Recumbents and all feet forward machines
I have built a new suspension trike. Like my last design, it uses Firestone 4001 rolling lip airbag springs and a single rear damper.
Front track 90cm, wheelbase 102cm, seat height 34cm (loaded)
Front suspension is unique: leading arm, torsion tube, beam axle with Watts Link. Rear suspension is space-frame trailing arm.
7 inches rear travel, 6 inches front travel for single wheel bumps, 4 inches for simultaneous two wheel bumps.
Front kingpins are Greenspeed GT3 mark 2 with their negative scrub radius, swapped left to right. I run more static castor. Steering is a variation on Greenspeed â€˜non-crossoverâ€™ steering optimised for ratio, Ackermann and bump steer.
Wheels are 16 inch.
81 gears, front drum brakes, rear hydraulic disc brake.
All chrome moly steel, nickel bronze brazed together.
Disassembles to package about 80 x 40 x 50cm.
First testing is very encouraging. Ride quality as good as previous (20 inch wheel) design, steering better, maximum lateral cornering performance better (10cm lower seat height).
Next steps are a lot more testing, and construction of carrier, then testing in fully loaded touring configuration.
Unfortunately, I can't post a link to pics - perhaps someone who has more posts than I have can link to the pics able to be found at photobucket user: JulianEdgar
Pics show prototype (just roughly painted black) with suspension at full droop.
Welcome to the forum Julian.
Thanks for sharing Julian. You've put a lot of work into that, by the looks of it.
Last week I covered the description of the frame and seat, and said that I was about to start some serious testing. So what did that testing involve â€“ and what were the results?
Since Chalkyâ€™s major purpose in life is to be a touring machine, the first major testing was to load up the beast. Not having yet built the rear carrier, I used the one from my Air 150 recumbent trike - this fitted straight on.
I then attached two Arkel RT40 panniers and loaded them and the carrier with no less than 43kg of water, contained in plastic bladders and a plastic container. The load of 43kg is a LOT â€“ I expect my normal touring load to be under 30kg, so 43kg represents a 30 per cent overload.
The test course was out of my workshop onto the front yard (a 60mm step), across the bumpy yard and then around the bitumen cul de sac on which I live, running at times onto the bumpy dirt verge.
But I didnâ€™t get as far as the bitumen â€“ an ugly sound occurred from underneath and I found that the front of the rear suspension arm had spread a little, allowing one of the two bearing carriers to pop off the bearing. I added a brace to tie the front of the rear suspension space-frame together, then went testing again, still with the 43kg extra load.
This time all seemed fine over the bumps.
Throwing the machine around (two wheeling, full steering lock at speed, hard braking) showed that the extra â€˜touringâ€™ weight was located higher than desirable: the trike rolled a fair bit and, moreover, could be provoked into lurching with the weight transfer. Clearly, the borrowed carrier was positioning the loads higher than they needed to be, so making the correct carrier became a priority. However, how would the machine handle at speed, even with the loads mounted so high?
I rode some local roads â€“ very steep and bumpy bitumen. At speed over long wavelength, low amplitude bumps, the trike could develop a corkscrewing motion â€“ the lack of roll damping, and the lack of front suspension damping, making themselves felt.
Under front-wheel braking, even with the touring load (but worse without it), the rear suspension extended abruptly â€“ more rebound damping was needed.
But these were about the only dynamic deficiencies.
Over sharp bumps the ride comfort was extremely good. With the heavy load, tyre pressures needed to be high (otherwise tyre scrub, especially of the back tyre, occurred in really hard cornering), but the steering was good and there was none of the twitchiness that could characterise the Air 150 running similar loads.
The next test was to ride the flat cycle paths and roads of the Gold Coast, pulling a trailer with my four year old son Alexander in it. I used a Chariot Cougar 1 child trailer; Alexander weighs 19kg. The trailer draw bar was attached to the rear wheel of the trike, not the suspended frame.
In this form the trailer towed well, but at full speed, the direct connection of the draw-bar to the rear wheel could be felt to be steering the trike a little. That is, any yaw of the trailer was directly transferred to the rear wheel. The ~3kg downforce of the drawbar also effectively added to the unsprung mass, giving a poorer rear wheel ride, especially over short, sharp bumps.
