date: 28 Jul 92 01:50:35 GMT # From: truesdel@ics.uci.edu (R. Scott Truesdell) # Newsgroups: rec.bicycles # Subject: Re: Suggestions: SPD pedals or Front Shocks upgrade? [....] Regarding suspension, the differences between the two leaders is pretty well established so I'll touch only briefly on them. Rock Shox: Pressurized air shock absorption with oil damping. Manitous: "Rubber bumper" shock absorption with nylon bushing damping. Rock Shox work well for faster, staminaed riders and big bumps. Manitous work well for slower riders and smaller bumps such as fire road wash board. Rock Shox work best on the roughest 1/4 of off-road riding conditions. Manitous work best on the smoothest 3/4 of off-road conditions. Rock Shox are preferred by riders who "don't want suspension". Manitous are referred by riders who want the comfort of suspension to be active over a broader range. Rock Shox are virtually unridable under air or oil seal failure. Manitous are still very ridable under complete failure (bumper disintegration). Rock Shox require more frequent maintenance or air pressure and oil seals. Manitous have lower (almost no) maintenance. Rock Shox will 'save yer ass' if you frequently ride on the ragged edge. Manitous will make all your riding more comfortable and safe. -- end of encl. matter From: " pardo@cs.washington.edu" Date: Thu, 30 Jul 92 15:32:04 -0700 Subject: Re: suspension in 'bents >[Would a suspension be even more useful on a 'bent than on uprights?] I believe suspensions are more important for 'bents than for uprights. I base this both on theory and experience. 'Bents typically have at least one small wheel. A small wheel rises over a given bump in a shorter time (shorter forward distance at a fixed forward velocity) than a larger wheel. That requires greater vertical acceleration to get the wheel over the bump, and the wheel is more likely to go airborne rolling down the down side of a bump. On 'bents it is also impossible to `stand on the pegs' as on a conventional bicycle or motorcycle, which exagerates the bump problems on rough surfaces. These problems mean that a suspension can improve both comfort and efficiency. I have gone trail riding on a Hypercycle-style SWB. At speeds that other riders found comfortable on upright bikes, my head was being jarred so abruptly that I had two or three simultaneous images of the world around me. >[Why do existing bikes have rear suspensions, though front may > take more load?] I don't know but can hypothesize: On SWBs, vertical space is at a premium. Adding a suspension uses a bit more of the already cramped space. In addtion, ready-made forks are designed almost all for 26" wheels. The rear suspension is simpler to design and build: the wheel has only one degree of freedom. My neighbor Grant Bower has built several fully-suspended recumbents. They are SWB and the front suspension is a significant advantage. On a LWB recumbent, I would expect a suspension to help more with traction and less with comfort. However a good design requires a very lightweight unsprung section becuase there is little downforce on the front wheel of a LWB. ;-D oN ( Springing in to action ) Pardo From: R.Stclair@EBay.Sun.COM (R D St.Clair) Date: Thu, 30 Jul 92 17:40:59 PDT Subject: Re: suspension in 'bents I have built a SWB with suspension front and rear so here are some comments on bent suspensions. Mountain bikes most need the suspension in the front because that is where the fine control takes place. Also the rear is effectively suspended by standing on the peddals. Recumbents are a whole nother issue. First they rarely leave the road. 2nd, you can't stand up. In my experience suspension on recumbents has 2 major uses. One, it reduces snake bite flats on high pressure narrow tires. Two, it improves control by keeping the wheels on the ground a little more. It does not help when climbing curbs. I wouldn't even try it with my 16" x 1 3/8 front tire. You can't pop a wheely on most recumbents so I just don't try. I designed my suspension to remain stiff except for major bumps. The total travel is only about 1.5" front and rear, but that is enough for on the street. It does not smooth minor bumps, nor do I want it to. Absorbing minor bumps would disipate energy, and I am a poor enough source of energy as it is. I believe front suspension is more important than rear on SWB's as the crank bounces higher than the front wheel (Assumed rotation about the rear wheel axle). Also the smaller tire tends to be up front. Thanks, R.D. From: dangold1@iastate.edu Date: Sat, 26 Sep 92 13:32:55 -0500 Subject: Re: suspension in 'bents From: dangold1@iastate.edu (Daniel M Goldman) Organization: Iowa State University, Ames IA Date: Sat, 26 Sep 1992 17:20:39 GMT Subject: side by side tandem I am not an engineer, folks, but perhaps some of you who are will comment. You've read about side-by-sides before, but this might be new. I propose that two totally independent bicycles could be made so that they can be linked together, with certain bracket or clamp members built or desiged in. Why bother? For fun. After riding to work all week on my bicycle, me, my wife and infant daughter go to the park for a gentle ride on the nice, wide, paved bike paths. The two bikes clamp together, with a basket or seat in the middle (or to one side) for the child and a backpack with snacks. We can all talk without using rearview mirrors, radio headsets, or turning around. One can pedal while the other relaxes. When both pedal, they choose their own gearing. At the end of the day, we are back to two, independent bikes. I'd really like to see this done with two recumbents. Why else? If you can sell me one cycle or recumbent which can be attached to another, I'll buy TWO, not just one. What *I* get is THREE. Two independent units, and one tandem. The sales department sells twice as many units, and the customer is 33% happier. OK. Then there are all you na sayers. 1) "It can't be built stong enough." Come on you guys. With the materials and physics available to you today, is this really true? Perhaps it needs to be made somewhat flexible, the way they are making huge farm implements now which follow the land contour. 