Air Shock Info... for an unusual use....

jimcroisdale
jimcroisdale Posts: 11
edited January 2019 in MTB workshop & tech
Hi all,

Really hoping to get some help from the community on the subject of air shocks. I'm building a 4 wheeled vehicle - a gravity racer - and the design currently has a rear subframe that pivots off the main frame. We need a shock to do the business of keeping it all in check.

The completed vehicle will weight 250kg all in, and will be a 50-50 weight split front and rear. Unusually, the shock will be mounted at the rear of the car, so the sequence will be pivot > axle > shock, and the shock will be mounted pretty much straight up and down.

We want probably around 25mm total movement at the wheel, including the static sag (7.5mm at 30%). As the shock is mounted approx as far away from the axle as the axle is from the pivot, this will give a shock travel required of 50mm, with 15mm static sag.

Given these numbers, where would you start?

Jim

Comments

  • cooldad
    cooldad Posts: 32,599
    You've been 'building' it for a bit of a long time...

    viewtopic.php?f=10004&t=13071316&p=19980317#p19980317

    Sorted the wheels out yet?

    What's unusual about shocks at the rear. My cars all have rear shocks, and front shocks.
    I don't do smileys.

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  • The Rookie
    The Rookie Posts: 27,812
    Find any shock with 50mm travel (well 50.8 is common), a 190/51 is a very common size, you'll be able to get almost any make and model of shock you want in that size.
    Currently riding a Whyte T130C, X0 drivetrain, Magura Trail brakes converted to mixed wheel size (homebuilt wheels) with 140mm Fox 34 Rhythm and RP23 suspension. 12.2Kg.
  • jimcroisdale
    jimcroisdale Posts: 11
    Haha, indeed. That was MKI. Went with different wheels in the end, and the car had a successful season last year, and became a test bed for the running gear for a world record attempt car, which is what we're building now.

    The road in Canada where the new car will be running is getting a little more rutted as time goes by, so we want a little more compliance than the types of chassis that have run there before. Canada is a long way to ship a car to find out that the winter has screwed the road so bad that it's too rough for a rigid chassis.

    I thought it was unusual cos it's a car, and this is a bike forum :-) Also, I think the placing of the shock is a bit unusual, but perhaps not so much.

    Jim
  • jimcroisdale
    jimcroisdale Posts: 11
    The Rookie wrote:
    Find any shock with 50mm travel (well 50.8 is common), a 190/51 is a very common size, you'll be able to get almost any make and model of shock you want in that size.

    My initial assumption is that because air shocks are adjustable for pressure, we'll be able to tune these to suit our car in a way that may not be the case with springs. Does that sound reasonable?
  • jimcroisdale
    jimcroisdale Posts: 11
    This was MKI setting a new unofficial UK record of 71mph last year:

    https://www.youtube.com/watch?v=F8k-VC6pUnI
  • steve_sordy
    steve_sordy Posts: 2,443
    jimcroisdale wrote:
    .........

    My initial assumption is that because air shocks are adjustable for pressure, we'll be able to tune these to suit our car in a way that may not be the case with springs. Does that sound reasonable?

    Yes, but don't forget that the shock will also have rebound damping (and maybe compression damping too). That will stop the rear wheels bouncing up and down like a pogo stick. If you ride an mtb you will know exactly what I'm talking about.
  • whyamihere
    whyamihere Posts: 7,702
    Bear in mind that most MTB shocks will have a max pressure in the 250-300psi range. Depending on the leverage ratio, that may well not be enough for an all-up weight of 250kg, so running a matched pair might be a better idea.
  • The Rookie
    The Rookie Posts: 27,812
    I'm with WAIH, I'd run a pair so the weight supported is closer to that intended (most MTB's will be running between a 2:1 and 3:1 axle to shock travel ratio) plus the damping will be a better match for the weight/spring rate, they are tuneable, more so the more you spend but I'd suggest one with a medium compression damper, you'll have adjustable rebound, you don't need pedal platform and they can be adjusted for spring progression via volume spacers (Fox) or Bottomless tokens (Rockshox) or just adding a dollop of grease in the air can.

    A Basic Monarch R would do the job and can be picked up from £150 new or about £30 used.
    With mid tune compression https://www.bike-discount.de/en/buy/roc ... 63EALw_wcB
    Currently riding a Whyte T130C, X0 drivetrain, Magura Trail brakes converted to mixed wheel size (homebuilt wheels) with 140mm Fox 34 Rhythm and RP23 suspension. 12.2Kg.
  • jimcroisdale
    jimcroisdale Posts: 11
    The Rookie wrote:
    I'm with WAIH, I'd run a pair so the weight supported is closer to that intended (most MTB's will be running between a 2:1 and 3:1 axle to shock travel ratio)

    Thing is, my ratio would be more like 1:2, so I think it will be a case of finding a shock that can me made soft enough, not hard enough, even though the car will be 250kg all in, and a bike might be 110kg with rider. This is why I thought an air shock would be most suitable as I can just let the air out until it's right - damping etc aside.

    Because of that, i thought one shock may be better than two?
  • me-109
    me-109 Posts: 1,915
    250Kg would be more motorcycle territory. I wouldn't think an MTB air shock would take enough air pressure to support the weight without blowing seals, particularly once under compression from being activated.
  • jimcroisdale
    jimcroisdale Posts: 11
    Assuming 50/50 weight split:

    MTB and rider = 110kg
    Weight on back end 55kg
    3:1 shock ratio
    = 165kg on the shock

    Car and driver = 250kg
    Weight on back end 125kg
    1:2 shock ratio
    = 62.5kg on the shock

    See my point though?

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