The trailer obviously needed to be connected to the main (suspended) frame, not the wheel.
After this ride, the next day I had a slightly sore neck and shoulders â€“ the seat was a bit too reclined for my best comfort.
So this testing showed that needed were:
â€¢ a new towbar
â€¢ a new carrier
â€¢ less reclined seat
â€¢ more rear damper rebound damping
I revised the rear spring and damper mounts to position the seat in a more upright position. This also placed more of the mass between the front wheels. I refilled the damper with more viscous oil.
I then went testing. Both because of the more forwards weight bias and the more upright seat (in which I felt more comfortable), I found I could throw the machine around quite phenomenally. I havenâ€™t yet measured max lateral acceleration but I would think it's the highest of any recumbent trike I have ridden.
In fact, to get it up on two wheels and hold it there I found quite difficult.
However, by swerving dementedly back and forth I could throw it from being up on two wheels on one side, to two wheels on the other side, alternating back and forth, one side of the front suspension compressing hard each time a wheel came back to earth.
This is obviously immensely hard on the structure of the trike â€“ and something failed!
The rearwards mount for the front suspension â€“ through which all the anti-roll forces are fed â€“ started to crush the main tube and the braze weld to fail. Iâ€™d thought that the long length of the brackets would spread the load sufficiently â€“ but that was not the case.
See photo bucket album for pic of failure.
More work needed!
Full load testing of the suspension trike â€“ two RT60 panniers filled with water bladders, one tent, three filled water bottles â€“ cargo load 37kg. Rolling down steep hill swerving from side to side until wheel lifts.
Pretty impressive, but remember you're putting a huge amount of lateral stress on your wheels (especially the rear) which they are not designed for. You want well constructed wheels for trikes.
True, but trikes aren't designed to do that, especially fully loaded. Anyway, it's your trike - you treat it as you see fit!
It's crazy (and so are the stunts!) I like it.
"...for many people your life is not worth the effort it takes to pay attention or the extra few seconds they may need to wait before they can safely get around you."-BikeSnobNYC
You'll get it confiscated for three days if you pull moves like that in Vic, nice one.
...whatever the road rules, self-preservation is the absolute priority for a cyclist when mixing it with motorised traffic.
London Boy 29/12/2011
The first model was great but this one is awesome.
Why have you used Watts link at the front?
Easier to build/different handling?
Congrats on an awesome trike.
Optima Baron Raptor
Low Racer - Recumbent
Easiest to post from the forthcoming AutoSpeed series:
Designing a light, durable and long-travel suspension system for the front of a tadpole HPV trike is extremely difficult.
Traditional (read: automotive) design approaches like double wishbones are too heavy (four arms and normally a massive 12 pivot points), and approaches like MacPherson struts are also problematic â€“ this time primarily because no lightweight off-the-shelf combination spring/dampers are available that are designed to cope with the bending forces involved.
On my previous Air 150 I used a unique front suspension design â€“ semi-leading arms with high pivot points. The semi-leading arms can be thought of as being like automotive semi-trailing arms (eg on the back of an old BMW or Datsun) but facing forwards. The high-mounted pivot points resulted in a high roll centre and a relatively short virtual swing arm length. A very stiff anti-roll bar was used, and suspension travel was 6 inches.
During suspension movement, the Air 150â€™s track altered (so providing damping without the need for external dampers), the camber changed, and - to a smaller extent â€“ the castor also varied.
Despite going against every prevailing suspension textbook wisdom, the suspension worked extremely well.
However, the development of this suspension design over two machines (an initial prototype Air 130 and then the final Air 150) showed that the anti-roll bar (arrowed) needed to be so stiff that the suspension was, to a large degree, no longer an independent design.
(Note: on a three wheel machine, the roll stiffness is provided solely by the end with two wheels. With long-travel, soft suspension, a large amount of cornering roll will be generated â€“ even with a suspension design that uses a high roll centre. Therefore, roll stiffness becomes a very important part of three-wheel vehicle suspension design.)
So on the Air 150, the front suspension design comprised two semi-leading suspension arms, each with an inboard pivot and an outboard kingpin; an anti-roll bar supported on two pivots; and two anti-roll bar links using a total of four rod-ends (rose joints).
