HPV Trike front suspension - assistance needed 3

[steinmini]

Hi to all,

Before I start asking specific questions, I'd like to ask one, is it appropriate to do it here? This seems to be the best place, but it's not about a car suspension, but for a full suspended Human Powered Vehicle (HPV), tadpole config, steering on front wheels, rear (single) wheel drive, ackermann incorporated...
If you say it's acceptable to talk about it on this forum, I'll proceed, otherwise, I'll go elsewhere...

 

[MikeHalloran]

Close enough.
Just don't waste another question with any more "Can I ask a question?" type fluff. Thanks.



Mike Halloran
Pembroke Pines, FL, USA

 

[steinmini]

Ok.

I read just about everything I found on front suspension design. I'm still far from being an expert. So, here's what bothers me.

The complete weight of the vehicle should be as low as possible, within reasonable limits, it should be capable of handling rider's weight up to 150kgs.
The problem I have is the tire wear at high speed cornering. Need to reduce it as much as possible while keeping the steering/handling performance. The suspension system will be a double wishbone (two A arms) with the shock absorber (suspension element) attached between the two arms, unlike it's usual in automotive applications (lower end on the movable part, the other end fixed to the chassis)

The previous design I made was designed to accommodate use in extreme conditions (off road use) and tire wear was not such an issue. This new design is intended to be used for a comfortable ride on normal paved roads and the suspension should only make the ride comfortable and allow for predictable and reliable handling. The attached image shows the offroad version. It's a parallelogram with four rod ends at the end (holding the kingpin) and two movable pieces allowing the kingpin axis to be at desired angle - about 17 degrees from vertical to meet the tire contact patch on the road/surface.

The new design should have only two rod ends holding the kingpin (one on top, one at the bottom) meaning I need to make the upper and lower arms of unequal length. Need to get the kingpin inclination at proper angle which gives me a rough idea about the length difference between the upper and lower arm, but at the same time, need to minimize the track width changes at the lower end of the wheels as the suspension works to reduce the scrub during suspension travel. Tire scrub at cornering can be addressed with the proper setting of the Ackermann compensation and I'm aware that some will be present.

My confusion starts with choosing the roll center and reaction point (according to

http://www.rqriley.com/images/fig-12.gif

) and basically I could use any of these, but I'd prefer to pick the best solution for the purpose.

I suppose I am missing some obviously better ideas and solutions, but have no doubt someone will turn my head in the right direction.

The offroad version has about 10cm/4" travel (wheel/frame), the road version will be fine with about half of that.

The trike will be pedal driven but will have an option to use an electric assist engine in the back. (rear hub/wheel)

 

[GregLocock]

Bike tires are not designed to scrub laterally, therefore your primary focus should be on minimising cp lateral migration in jounce and roll. and presenting the outside tire to the road at the correct angle.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[steinmini]

With the offroad version there's no significant lateral scrub problem, most of the time, the trike is used on relatively soft ground. Asphalt OTOH might be quite rough of skinnier tires. Is there a relatively simple solution to minimize differences in track width at cp with various amounts of suspension compression? A sway bar maybe?

 

[MikeHalloran]

Have you actually built that offroad model, in hardware, in real life?

I'm curious about how you assemble the paired rod ends at the distal ends of the control arms, specifically how you swing one past the other while screwing the stems in, and how you phase the stem threads so that a bolt can go through both balls and the spindle without breaking something as you tighten the bolt.




Mike Halloran
Pembroke Pines, FL, USA

 

[steinmini]

Mike, the (3Dmodel) image I posted is a revision model ("sort of" a new model) based on already well proven product I designed back in 2005. Since 2006 it has proved to be one of the best and most popular HPV trikes with full suspension on the HPV market. We made and sold about 550 units so far with minor changes and improvements during the past five years. The basic concept is the same for the narrow and wide track models (about 8cm difference in track width) - but I prefer not to do any improper advertising here, I'd prefer to learn more about the subject. I am completely aware that I had a lot of luck during the design/development process with the off-road model... Google images for Steintrikes Mungo/Meerkat and MadMax. I think it will provide sufficient information on solutions used... Then, if I can provide additional detailed information, I'd be glad to do so.

 

[MikeHalloran]

That was sufficient; thanks.


Mike Halloran
Pembroke Pines, FL, USA

 

[GregLocock]

I'd do it with kinematics.

I suspect that a beam axle is actually the correct solution.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[MikeHalloran]

Unfortunately, a beam axle is a hard sell against 'IFS!'.

