Snow skis as the basis for the leaf suspension

[Minikettwiesel]

Hi. First of all let me apologise for writing in English! I hope that the readers of this thread can understand me. I am very interested in the suspension design of the Hi Trike, and wish to find out more about it. The one forum thread I have been following on the recumbent.com forum, has been fairly negative about the concept. One poster has been dissing the concept by saying that there will be braking problems causing the suspension to wind up, or roll over. Even suggesting that there should be track rods to stabilise the front end!
Anyway, I am looking at modifying my homebuilt trike with coroplast velomobile body. It currently has no suspension, and I am over the rough ride!
I would like to replicate the system at least in principal. The current idea is to use second hand snow skis as the basis for the leaf suspension.
I realize that this May not be as refined as the Hi trike suspension system, but will it work?
Any input will be appreciated! Thanks in advance, Rob

Dear @Freddyflatfoot , as my English ist not the best I hope that I chose the right title for your thread ...

 

[oDKo]

Hi,
I don't know anything about snow skis, hence I can't tell you if your idear works. But I would like to comment on the concerns regarding breaking behaviour you found in the other forum:

In principle tadpole trikes get their breaking stability by a low center of gravity that should be as far back from the front axis as possible. On the other hand it can be shown that side-stability while tuning requires the center of gravity (CG) to be as close to the front axis as possible. So, a good trike construction is a compromise of these two requirments. In reality this results in most cases in the CG being located low and relativly close to the front axis to meet both criterions but far enough back to prevent the trike from flipping over during hard breaking.
(In this thread there are some more detailed explanations and some pictures/drawings: https://www.velomobilforum.de/forum/index.php?threads/praxiserfahrungen-bremsfähigkeit-vs-kippstabilität-delta-vs-tadpole-in-gefahrensituationen.40824/ )

So far the theory without suspension...

If you have now a front suspension breaking results in deflection of the front suspension. As mentioned above the CG is close to the front axis and that results in a slightly down-movement of the CG - which invokes, with regards to the above criterions, a positive effect to the stability (CG comes down a bit).

So in general one can say with regards to breaking stability you want to have a front suspension. It makes a trike (a bit) safer.

 

[Freddyflatfoot]

For starters, thanks for starting this thread for me!
As I am modifying an existing trike, the CofG is already sorted. I have a reasonably long wheelbase, and low seat height. I have done quite a few thousand k's on her already, and braking stability has never been an issue. What has been an issue, is the stability of the trike at speed, and sudden bumps on one front wheel, causing shock and a rough ride!. I believe adding suspension will tame the front end down, and provide for a much smoother ride. I was planning on a double A frame front suspension similar to Steintrike. However, I felt there was too much travel, especially for a velomobile body. Enter the Hi Trike leaf design.
I love the simplicity of the design, and the ability to route the chain with minimal interference. I also appreciate that the amount of suspension movement will be about 30% of the Steintrike? Or maybe less. Of course this will depend on the stiffness of the snow skis I intend to use to try the concept.
To this end, I have picked up a total of 4 sets of skis, all of varying thickness, and springiness. This should give me the ability to fine tune the suspension characteristics by changing leafs.
I will start by using the lightest set of skis that I have, and see where I need to go from there.
I will, at the same time, change the steering from direct steer, to side stick steering. This should enable me to slow the steering down a bit, and allow the cabin room to be roomier, with a less intrusive steering system.
I have test cut, one of the skis, and it is a composite construction. The skis are thicker in the middle, and taper off both to the back and the front. As the taper is from the top of the ski, to the base, I suspect the spring characteristics vary depending on which way the ski is actually flexing. There is also a curve built into the ski. My first thought, is to reverse one of the ski leafs, to see, if there is a variation in the spring rate, that I might induce a natural dampening effect. At the same time, I could utilise the initial overall curve, to build in some pre tension into the suspension system, by pulling the leafs together at the kingpin. Of course, I am just dealing with supposition and hypotheticals at this point, but the one thing I am confident in, is that I will build a system that will provide some suspension, and be robust enough for normal road use.