Over the Air 150, the design goals for the front suspension of Chalky were for a lighter design that integrated the anti-roll properties into the suspension structure â€“ ie, did not need an â€˜add-onâ€™ anti-roll bar. Suspension travel again needed to be around 6 inches.
Front Suspension Design
When looked down on from above, Chalkyâ€™s front suspension is shaped like the letter â€˜Tâ€™. The top of the â€˜Tâ€™ is a solid axle that joins the two front wheels, while the upright of the â€˜Tâ€™ points towards the back of the machine.
Solid (beam) axles are widely regarded as being very low tech â€“ suitable only for agricultural machines like trucks. However, the use of a beam axle in this application actually results in a very light-weight design.
The upright of the â€˜Tâ€™ is a torsion tube. It is pivoted at its base such that the wheel axle can rise and fall, but twist (eg when the body rolls) is resisted. That is, the torsion tubeâ€™s pivot point is a horizontal axle, with its axis at right-angles to the length of the trike.
To prevent the wheel axle moving laterally (ie by bending the torsion tube), a Watts Link is placed at the mid-point of the beam axle. This prevents the axle from moving sideways, while still allowing free vertical movement.
Firestone model 4001 rolling-lip airbag springs are located a little inboard of the 16 inch wheels. These air springs are light in weight, have no stiction, can be easily optimised via internal pressure for differing load carrying conditions, and can be mounted without the ends needing to stay parallel through the full suspension movement.
This apparently simple suspension design is actually quite complex in the way it works. Letâ€™s take a look.
â€¢ Two-Wheel Bumps
Over two-wheel bumps (eg a speed hump), the front wheels rise and fall together. That is, the wheel travel and the spring travel are the same (ie a 1:1 motion ratio). The specâ€™d travel of the air springs is 3.6 inches (but in this front application I run them at about 3.8 inches), so over a two-wheel bump, the maximum front suspension travel is 3.8 inches.
Over this type of bump, the torsion tube acts just as a suspension member â€“ it doesnâ€™t twist. The Watts Link ensures the axle rises and falls without any sideways movement.
Steering castor varies with wheel movement, with the castor increasing as the suspension compresses.
â€¢ One-Wheel Bump
Over a one-wheel bump (the vast majority of bumps â€“ letâ€™s imagine a stone on the road) a single wheel rises.
Because the axle is supported at the opposite end by the other wheel, and because the spring is mounted inboard from the rising wheel, and (finally) because the torsion tube can twist (the inertial mass of the rider resists being rotated, so the torsion tube twists instead), the compression of the spring on a one-wheel bump is less than it would be for a two-wheel bump of the same size.
Therefore, compared with a two-wheel bump, the spring rate is effectively lower, and the maximum travel is potentially higher â€“ probably at least 6 inches.
In addition to the air spring, the torsion tube also resists the one-wheel bump (remember, it acts as an anti-roll bar, and anti-roll bars resist one-wheel bumps). One-wheel bumps attempt to cause some sideways movement of the axle, but this is prevented by the Watts Link.
When cornering, the body rolls.
Resisting this is the air spring on the outer wheel and also the longitudinal torsion tube. The spring has a slightly rising rate (it gets stiffer as it is compressed more) but the torsion tube is linear in its torsional behaviour.
Itâ€™s in cornering that the Watts Link is subject to its greatest forces, with its action stopping the axle being pushed sideways.
The suspension sees dive much like a two-wheel bump â€“ the spring rate is effectively higher than it is for one-wheel bumps.
â€¢ Steering Lock
Another characteristic of this suspension system is not in its behaviour but in its shape â€“ there is a lot of room for steering lock to occur.
â€¢ Spring Motion Ratios
As implied above, the spring motion ratios are kept low. Compared with motion ratios where the spring moves far less than the wheel, this approach reduces bending stresses in the suspension arms. In the case of the air springs, it also allows a lower inflation pressure to be used (giving better headroom to the max allowable pressure, and so also permitting the machine to cope with bigger load variations).
This suspension design uses about 25 per cent less tube than the Air 150â€™s semi-leading arm design. The front suspension arm, complete with Watts Link and rear bearings, weighs 1.7kg. With the two air springs, this rises by 700 grams for a total of 2.4kg.