Another possibility is parallel fore/aft arms, like in old VWs. I would turn them around, to be leading arms, with the chassis pivots a little below the spindle pivots. Ford's Merkur XR4Ti used something along those lines, where the lower 'arm' was actually a huge anti-roll bar.



Mike Halloran
Pembroke Pines, FL, USA

 

[GregLocock]

OP needs to decide on what what contact patch and body locations/angles he wants when cornering and straight ahead, and then work back to a mechanism.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[steinmini]

Unfortunately, neither the beam axle, neither the trailing/leading arms (old WV) do the job in this specific construction. I suppose that I don't have to explain to you guys why. As mentioned, the overall weight of the whole vehicle must remain low. The design should be as simple as it's possible, but not simpler than that. The ideal case would be a tilting (leaning) trike , doable, but ends up with several problems, complex for both cost effective production and everyday use. There are some out there, but all ended up being too expensive and exotic and made in small numbers. The goal is to design a trike that would be comfortable, light in weight, reliable, with good handling, practical for use and affordable. Eleven years ago, when I started building them, someone said that I'll be the "new H.Ford of recumbent trikes" which is an overstatement, but I'm sure that since that time, the prices went down significantly, or at least, became less exotic. Most of manufacturers went to far East to cut the production cost and realized that the market will not grow if the trikes remain in the "luxury" niche...

My goal is to keep making them better and more affordable. That doesn't mean I'm a silly/crazy "treehugger", I'm just trying to do something useful in my life and doing my best to design reasonably priced vehicles for individual transportation...

 

[GregLocock]

So, do you understand enough about what you want your tires to do to define their required kinematic behaviour?

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[WolfHR]

This might be a bit daft question, but seeing that bike tyres are used wouldn't the 'leaning' concept be the way to go? I presume that bike and motorcycle tyres are not designed to deal with any significant amount of lateral loads, so from performance point of view it might be the best to use them in a way to keep normal loads (from tyre 'perspective') maximized and lateral loads to a minimum?

 

[steinmini]

@Greg:

Not quite. I know what I would like, but still not enough to be sure it's really what the design needs to be well done. At this time, I'm fiddling with dimensional stuff, need to take care about disc brakes, steering and suspension linkages and stuff that all must pass each other at all times without conflict. Once I'm completely done with that, I'll have the limitations defined and I can start tinkering with the options I can have within the limitations that certain parts/assemblies require.

I'm aware that I will not get zero scrub at all times, but if I get to the point where most of the parameters are good enough most of the time, then, I think I'll be able to say I did the best to achieve the best that could be done within existing limitations. Nobody will complain if the tires get worn after 2000 miles. Everyone will complain if they start showing wear after 200 miles. That's what I'm trying to prevent. And it's possible. I just have to keep learning and working on it. Fortunately, I'm my own boss and made no promises or commitments to anyone, so, I still have enough time to do it the right way.

 

[steinmini]

@Wolf:

The leaning concept is ok, but most of the people who look at our way don't like it. There are some fantastic designs, concepts and products out there using this feature, but for some reason(s), eight of ten people on fairs don't even bother to ask for a test ride... Tripendo is a defunct German company which made some beautiful leaning trikes, but the price was too high, weight figures also, and somewhat "overengineered" which lead to their end. We base our future models and designs toward pleasing the user... Shouldn't everyone get the best? Yup, but people often don't know what is the best, they rather pay for what they think it's the best for them... And the customer is always right... Even then, when he/she is not...

 

[GregLocock]

When the body rolls it drives a camber change and a lateral motion at the cp.

The latter is what you want to minimise. Effectively that demands a fvic at ground height, ie a RCH of 0. The length of the fvsa will then govern the camber gain.

Another theory is that the tire wear has nothing to do with lateral motion at the cp, and everything to do with using a bike tire as a car tire. Presumably you have checked that out.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[steinmini]

Figured a few things (cp to stand for contact patch) but I'm lost trying to find what FVIC, FVSA and RHC are. Guess I'm right believing F would be Front, and then... Sorry, I'm not an automotive engineer and English not being my mother tongue, just one of six languages I'm occasionally using, doesn't really help, neither does Google in cases like this one. (blush)

 

[WolfHR]

Steinmini, you are correct about contact patch, and fvic is front view instant centre (virtual point around which the wheel is considered to rotate in bump/rebound), whlie fvsa is front view swing arm (the horizontal distance of line connecting the fvic and cp). RCH is roll centre height (distance of roll centre from the ground plane). HTH

 

[steinmini]

Wolf, thanks for clarification. Now Greg's post became valuable, at least for me.
Now I'll need some time do digest it