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...

 

[patprimmer]

This is not my area and this may be a stupid suggestion, but could a huge castor angle and minimal king pin inclination be used to create camber changes to see leaning of the wheel while cornering? Would this in fact help? I know there are other complicating issues like track change, scrub radius, ackermann and bump steer and maybe even steering input force required.

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
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[GregLocock]

The aero advantage of a recumbent trike is very significant, as is the cool factor. It is possible to have a recumbent two wheeler but they are not for the faint hearted. If you use proper bike wheels they are /very/ long, or the short wheelbase ones use little front wheels, and look rather Heath Robinson.



Cheers

Greg Locock


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[patprimmer]

OK I was a bit naughty, now back on topic. Along a similar line to Mike, but slightly simplified, would a torsion beam axle work

Regards
Pat
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[steinmini]

After trying to get somewhere with the advices provided here, guess I'm back to square one. Need to re-think what I want to get what I need. The figures I had (regarding required clearances) simply don't add up... Funny, though, I thought that reducing expectations from a design (travel suspension and less frame/ground clearance for example) would come with somewhat easier ways to get to the desired goal...

 

[steinmini]

Ok, I'm pretty much of a "visual" type, I easily figure stuff I see, not so much things I read. Made a mock-up sketch with the basic arrangement and dimensions of the front wheel, disc brake and a sheet metal plate representing the kingpin. To get it where I need it (clearance thing) got about 18 degrees kingpin inclination from vertical which results in about 25mm/1 inch difference in distance measured horizontally from the middle of the chassis to the centers of the rod end balls which will hold the kingpin. That would be the outer limitation. The lowest end of the chassis should never get lower than about two inches from ground (suspension fully compressed) and without load, about 100mm/five inches from the ground. That places the lover arm's inner rotation point to about 120mm from the ground. The other dimensions are on the sketch. This would be some sort of basic starting set up with the possibility to change some positions on the chassis side, although not too much because of clearance issues, but I'll try whatever suggestion I get.

 

[GregLocock]

Can you redimension it so that each hardpoint has a horizontal and vertical dimension?

http://www.v8mongrel.com/_i/slarck.xls

allows you to look at the geometry.



Cheers

Greg Locock


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[BrianPetersen]

Your instant center is outside the vehicle and very high on the "wrong" side of the wheel. I see a whole lot of camber change in the wrong direction with body roll and a whole lot of trouble with steering kickback on bumps (due to sideways motion of the contact patch) with that arrangement.

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.

 

[steinmini]

@Brian:

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/

 

[GregLocock]

Does the single rear tire wear at the same rate as the fronts?

Cheers

Greg Locock


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[steinmini]

Depends on the riding style, "normal" riding (commuting, recreational riding, etc) gives no noticeable wear on the rear tire. In average, even on well designed no-suspension trikes, the front tires always wear more than the rear tire. Most of the time, improper steering design or toe-in setting is the problem causing this. Enthusiastic riding is what wears them quickly, but people who like that sort of "rush", don't complain about it, they know that the extensive tire wear is the result of the way they ride their vehicles...

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/

 

[BrianPetersen]

Rules of thumb with double wishbones. In all cases I am referring to dimensions to pivot points (i.e. between the pivot axis at the chassis side and the ball joint). The arm itself might be at a different angle but if the ball joint is bolted to it and sticking out the top, what counts is the dimensions involving the pivot axis on the chassis side and the center of the ball joint pivot. With that out of the way.

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.

 

[GregLocock]

Those are good rules of thumb for cars, but on recumbents the CG is much lower compared with the suspension and antidive and so on become irrelevant.

Cheers

Greg Locock


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[BrianPetersen]

Sure, but regardless of C of G location, there's still no way that it should be designed so that the camber goes drastically the wrong way with bump travel, which would have been the case with the originally-proposed design.

 

[steinmini]

Did the math on steering and some details, and this rework allows me a setup that should be a better starting point to get a decent design. I hope that this satisfies the rules of thumb
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/

 

[steinmini]

Is there a "rule of thumb" on approximate FVIC (desirable) distance from the chassis center - or from a different side, a distance ratio between pivots on the chassis side to ball joints on the outside? I can reduce the distance between the ball joints to move the FVIC farther from the chassis center...
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/

 

[GregLocock]

Milliken said something like if you design a car using rules of thumb, you'll design a thumb, not a car.

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


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[steinmini]

I think that I'm pretty much done with this setup, looks fine to me for a prototype. I agree that math could be very useful, however, with two many unknown factors, I usually build up a prototype (or a few versions) and do a real life testing. I know how it should behave when I sit on it and take it for a spin... Now I'm going back to SW and prepare the parts to be manufactured and check how it behaves. Fortunately, this is a time when I do have time to fiddle with working on new stuff. Sort of "dead season"... Not that I need to do much welding and manufacturing when it's full season, and sometimes I miss that part a bit... Thanks, I'll let you know how well this went. BTW, the trike should hit the market in 2013, so I'll have quite enough time to refine the details. Anyway, I did get what I asked for, assistance and not a completely defined design. So, back to designing a suspended trike and not a thumb

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[moon161]

Things may have changed, but when I raced bicycles (20 + years ago)tires were basically round sectioned. You may not need to worry about compensating for body roll w/ camber like on a car.

 

[GregLocock]

You are absolutely correct and miss the point entirely. Well done.

Cheers

Greg Locock


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