The leading, torsion tube, beam axle with Watts Link design also has some shortcomings.
Firstly, this suspension design doesnâ€™t have a dynamic negative camber increase on bump â€“ that is, the outer loaded wheel does not adopt negative camber during cornering.
But, in fact, it actually does!
This is achieved not by the suspension but by the steering, that runs a lot of castor and so automatically develops neg camber when steering lock is applied. Furthermore, as the outer spring is compressed in cornering, so the castor (and therefore neg camber) increases.
Secondly, this suspension design doesnâ€™t have the â€˜track changeâ€™ damping of the Air 150â€™s semi-leading arms.
After a lot of experience of the track change design I still cannot make up my mind as to whether track change damping is a good idea or not. On the Air 150 it worked very well in terms of damping, but when the machine was ridden really hard, front tyre wear could also be dramatically high. (But then again, when ridden normally, tyre wear was fine!).
Itâ€™s impossible to have a beam axle and have track change with suspension travel, so once a beam axle was decided upon, track-change damping as an option went out the window. Damping on Chalky will either be via hydraulic damper(s) - or no damping used at all. Initial testing indicates that dampers may not be needed (in part because the natural frequency of the suspension is so low, and so it is less easily excited by short-spaced bumps.)
Having the steering wheels on the one axle also has the potential to give bad steering shimmy, a problem that often occurred in cars of the 1920s and 1930s equipped with solid front axles. (It also can occur in unusual bicycle designs.) However, shimmying always seems to be able to be overcome, either by revising stiffness of bushes (etc), or by changing the steering stiffness and/or geometry. I also wonder if shimmy of those old cars wasnâ€™t exacerbated by the zero scrub radius, low castor angles and positive camber then often adopted.
The roll centre of the suspension is low. This was required for packaging purposes, but it also means that the amount of body roll that occurs for a given cornering force is higher than it would be if the roll centre was further above the ground.
Finally, the pitch centre is also low â€“ which could cause dive problems when braking (but at least dive is resisted by both springs) and allow rear suspension extension, again when braking.
www.autospeed.com series starts in about 4 weeks. For anyone interested in recumbent suspension trike building, my two previous attempts are covered in AutoSpeed in great detail - site search under 'human powered vehicle'.
Nicely built - like what you've done withthe front kingpins to get the steering pivot point to go through the contact patch - good thnking.
Seems to me, though, you could do with a substantially lower CofG. The seat seems high compared to the roll centre, which is at ground level at the rear, and at the centre pivot on the Watts link at the front. Can't get your bum closer to the ground? Hmm, Probably not on this particular build. If you can't manage that, any scope to go wider at the front?
Last edited by trailgumby on Mon Apr 06, 2009 9:10 pm, edited 1 time in total.
Sorry if the first edition of my post was a little brusque.
Moving the CG down closer to the roll centre will reduce the roll moment, and therefore the propensity to roll and then ultimately tip over. You then wouldn't need to worry so much about damping or roll stiffness. I acknowledge it would be difficult without a major re-engineering of the current design, but it's worthwhile if you can get it even a little lower becasue a low CofG helps improve handling manners in many many ways...
There's nothng you can do to increase roll stiffness at the rear apart from dropping the CG, but on the front you could try lifting the pivot point for the Watts link to bring the roll center up closer to the CG. Unfortunatley that is limited in how far you can go before you start having to fiddle with pivot mount points to maintain link geometry, and past a certain point it also means you'll just lift a wheel in the air sooner.
If you're interested in the physics of car design (whihc is what you've built, strictly speaking, since it can no longer lean into the corner), you may find Carroll Smith's book Tune to Win very, very useful. Another good book is Paul van Valkenburgh's Race Car Engineering and Mechanics. They're about $60-$75 each and well worth it if you're building and interested in getting good (and safe) handling. Another goodie by Carroll Smith is Engineer to Win. Some good stuff on metallurgy, brakes, etc. Hope this helps. Steve Smith's Advanced Race Car Suspension Development has some great worksheets for working out things like CG heights, roll centres, roll moments, roll axes - again, all extemely useful stuff if you're chasing improvements in handling manners.