2) "its too wide" Remember, with the turn of a few wingnuts, snap of a few clamps or other such clever devices, you're right back to two wheelers if the road is too narrow. Where the path is sufficiently wide or the traffic sufficiently light, go back to tandem. 3) "The steering won't work". You mean a few linkages can't be designed to do this easily and simply? I am inclined to agree that for you power riders or off-the-road people, this might not be an attractive item. But it is to me, and I think there are many other people who would be attracted to this. I realize I have oversimplified the problem from an engineering standpoint, but I also don't believe it is anywhere near as impractical or unmarketable as I have been told. You people work on bikes-is this really such a foolish desire, or could some clever, innovative person really go-to-town on an idea like this and DO something with it? I'm sure this isn't an original notion-if so, is it worth reexamining? -DmG From: Torsten.Lif@eos.ericsson.se (Torsten Lif) Date: Fri, 31 Jul 92 08:47:36 +0200 Subject: Re: suspension in 'bents For my own part, I'd definitely like some sort or rear suspension on my (LWB) 'bent to take up those bumps where you can no longer lift your butt of the seat. "Poor-man's suspension" is fat tires. I'm currently up to 37 mm on my rear wheel. Suspension design on a bent should be easier than on an upright since you can't stand up. On an upright, standing up changes the entire dynamics of the bike - the whole bike becomes a "boggie" under the mass of the rider and the legs act as primary suspension/shocks. The bike's own suspension then has to deal with a completely different system compared to when the rider is sitting down. This is why it seems hard for MTB designers to come up with suspension systems that work well under all riding conditions. But on a 'bent you sit down all the time. There's no weight shift from a heavy rider moving up and down as he pushes up a hill and any "unsmooth" pedalling that leaves a momentum component on the crank-rider axle is mainly horizontal and does not affect the suspension. What remains is the torque of the chain pulling on the rear triangle and that can be minimized by proper geometry. The Linear with the folding rear wheel assembly could be an easy way to start for anyone interested in fiddling with suspension systems. IF they are prepared to tackle the problem of aluminium wear in the joint first :-( Front wheel suspension would be a boon too since the front wheel is so small. Something small and simple should be sufficient - the weight on the front is ridiculously low. The obvious source for parts would be that world-renowned maker of small-wheel suspended bikes - Sir Alec. Anybody have a spare Moulton I can cannibalize the front fork from? :-) Another possible way to achieve better comfort on a 'bent could be to have a sprung seat. Unfortunately, on most 'bents the seat is an integral part of the frame and having it move up-and-down would either weaken the structure or make it heavier. Maybe I should just make a thicker foam pad for my seat. :-} /Torsten From: DAVID WITTENBERG DTN 226 5276 Date: Fri, 31 Jul 92 10:00:42 EDT Subject: RE: suspension in 'bents ---------------------Reply to mail dated 30-JUL-1992 17:02--------------------- The presto's suspension is on the rear half of the seat, not on the rear wheel. It was a very cheap way to suspend a lot of the seat, and I think they chose the rear on that basis. --David Wittenberg From: Gregory Snead Date: Mon, 03 Aug 92 09:04:05 MDT Subject: suspension in general I would like this group to spew forth all it's knowledge of suspension systems! Let me begin by doing a brain dump of what I know. (Please excuse my ignorance of the the terminology!). Kinds of suspension: - solid object, e.g. a rubber ball or cylinder like under the seat of a Presto. This seems to be this simplest method. The problem is controlling how springy or tight it is. Also, does the ball start to loose it's shock absorbing properties over time? - cylinder, e.g. rock shocks fluid (oil, hydraulic fluid?) air leaf springs Much harder to incorporate in the design when making a home built. Also, more expensive. Places to put it: - front - rear - seat Problems: The two problems I'm worried about are energy wasted on stuff beside bumps, e.g. pedaling, and weight. On the energy issue I have a couple of examples. - On the Cannondale MTB that has rear suspension, doesn't the torque caused by pedaling put the rear wheel up? - One of the advantages of a 'bent is your back is supported enabling greater power output through the legs. Doesn't the seat suspension on the presto absorb some of that energy? I talked to a guy at the Seattle Bike Expo who makes trikes (sorry can't remember the brand) who made sure that there was no give in the seat. For comfort he added an air cushion on the bottom of the seat. On the weight issue, I'm mean the weight of the actual suspension system. Of course, the more the rider weighs, the larger the suspension system will have to be. I know that the fastest way to improve your speed (if speed is an issue) is to loose ten pounds of body fat. Weight is not that critical of an issue with me (my daily ride bag weighs 13 lbs.), but making a clean and efficient design is. Another problem is getting analysis paralysis with suspension and not getting my bike built. Most likely, I'll make my first without suspension. Solutions & Preferences: I don't like the idea of dual shocks, e.g. rock shocks, I'd prefer to have just one shock per wheel. One of the designs I've toyed with in my mind is not having a fork over the wheels, but a single arm going to the hub. Some motorcycles do this. I like the idea of air in the cylinder. It allows me to replenish the supply and to vary the tension. ? ================================================================= "What necessarily follows from something necessary is itself necessary" -Unknown [Gregory Snead (snead@mdd.comm.mot.com)]