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Front suspension for DIY scooter

  1. Sep 11, 2016 #1
    Hi
    I am new to this forum and I was referred to it by a friend.
    I am building an electric scooter and want to know how to add suspension/shock absorbers to it.

    I know the terrain I wanna go over. Bumps of about 1-2 inch. My wheel size will be pretty small about 7-8 inches. I was looking to buy online small shocks for pockets bikes or mini scooters. What I couldn't figure out was the spring stiffness or spring rate to choose.
    Weight of the scooter with me would be around 130 to 150 kg. Say it is equally divided for both front and back. So front shocks have to take about 65 to 75 kg of static load. Assuming I use only 1 shock in front, it should be able to take 75 kg of static load plus force due to bumps. Bumps would cause displacement of 1 inch . Force will be 75kgf plus (spring stiffness x bump height/depth). What should be the stroke length? 1 inch for the bump plus deflection due to static weight (spring stiffness x 75kgf) plus a clearance.

    It all comes down to what should be spring stiffness or spring rate?
    I am not even considering effects damping. Any idea how I should approach this problem. Is there an online calculator or a design guide book (couldn't find one) for free or nominal charge. What should be the extra clearance I should have so that I know my full stroke length. Please guide me for the same.

    Thanks
     
  2. jcsd
  3. Sep 11, 2016 #2

    jack action

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    You can start with the 6-part technical paper on spring & dampers found here. Then I would suggest some more that repeat or add some information:
    These will probably raise more question than it will answer, but it will give you the terminology to look for and help you ask more specific questions.
     
  4. Jan 18, 2017 #3
    Sorry, please accept my apologies. I saw your reply long ago but did not thank you.
    I got busy but I am coming to this project. I am still debating whether it should be 2 wheeled or 3 wheeled (tilting trike type).

    I have hit a road block. My scooter would not weigh more than 30-40 kg (based on battery/motor specs etc).
    With Rider, or even 2 riders the weight should not be more than 180 kgs. Say the weight distribution for front and rear is 40%/60%.
    So for front suspension the effective mass would be about max 72 kgs. For ride frequency of 1.5 Hz, the front shockabsorber stiffness (wheel rate) comes around 6389 N/m or 36.82 lbs/inch.
    I may get shock absorbers of 50 lbs/inch and with manipulation of Motion ratio I can probably achieve the calculated value.

    But the deflection of the shock absorber will be more than 4 inches, under static conditions. Is that normal? I have been trying to find out what cycles and mountain bikes use for front. For rear they usually have springs ranging from 200 lbs/inch to 1500 lbs/inch but through the specific geometry, they can vary motion ratio to get the desired wheel rate.

    Ofcourse I am ignoring damping effects (for one reason that no standard shock absorbers mention anything about the damping coefficient). Also ignoring unsprung mass (which I don't know right now). Which will probably reduce the effective mass, inturn reducing the stiffness/wheel rate.

    Again thank you so much for the links. Helped getting started. And again please accept my apology for not acknowledging your reply.
     
  5. Jan 18, 2017 #4

    jack action

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    There is also the concept of preload. If you pre-compress your spring by 4 inches when installing it on the vehicle, you will notice that the suspension will not drop under the vehicle's weight. That is because the pre-compressed spring is already putting a force equivalent to the vehicle's weight, forcing the suspension links to stay fully expanded. So preload is another suspension characteristic that can be adjusted.
     
  6. Jan 20, 2017 #5
    Thanks a lot Jack.
    So yes I can add preload so I don't have to maintain a very high ground clearance. But it will also change traction right, specially for a car/4 wheeler?
    Like since the shock absorber won't expand beyond the preloaded value. On other hand in a normal non preloaded case, it may extent more.