 

[oDKo]

Do you have some pictures of the skis or maybe some sketches of the whole thing? Would be interesting.

 

[Minikettwiesel]

@Freddyflatfoot as you obviously do not understand German at all:
Top right you see your Name (left), Personal Notes/PN (Unterhaltungen, middle) and Hints (Hinweise, right).

If you got PNs or hints you will see a small red field with a number within (number of PNs or hints).

Minikettwiesel

 

[Gear7Lover]

... and on the bottom left you can change from Deutsch (Du) to English (US)

 

[Freddyflatfoot]

Ok, I have made some minor advances on the design. I have ordered 4 spherical bearings to place at the ends of the ski leafs to contain the front wheel kingpins. I am using my existing kingpins/steering system, so no further modification there. The axles are placed beneath the kingpin, so sit low, and will actually have the axle just below the lower suspension leaf. My only difficulty here, is that the lower leaf will be very close to the drum housing. I have calculated that the hole required for the spherical bearing will be only 5mm from the edge! Maybe a point of potential weakness. The spherical bearings will be retained by a combination of aluminium discs, and an 'o" ring spacer. The aluminium discs will be screwed directly into the skis.
I have already made all my aluminium discs. The lower ones are shaped differently to allow for the reduced width near the brake housing.
I cut off the end from one of the skis, and it is a full composite, no timber in sight! It cut very easily and appears it will be very easy to work with. To further test the offcut, I cut a 25mm hole approximately 5mm from the edge, and also drilled and tapped an M5 thread. Surprisingly, the material appears very strong! I haven't fully abused the edge near the hole, but it appears as if I may get away with this! I inserted an M5 bolt into the thread and fully tightened it with a spanner. I didn't strip the thread, so I am confident that I can use a thread in the ski to hold my retaining discs!
So far, I am fairly happy with the way things are working out!
My next decision, will be in the actual mounting of the leafs to the main frame. I have two choices, The first is to bolt the leafs directly with mounting holes. My first thought is this may create a potential weakness point? Maybe an area of high stress? Although the Hi Trike Pi is mounted in this fashion.
The alternative, is to use a clamping system, where a bar is used to clamp across the width of each ski leaf.
After I have mounted the ski leafs, I will work on the revised steering system.
I'll try and add some pics later.

 

[Freddyflatfoot]

Here is a pic of the test po rtion of my ski.

 

[Freddyflatfoot]

Managed to get some work done today! Cut down the skis, mounted my spherical bearing retainers, and got all my frame mounts all made up. Next step is to weld the mounts to the frame. Looks like the leafs are going to be quite stiff, and these are my lightest ones!

 

[Gear7Lover]

Cross country skis are much lighter. They are sold in different stiffnesses. Means, the body weight all on one ski must flex it to make the center part touch the ground. While with the weight on both skis the precamber makes the center lift off the ground. When you rent or buy those the first question is what you weigh. This refers to classic style cross country skis, not skating style.
The downside: They are much easier to break, since they are not built for the high forces that occur in downhill skiing. And they are much narrower.

 

[Freddyflatfoot]

Ok, a bit more advanced. I have my leafs done, bearings mounted, frame mounts done! Just need to do some fine tuning to get the fit right, and then make up my new steering! Need to take some more pics! I have used the lightest skis I had, maybe ladies or child? But they look they will be plenty stiff, I hope there will at least be some suspension movement!

 

[Freddyflatfoot]

Ok, here is a pic with my leafs mounted.