Edit: another thought - you could also try moving your springs further outboard on the front. Failng that, an anti-roll bar of some description.
I am aware of all these ideas, and have most of the books you have mentioned. The increase in roll stiffness is not needed for the reasons you suggest - as I already suggested, there are no problems at all with cornering. In fact, the COG is very low - it cannot be much lower with this much suspension travel.
The roll stiffness is easily increased by changing the diameter of the leading torsion tube; the reason I didn't do that from the beginning is that it also increases the stiffness of one-wheel bumps.
This is my third suspended trike, and I have used different front suspension designs on each with differing roll centre heights, differing camber changes, differing roll stiffnesses, and differing anti-dive.
Sure. I guess that wheel in the air kinda waves flags for me somewhat, but it's your project and you've got far more experience than I with trikes and recumbents. The thought I had was to leave your feet and knees where they are and try to get your bum down a bit lower - moving a little toward an F1-style sitting position, come to think of it.
If you've got the books I mentioned and understand them from a practical perspective, then you're more than halfway there and there's not much more I can contribute.
I used to race nitro R/C cars for more years than I'd care to admit. The cars were fully adjustable - camber, track, toe, spring rates, roll bars, ride height, droop and bump travel, damping, shocker angles and positions, caster, some even had adjustable wheelbase, plus a bunch of drive train adjustments and some rudimentary aerodynamics. It was a great playground for learnign about vehicle dynamics - the physics is identical, changes were very quick to make and the feedback was pretty much immediate. I wasn't particularly quick as a driver, but my car more than made up for it - thanks almost entirely to what I learned from those books, especially Tune to Win. I even managed to snag a State Champs or two - against drivers of vastly greater talent.
Hence, I'm a bit of a Carroll Smith tragic.
Good luck with it.
I am currently experimenting with the air springs being used as fluid displacers. In this approach the miniature Firestone airsprings are filled with water. The displaced water moves through hoses to plastic bottles pressurised with air.
This approach allows suspension interconnection and also allows damping valves to be placed in the interconnection lines. It is very similar to the 1970s BMC Hydragas system, except using air pressure rather than nitrogen pressure.
I am currently running two 600ml bottles for the front (non interconnected, ie one for each displacer) and a 300ml bottle for the back.
With this system damping can be very easily driver adjusted, ride height can be easily adjusted, and damping can be configured to remain constant in rate with differing loads.
I have been comparing the ride comfort of my pneumatic / hydraulic suspended trike (â€œChalkyâ€) against a Brompton folding bike (16 inch wheels, sprung seat, small-travel rear suspension) and a Gazelle Medeo Plus (high quality commuting bike, 28 inch wheels, sprung seat post, sprung front forks). Chalky has 16 inch wheels.
The tests were conducted at a fairly slow pace over a bumpy grass-and-dirt surface.
This pic graphs the RMS values for accelerations in X, Y and Z planes of the three machines over the test course.
This shows the actual traces for the measurements.
I found the software very good, especially when attempting to gain best compromises between conflicting requirements â€“ eg reducing body roll by the use of a stiffer anti-roll bar versus the resulting increase in the wheel rates in one-wheel bumps. Spring rates, suspension interconnection and damping rates were all also developed using the data.
The above move to using the air springs as water displacers occurred after a trip.
A last minute change of plans (a major bushfire in the area Iâ€™d intended to go to) meant I went straight over the Great Dividing Range on what turned out for much of the distance to be a fire trail through a national park. (From Hoskinstown to Braidwood, New South Wales.)
With the longer trip in mind I was packed for 4 days of self-supported touring, including 15 litres of water, food, spares, etc - 39kg of stuff.
Was very tough - only about 20 kilometres for this section but very steep, rough, and often wet and slippery dirt. Camped overnight.
I didn't expect the road to turn out to be a fire trail (had poor map and no phone reception for nav) and I certainly didn't expect it to be so deserted - no one passed me for 18 hours or so. So I was a bit disconcerted by the difficulty and lack of other people. Knowing what I now know, I would do it again, either in the company of others or with an epirb (even a sprained ankle would have been very dangerous).
Blew seals in all three suspension dampers (thus the change in direction in damping approach) but had no punctures and no other failures.
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