Leaner Sidecar/ Vintage Vespa

Discussion in 'Hacks' started by Charlieman22, Jan 14, 2020.

  1. Charlieman22

    Charlieman22 Adventurer

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    Question to add to above 1-3.
    4. If I tack it all together - should I be able to feel the difference of resistance between front pivot heights by just leaning the bike - while it sits still - or would I need to be riding it to get a sense of things?

    Thanks!
    -CM
    #21
  2. brstar

    brstar Long timer

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    Went and had a look at mine then went into cleanup mode for a bit.
    Got a coffee now so here's some more to add to the murkyness.
    The steering linkage in a car works in such a way that the inside wheel turns tighter than the outer one.
    Reason? the turning radius for the inner is less than the outer.
    Capiche?
    Ok then.
    The 9deg out of level for the mounting points really only varies the toe in of the sidecar wheel (which should be minimal).
    It could not be called steering as such.
    Again when you turn hard at low speed you can get the bike down as far it can go
    (40deg?) And it will be turning much tighter than the sidecar wheel is.
    Hence some scrubbing for that reason.
    There's also the sideways push as the bike leans and stands straight again.
    Try it stationary.
    You would wear flat spots on the lighter wheel (sidecar unless the missus is huge )
    Now as speed picks up the lean of the bike results in a progressively shallower turning radius for the bike.
    Therefore the scrub becomes minimal if the basic setup is good.
    Somebody with more smarts than I could fit electric power steering to the sidecar wheel along with speed and lean sensors coupled with a little puta to control it all.
    Then the sidecar would be in harmony with the bike and zero scrub while moving.
    #22
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  3. Hellracer.nl

    Hellracer.nl What the hack???

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    Scrub is quite simple to explain.
    Think of the contactpoint of the wheels of the Vespa on the ground as hinges. You lean the Vespa on these points to the left or right.
    Now look from the front to the Vespa (wheels in line).
    Attachpoint at 10cm from the ground and lean the Vespa to 45 degrees to the left, you'll get a displacement of the attachpoint of 10cm to the left. The sidecar wheel needs to scrub 10cm to compensate this.
    Now attachpoint at 20cm height from the ground, again 45 degrees lean angle and the scrub goes to 20cm.
    The higher you mount the attachmentpoints, the more scrub you'll get. Scrub means steeringslapping, you don't want that.
    #23
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  4. Charlieman22

    Charlieman22 Adventurer

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    Alright gents. I'm crawling - but I'm getting there.
    So two things I am working with.
    1. Different radius on sidecar wheel vs bike
    2. Height of second set of hinges created by pivot points distance from ground (ground being bikes hinge point).

    Editors note: Tank slap scares the crap out of me.

    Re-thinking front mount.
    I see two options.

    1. It's possible I could snake a front connection bar from scooter to sidecar with some twists and turns from between floor board and front tire. Not sure. This would put front mount (pivot) at similar height to rear. I might be able to sneak it down a little - creating differential by being LOWER than rear pivot point instead of higher. Would this also work?!
    2. Keep above floor board mount and bring it down as close to the scooter's center tunnel as possible. This would limit my height differential between pivots - maybe 4-5 degrees instead of 8 I had targeted.

    I think I will tack together some mounts and test the options.
    I can tack a front mount above the scooter's center tunnel (as shown in my cardboard mock up).
    This can have two heights to test both higher and lower.
    I can also tack together some form of lower mount to see if I can snake a connection to the side car out without interference.

    Option 1 - will likely compromise pivot point height differentials - optimize for low to ground pivot height.
    Option 2 will allow better differential - but create higher pivot points

    Feel free to opine on your preferred solution and why.
    Input has been highly valuable so far. Thanks!

    Oh - and BTW - this made me chuckle: "You would wear flat spots on the lighter wheel (sidecar unless the missus is huge )". Good point.

    #24
  5. brstar

    brstar Long timer

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    More random.
    Front pivot point.
    Is there room immediately behind the front wheel?
    This is brought on by the potential for the rear pivot point to be lowered.
    Rear pivot point.
    With the suspension compressed and the pivot point as close to the rear wheel as feasable.
    Being then shielded by the tire it only needs to be high enough to clear speed bumps gutters/potholes and curbs.
    That could/should be your starting point.
    Just cover it with a mudflap.
    #25
  6. Charlieman22

    Charlieman22 Adventurer

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    After a day to ponder - here is my boil down from the feedback:
    1. Low to the ground pivot points is critical to smooth function.
    2. Differential between front and improves handling.

    The above dictates that the front pivot point must move in front of the leg shield.
    I think I can shoe-horn the mount in between the leg shield and front wheel - as bstar suggests.

