HPV Trike front suspension - assistance needed
HPV Trike front suspension - assistance needed
(OP)
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... :(
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... :(
RE: HPV Trike front suspension - assistance needed
Just don't waste another question with any more "Can I ask a question?" type fluff. Thanks.
Mike Halloran
Pembroke Pines, FL, USA
RE: HPV Trike front suspension - assistance needed
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)
RE: HPV Trike front suspension - assistance needed
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: HPV Trike front suspension - assistance needed
RE: HPV Trike front suspension - assistance needed
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
RE: HPV Trike front suspension - assistance needed
RE: HPV Trike front suspension - assistance needed
Mike Halloran
Pembroke Pines, FL, USA
RE: HPV Trike front suspension - assistance needed
I suspect that a beam axle is actually the correct solution.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: HPV Trike front suspension - assistance needed
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
RE: HPV Trike front suspension - assistance needed
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: HPV Trike front suspension - assistance needed
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...
RE: HPV Trike front suspension - assistance needed
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: HPV Trike front suspension - assistance needed
RE: HPV Trike front suspension - assistance needed
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.
RE: HPV Trike front suspension - assistance needed
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...
RE: HPV Trike front suspension - assistance needed
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: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: HPV Trike front suspension - assistance needed
RE: HPV Trike front suspension - assistance needed
RE: HPV Trike front suspension - assistance needed
Now I'll need some time do digest it :)
RE: HPV Trike front suspension - assistance needed
I know this does not help, but personally I see two wheels as most efficient for a HPV, with some flex in the front fork as the only suspension.
Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
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RE: HPV Trike front suspension - assistance needed
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: HPV Trike front suspension - assistance needed
Regards
Pat
See FAQ731-376: Eng-Tips.com Forum Policies for tips on use of eng-tips by professional engineers &
http://eng-tips.com/market.cfm
for site rules
RE: HPV Trike front suspension - assistance needed
RE: HPV Trike front suspension - assistance needed
RE: HPV Trike front suspension - assistance needed
www.v8mongrel.com/_i/slarck.xls allows you to look at the geometry.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: HPV Trike front suspension - assistance needed
The normal upper-and-lower-wishbone arrangement has the lower arm almost horizontal and the upper arm shorter than the lower and its inner attachment point on the chassis lower than the attachment point at the spindle, which puts the instant center on the inside and relatively low to ground level. Explain what your rationale is for doing it differently because I don't see it.
RE: HPV Trike front suspension - assistance needed
There is no "rationale" for doing it the completely opposite way. The sketch defines the starting points (not fixed) in space which allow for sufficient clearances. The "hard" lines and points are something that defines the minimum, the dashed/construction lines are just to show the awful result.
I'll try to move some of the points/parts to get dimensions allowing me to get the IC to the proper location...
http://www.storyofstuff.org/
RE: HPV Trike front suspension - assistance needed
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: HPV Trike front suspension - assistance needed
BTW, did some fresh calculation on steering, displacing the steering elements to back, now I can recalculate the required clearances on the chassis side to get the desired position of the instant center... Someone might find these free xl calc sheets to be useful, created by Peter Eland: http://www.eland.org.uk/steering.html
I'll be back soon with a fresh sketch, just need to do some math and 3d visualization to check for clearances.
http://www.storyofstuff.org/
RE: HPV Trike front suspension - assistance needed
In front view, the lower control arm at nominal ride height should be horizontal give or take. (purpose: avoid excessively high roll center and tire scrub with suspension movement)
In front view, the upper control arm at nominal ride height should have the ball joint at the steering knuckle higher than the pivot axis at the chassis side. (purpose: get the instant center in a reasonable place so that body roll results in camber going in the correct direction)
In front view, the upper arm should be shorter than the lower one. (purpose: increase camber gain with suspension compression i.e. body roll)
In front view, the lower ball joint should be only slightly lower than the axle center line and the upper ball joint should be well above the axle center line. (no purpose behind this, it's just the way it normally works out if you want decent ground clearance, but it has something to do with what happens next ...)
In side view, the pivot axis of the lower control arm should be more-or-less horizontal. (In combination with the paragraphs above and below, this minimizes fore/aft movement of the wheel with suspension travel.)
In side view, the pivot axis of the upper control arm should slope downwards towards the rear. (purpose: this introduces a side-view rotation component to the steering knuckle with suspension movement, and this has the effect of producing an anti-dive effect when the brakes are applied, offsetting suspension compression due to load transfer when applying the brakes. There is a side effect that the trail and caster angle will change with suspension motion, but see below for an optional feature that can minimize this.)
In top view, the pivot axis of the lower control arm should be parallel with the vehicle centerline.
In top view, OPTIONALLY, if you make the pivot axis of the upper control arm further apart at the back than at the front, it will increase the anti-dive effect with suspension compression while allowing the change in caster angle and trail with slight deviations from nominal ride height to be minimized. Not all designs use this but several good ones do.
"By how much" you make all of these features ... depends on the situation, depends on the weight distribution, depends what your objectives are, depends how much space is available.
RE: HPV Trike front suspension - assistance needed
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: HPV Trike front suspension - assistance needed
RE: HPV Trike front suspension - assistance needed
The drawing shows the front view, one half, no load. The lower arm is almost horizontal, about one degree off from horizontal...
http://www.storyofstuff.org/
RE: HPV Trike front suspension - assistance needed
BTW, the lowest point of the chassis is significantly lower from what the sketch would suggest - the frame doesn't follow the mounting points/pivoting points on the chassis...
http://www.storyofstuff.org/
RE: HPV Trike front suspension - assistance needed
Long fvsa length implies gentle curves, lower RCH, less camber recovery.
As I said before, if you are sure that your tire wear is driven by track change in roll, then SLARCK or CAD will do the job.
However, I suspect from a bit of poking around that much of the tire wear is caused by aggressive corner entry and drifting in corners.
The way I look at kinematics is to decide what the important parameters are, and then write down a set of target values, and then see if I can fiddle the geometry to get them. If not then I know I need to change the architecture, or trade off some of my targets.
So, do you have a target for camber gain? Typically on a road car it is around 30-100% (camber/roll), whereas for a race car more is often appropriate.
If you specify camber gain and track gain in roll=0 then I suspect you have pretty much defined your upper arm geometry, for a given lower arm.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: HPV Trike front suspension - assistance needed
http://www.storyofstuff.org/
RE: HPV Trike front suspension - assistance needed
RE: HPV Trike front suspension - assistance needed
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?