    For example in case of a car:
    Say my suspension is 8 inches of travel. 50lbs/inch of wheel rate (and some damping that we are choosing to ignore right now).
    When I sit on the vehicle, about 200 lbs of weight is acting on the individual suspension. It compresses 4 inches and can compress 4 inches further. When cornering in opposite direction, it can compress 4 inches more (centrifugal force acting on it). Say turn is made in its side so that centrifugal forces tip it in the opposite direction. The tendency of this wheel will be lose down force on it, lose traction. In such a case will preload be better or worse. Without preload, it can expand 4 inches before losing traction. With preload, it cannot expand completely.
    I am guessing it will be worse wouldn't it? Even though the shock absorber spring is still under compression, it will not give in a reaction to the wheel? My experience of preload has been by playing around a typical cycle shock absorber.
    Is my above theory right? Or force acting on each side be more or less the same and tip-over characteristics be the same?

    In bikes, cornering issues won't be so easy to discuss (front what I have read online). But preload will effect going over a bump/pothole.

    Sorry for abruptly changing the topic to 4 wheelers
     
  7. Jan 20, 2017 #6

    jack action

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    Your suspension design doesn't have to be done around your spring. A real example I know:

    A 1970 Chevelle has a front corner weight of about 875 lb (one wheel). The spring it uses is a 293 lb/in and the motion ratio is about 1.7.

    So the natural deflection under load is 875 / 293 * 1.7² = 8.6 in. This corresponds to a natural frequency of 188 / √8.6 / 60 = 1.06 Hz.

    But the actual suspension motion range is about ±4 in. So the spring is pre-compress about 2.7 in, which give 4.6 in at the wheel (= 2.7 * 1.7). When the weight of the vehicle is put on the suspension, the suspension drops another 4 in ( = 8.6 - 4.6) and the wheel is set right in the middle of its allowed range.

    There is a limit with that, as if you use too soft a spring for the suspension range, the spring will not be allowed to the full range of its natural up-and-down motion and the suspension will constantly hit the bump stops. You have to set a range that is realistic for the purpose of the vehicle.

    That being said, there are special cases where you prefer having lots of bump and no droop at all. For example, an off-road vehicle that jumps in sand dunes will often have no droop. The importance of absorbing the huge shock caused by the entire vehicle dropping on the ground prevails over the nice sine wave motion you could expect from a normal vehicle. Here's footage from a dirt bike where you can see how the 'normal' ride height is set with no droop:


    It's all about usage and you have to base your design on what others have already experienced to select the proper set up. With racing - where 1 tenth of second makes a difference - they adjust the set up for every track.
     
  8. Feb 14, 2017 #7
    @jack action :
    Once again thank you so much for your detailed reply. And sorry for my delayed response, AGAIN.

    I just have one more query. I know it's probably not connected directly to suspensions but do you know anything about torsion springs, specifically adjustment done on torsion springs ( there are many videos online on how to adjust garage door springs). It seems all the adjust is preload. Am I right?
     
  9. Feb 14, 2017 #8
    And I almost forgot, 293 lbs/inch feels so soft. Like I have been playing with some cycle (non-powered) suspensions and the spring rates are so large comparatively (range of 750 lbs/inch to 1500 lbs/inch). I know you proved to me that it is fine and fits the requirements.
    Thank you so much.
     
  10. Feb 14, 2017 #9
    I used to race motocross (with motorcycles) and we would specifically set up our shocks to allow the height of the rear of the bike to "sag" (lower) by almost exactly 4 inches when we would stand on the footpegs with all of our weight. Our total suspension travel was 13" and we would take enormous impacts (sometimes bone shattering impacts) with those bikes.

    Also, use compression dampening to counteract the most abrupt impacts. The spring should not be the only thing resisting them and rebound dampening should be less than 2hz.
     
  11. Feb 14, 2017 #10
    Motocross bikes certainly do have droop. It is not apparent when looking at the fork because the rake of the front suspension defers the bulk of the static load to the rear suspension, which effectively droops the bike's height about 4" when the rider is on it. Droop is very important in off-road racing because it allows the rear tire to follow the shape of the ground on the small choppy bumps that are everywhere off road. If there was no droop or sag, the tires would abruptly leave the ground almost any time a small pot hole or bump came under the tire. That would be detrimental to gaining speed or maintaining control of the bike.