    There is advantage in making the front pivot - rather than the rear - the lower of the two points - based on the chassis geometry.
    The question that still stands then is:

    Will the toe-in, toe-out effect be the same if I reverse which mount is low and which mount it high in this manner (front lower then rear)?

    -CM
    #26
  7. brstar

    brstar Long timer

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    No to making the front lower than the rear.
    It would steer the wrong way.
    If they were level with the sidecar wheel having zero lead perhaps.
    Just a thought not having tried it.
    Or knowing any that have.
    But that has its own issues.
    #27
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  8. Charlieman22

    Charlieman22 Adventurer

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    Ahhh - of course. It would toe out when you want to toe in.
    Ha! Well - that would probably not be such a good solution.
    Back to the concept drawing board. I have my work cut out for me.
    Tks!
    -CM
    #28
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  9. Charlieman22

    Charlieman22 Adventurer

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    Gents - have a revised plan - but could use your insights so I better understand the gives and takes.
    Two questions:
    1. How many CM or Inches is recommended as my "low" or rear pivot point - assuming I could put it at any height I want?
    2. If I had to choose between 8° or 4° of differential height between rear and front pivot - which one would be best for lower speed - and which one would be best for higher speed?

    My suspicion is that larger differential reduces scrub at higher speed - and lower differential reduces scrub at lower speed.
    Can anyone confirm or correct this?

    Thanks - look forward to posting next round of pics of my mock up!
    Best,
    -CM
    #29
  10. Hellracer.nl

    Hellracer.nl What the hack???

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    The offset is not related to speed.
    It is related to the wheelbase of the motorcycle.
    #30
  11. Charlieman22

    Charlieman22 Adventurer

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    Helracer - I would think that you could optimize differential pivot heights for a certain speed - but it can never be optimized for all speeds - because you lean more at high speed then low. At least that was my reasoning... Perhaps I have it wrong.

    Bikes have to lean further over at high speed, then low speed - to achieve the same turning arch.
    So if I turn the same curve on a road twice in a row - but at different speeds, I lean differently each time.

    I figured the 8° differential between front and rear pivots had been chosen over time as a happy medium - but I suspect it might be more tuned for a bit higher speed then I will average. I was hoping that was the case anyway - so that I could justify having something closer to a 4° differential front to rear for my project.

    I have commenced building a prototype similar to what you did with your first.
    Simple tack welds mostly - and a little custom mount here or there...
    Below a few pics to give the idea of some of the challenges/ related to the legshield. IMG_3210.JPG
    Flipped bike over and found center line between wheels - slightly offset on this scooter from body.


    IMG_3220.JPG Welded up mount - the crappy quality of my welding work is apparent. I like to think of it as "aspirational"
    IMG_3212.JPG Mount sits like this on the bike - this was shot as I was laying it out.
    IMG_3221.JPG I put two mounting points on the front so I can experiment with height. Same at back.
    IMG_3240.JPG I will need to make some bends and turns to optimize the connection bar exiting the scoot. Note - the old frame is still on the side car and visible in this picture - it will be cut off.
    IMG_3251.JPG From the under side. here the connection bar is attached to the lower pivot point - there is an higher pivot point unused. I will experiment with both.
    #31
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  12. Hellracer.nl

    Hellracer.nl What the hack???

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    You are right that when you turn at high speeds, you lean more, but with leaning more, you increase the steering of the sidecar wheel. That is the only thing that you influence with changing the hight of the front pivot point, the level of steering of the sidecar wheel.
    If the steering level is correct at low speeds, the steering level is correct at high speeds.

    If you move the front pivot point too high, the sidecar wheel will push you in the corner, this gives a very unstable feeling, at high speeds and at low speeds.
    If you move the front pivot point too low, the sidecar wheel will not steer enough, hence you have to pull the sidecar into the corner using your weight. This feels heavy and will influence the light footed feeling of the motorcycle.
    The balancepoint is somewhere in the middle.
    I've been experimenting with this quite a lot before building my current version.
    #32
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  13. Charlieman22

    Charlieman22 Adventurer

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    That's very helpful.
    Now I can imagine the sensation - the heaviness from too low a front pivot and the unstable feeling if it is trying to stand you up due to pushing.

    Ok.

    Due to the scooter chassis shape - I have to make a choice - both are a compromise.
    1. Keep the front pivot nice and low - but have less differential from rear pivot height.
    2. Move front pivot higher - have better differential - but suffer any ills from high front pivot.

    I spent the day fabricating a test assembly.
    I've got multiple pivot attachment points front and rear - so I can play with different heights.
    I will look out for the sensations of heavy or pushing you describe - when I test ride.
    Very helpful.