 

[Stefan M]

Hi Freddyflatfoot,
acceptable suspension with GFK-material needs two-dimensional shape. Your choice will be great.
Regards Stefan

 

[Freddyflatfoot]

Hi. a lot more advanced this weekend. Fine tuned all the fittings for the ski leafs and did the final? assembly. Also put the side stick steering together. My first effort was a bit flimsy, and flexed a lot, I redid it in some heavier material. It seems much better. I managed a very short test ride around the bottom of my court. I was very surprised! Even though I couldn't see any suspension movement, the ride felt very supple. I managed some bumps, and even though I felt them, they were definitely smoothed out! One of my welds on my handlebars started to give way, so I'll need to repair that before I do any further testing. The side stick steering felt good, although a much longer ride will tell if its any good. I'll also need to recheck my toe-in of course! All in all, a very productive weekend and I'm happy with my progress!

 

[Stefan M]

I have thinner GFK-parts and connection between GFK and wheels is done by hinges.
Works perfectly.

 

[Freddyflatfoot]

Stefan, you have built this setup already? I had thought about a hinged arrangement.

 

[Stefan M]

Stefan, you have built this setup already? I had thought about a hinged arrangement.

Yes,its built. After some testing of different GFK thicknesses (2-10mm) and also combination of different thicknesses with stiff arrangement GFK-wheel biggest effort was achieved by using hinges.
Maybe your solution by using ski-material is still too stiff even in combination with hinges.
Regards Stefan

 

[Freddyflatfoot]

I went for a reasonable trial ride today, around 20 k's, after making further mods to the steering. The ride was not without issues though! The ride itself was fantastic! Deliberately rode over bumps and just floated over them. Impressed! Now for the bad. Back wheel must not have been overly tight in the dropouts, and jumped out of the right dropout, causing the wheel to go sideways, bending the skewer, and the left dropout! I went sideways and tipped over! Continued the ride at a more subdued pace, and made it home ok. Had a couple of other minor issues I wont go into here. The main issue I am noticing, is that under braking, there is some distortion of the leafs, causing the wheels to 'rotate' Bit disconcerting, especially if I brake on one wheel. If I brake evenly on both wheels, it is more manageable. I am assuming that this is a known issue, and I suspect that the remedy is to try for stiffer springs (leafs).
One bonus I noticed with the side stick steering, is that it is much more comfortable and relaxed than the direct steering. The indirect steering really isolates the handlebars from the steering shocks. I only maxed out at around 36 kph, so I can't say how the high speed handling will be, but I suspect it will be much less twitchy than with the direct steering setup. Now, if I can just stiffen up the front leafs a bit more, I think I will have a good system!

 

[Freddyflatfoot]

Ok. I'm about 90% happy with my solution! I added a vertical Aluminium strut plate between each pair of leafs, close to the kingpin. Angled top and bottom and bolted to each leaf. The theory being that the strut will resist the twisting torque. I have test ridden the revised design and tested the brakes, both singly and in tandem, and under heavy braking from around 25 kph. What I have found, is the brake distortion is greatly reduced, but not totally eliminated. With one brake, I have a small amount of brake steer, that is very manageable. With both brakes applied, there is a very slight detectable distortion, but I feel at this point, that it is quite acceptable. Under heavy braking, the trike behaves pretty much the same way it did with the solid crossmember. I feel I have made real progress here!
My track width is 730mm,(about 27.5") and I am running 5 degrees negative camber. The amount of turning angle is 22.5 degrees in each direction. The kingpin inclination is approx. 12 degrees.

 

[Freddyflatfoot]

I had a recent test ride down a steepish, high speed hill. I did not like how the trike felt, because I could not balance the brakes! The small amount of distortion on the leafs was quite disconcerting, and the trike kept changing direction! I tried hard to limit my speed to around 40 kph or less. So now I have discovered a limitation with the skis. They don't have enough torsional stiffness. I am going to try a dual pull brake lever to link both front brakes, to see if that settles the problem down. I also need to see if I can 'link' the two leafs together better, to counteract the torsional twist. Stefan, you mentioned a hinged system you used? I would like to see a picture of that to understand what you have done. Thanks.
Kudos goes out to the Hi Trike designers, they have pulled something off that I now have a much better understanding of! Their leafs must be exceptionally stiff laterally, while being compliant vertically. To have a system like that with no? torsional rotation is a true master stroke!