    This video illustrates the importance of rear wheel tractability. Watch the behavior of the bike as it rides across the small bumps. Keep in mind this is a world-class racer on a world-class bike and he most definitely sets his bike up to have sag or "droop".
     
  12. Feb 14, 2017 #11

    jack action

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    The difference between spring types is mostly the packaging. They can all be treated the same. They have a spring rate and a displacement range and they can be preloaded.

    That is the difference between wheel rate and spring rate. What you care about is the wheel rate, i.e. the spring rate as felt by the wheel. This depends on the motion ratio. Once you have found the wheel rate you want, you choose the spring rate according to the motion ratio of your suspension linkage design:
    [tex]MR = \frac{Wheel\ Displacement}{Spring\ Displacement}[/tex]
    [tex]Spring\ Rate= (MR)^2\ Wheel\ rate[/tex]
    My car example has a 293 lb/in spring rate and a motion ratio of 1.7, that gives a wheel rate of 101 lb/in. A bicycle that would have a 700 lb/in spring rate and a motion ratio of 2.67 (say, 4 in wheel displacement for 1.5 in of spring displacement) would give a 98 lb/in wheel rate. So this bicycle suspension would be just as stiff as the car suspension (although they don't support the same weight, so the bicycle ride would be harsher).
     
  13. Feb 15, 2017 #12
    @RogueOne
    I used to race motocross (with motorcycles) and we would specifically set up our shocks to allow the height of the rear of the bike to "sag" (lower) by almost exactly 4 inches when we would stand on the footpegs with all of our weight. Our total suspension travel was 13" and we would take enormous impacts (sometimes bone shattering impacts) with those bikes.

    Why exactly 4 inches? That may work for motocross but for something like a scooter or moped that'll be too much.

    Motocross bikes certainly do have droop. It is not apparent when looking at the fork because the rake of the front suspension defers the bulk of the static load to the rear suspension, which effectively droops the bike's height about 4" when the rider is on it. Droop is very important in off-road racing because it allows the rear tire to follow the shape of the ground on the small choppy bumps that are everywhere off road. If there was no droop or sag, the tires would abruptly leave the ground almost any time a small pot hole or bump came under the tire. That would be detrimental to gaining speed or maintaining control of the bike.

    This video illustrates the importance of rear wheel tractability. Watch the behavior of the bike as it rides across the small bumps. Keep in mind this is a world-class racer on a world-class bike and he most definitely sets his bike up to have sag or "droop".

    Thank you for above. Sounds very interesting. :thumbup:
     
  14. Feb 15, 2017 #13
    @jack action

    My car example has a 293 lb/in spring rate and a motion ratio of 1.7, that gives a wheel rate of 101 lb/in. A bicycle that would have a 700 lb/in spring rate and a motion ratio of 2.67 (say, 4 in wheel displacement for 1.5 in of spring displacement) would give a 98 lb/in wheel rate. So this bicycle suspension would be just as stiff as the car suspension (although they don't support the same weight, so the bicycle ride would be harsher).

    I think the motion ratio will be higher than 2.67 Don't think bicycle suspensions are so stiff.

    Anyway thank you so much for everything.
     
  15. Feb 15, 2017 #14

    jack action

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    I don't know much about bicycle suspension set ups, but based on your numbers and http://service.foxracingshox.com/consumers/Content/mtbspringratecalculator.htm [Broken], I thought it made sense.
     
    Last edited by a moderator: May 8, 2017
  16. Feb 17, 2017 #15
    The bike's geometry is optimized by the manufacturer for having 4" reduction in seat height from sag/droop. Since some sag is necessary for more consistent traction and tire contact, the manufacturers optimize the bike's geometry for having about 4" of sag. That is basically just an industry standard that was adopted and it is not precisely 4". That's just a general guideline that is adhered to +/- a half inch or so
     
  17. Feb 20, 2017 #16
    @RogueOne: I was just curious. I guess for the kind of set ups used in motorcross, it must be a thumb rule/general guideline as you suggested. Thanks you so much.

    @jack action: Could be. Maybe mountain bikes need to be stiff for rough rides. Thanks again
     
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