    Some pics below:
    IMG_3269.JPG you can see I have two options for pivot points. I put a bend in the bar - so it does not interfere with the mud guard or leg shield when the bike leans
    IMG_3281.JPG
    The frame is very quickly tack welded. The short two tubes in the middle are there to allow me to slide the side car tub closer and farther from the bike to experiment with lean angle interference.
    IMG_3284.JPG Here the sidecar tub is about 11" away from the scooter chassis. Wheel base is about 48". It is a little wider then I would like - but as you can see from the next picture - I can lean the scooter to full lean (gear box hits ground) like this.
    Here it is at 11" inches (48" wheel base) in the max lean position
    IMG_3285.JPG
    IMG_3291.JPG Here it is at about ~9". This gives me a 45" wheel base but limits lean from maximum. See below.
    IMG_3290.JPG Here is the lean angle with 9" distance/ 45" wheel base.

    As usual - welcome any comments/questions.
    Cheers.
    -CM
    #33
  14. brstar

    brstar Long timer

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    I must beg to differ here helracer.
    And I feel this is an important point to recognize.
    I use motorcycle racing as an example.
    On a race track there are many corners with greatly varying radii.
    Yet professional racers lean their bikes down to similar lean angles no matter the speed or turning circles.
    Same with this scooter.
    There will be a sweet spot for most every turning circle in regards to speed and lean angle combination.
    The sidecar wheel only turns slightly purely on the lean angle. Thus scrub.
    Hence it is better to tune for higher speed stability than lower.
    Cause the laws of physics don't care.
    As for mounting point height.
    Make the rear one as low as you can then work with 8/9 degree as a start.
    I will leave it at that.
    #34
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  15. Bobmws

    Bobmws Curmudgeon At Large

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    In your test lean pics the sidecar is not attached to the connecting bar. If I'm following this correctly, if the bar is attached when you lean, the hack nose will move as well, and might allow you to get full lean in the narrower position.
    Put a mark on the bar & hack frame, lean the scooter and move the hack frame to align with the mark to check clearance.
    I think I would want to limit lean to a point before I touched down anything on the scooter.
    #35
  16. Charlieman22

    Charlieman22 Adventurer

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    Bob - the sidecar frame is actually welded to the connecting tubes that run to the scooter chassis.
    I have the sidecar tub just sitting on those - so I can shift it around to test for clearance at different possitions.
    It does push out when I lean towards it - and in when I lean away.
    Pics might not show that so clearly - but it's happening.

    Gents - good discussion - I will have to attempt and see what I get.
    bstar - your suggestion is pretty well how I am approaching.
    The back pivot is established as low as I am comfortable.
    At the front - because its a scooter with a leg shield and central tunnel - I have to make a choice.
    If I want 8°-9°, I have to move the mounting point to the inside of the leg shield.
    You will see I have a mount tacked in place there (picture below).
    This gives me the 8° but helracer's input suggests this pivot point will be too high off the ground to be optimal.
    My second option is seen in the prior pictures - where the front pivot is in front of the leg shield.
    This looks more elegant - I like it - BUT - it means I have more like a 4° differential rear to front.

    One way to look at this is a choices of compromise:
    Put the front mount at 8° but relatively high off the ground
    or
    Put the front mount in front of the leg shield and nice and low to the ground - but only achieve 4° of differential.

    If you had to compromise height off ground or differential between rear and front - what would you choose?

    IMG_3270.JPG You can see the mount I have also tacked on in the green circle. I could use this one to get 8°, but it moves the front pivot about 12" above the ground (estimated - I have not measured)
    #36
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  17. brstar

    brstar Long timer

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    My feeling is the higher front mount will be the go.
    But it seems you have the capability to try both.
    Maybe a bolted front connection?
    It is all a compromise.
    #37
  18. brstar

    brstar Long timer

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    The 1st time you load it up will show you a lot.
    #38
  19. Hellracer.nl

    Hellracer.nl What the hack???

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    I totally get what you mean but I think you are forgetting that at the mentioned lean angle the riders are hanging next to their bikes at different positions because the bikes are at their maximum lean angle. This is not something that you will do on the Vespa I presume. Nor has it anything to do with the required steering of the sidecarwheel.

    You'll need to study the Ackermann steering model to understand what is going on.
    But easier is to just experiment with the height, you'll feel what you are doing when you change the height.

    This I agree with. The rear one should be as low as possible.
    Start with that and start experimenting.
    #39
  20. davet2

    davet2 Been here awhile Supporter

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    If you use the pivot inside the leg shield, do you know how much room your right foot and leg will have when leaning to the right?
    #40