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advice on springing a python trike ? long post !!!

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stormbird View Drop Down
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    Posted: 21 October 2017 at 5:37pm
Hi all

Well I have done about 4k miles on my first Python trike and it has performed well.



However as with all builds you think the next one will be even better ! and my priorities have changed somewhat since I started number 1.

So ride is very jiggly and uncomfortable a combination of:-
a) short w/b 36" about 1 m [ done to get it in the car and still use the front passenger seat ]
b) stiffening the rear upper seat mounts to combat bending in use
c) and the gradual drift from Big Apples to Marathons to achieve more speed.

Also it is very dangerous at times with both brakes on the front wheel , towing a trailer on wet pave/cobbles in Holland can produce a slide that ends at the scene of the accident Wink

So a new rear end is in order to lengthen the trike by about 10" to enable me to carry my camping gear instead of towing a trailer , rear wheel brakes and I though it would be a good idea to add simple suspension at the same time.

So I need the rear pair of wheels sprung however the mechanism has to be as compact as possible to sacrifice as little of the luggage space  as I possible can . I appreciate elastomer’s would help the space issue however I have loads of cheap shocks with various spring rates and they will give me more travel [ I hope ].

With only 5" of clearance under the frame there seems little possibility of getting the gubbins under the frame ?

Main frame is 30mm x 30mm x 1.5mm . the wood shown is approx same dimension , rear of trike to the left my existing front half will bolt to this new rear end on the right.

So I have thought of 3 possible solutions :-


  
or



Both the above would give me somewhere for the upper seat mount to fasten saving it having to go all the way to the main frame ?



This one is the least bulky and could be made to work without the shocker absorber body thus slimming it down even more.

This shows the use of space I am looking for in front of the pivot ?



Red line upper seat stays , blue line seat and green blob room to get tent under seat in the space before the  pivot.

I am sure someone on here can tell me the best solution ? and any pitfalls to avoid ?

regards Paul



Edited by stormbird - 21 October 2017 at 5:38pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GeoffBird Quote  Post ReplyReply Direct Link To This Post Posted: 21 October 2017 at 7:30pm
Hi Paul, The best solution is usually the one that packages and weighs the least. Steel springs aren't great from this point of view. Elastomers would be much lighter and cost very little - search for PU die springs, rather than going to a cycle company. They work best if you only use 20% of their length in travel, as this gives a near linear rate. They are also, to a certain extent, self-damping.

I could go on for hours about suspension design (but you will be relieved to hear I'm not going to!), but one thing you need to watch is your 'motion ratio'. This is the ratio between how much the wheel moves against how much the spring moves. For the same 'wheel rate' (suspension softness) the spring rate is the SQUARE of the motion ratio, so a high motion ratio will need a MUCH stiffer spring. Dampers also prefer low motion ratios - aim to have a similar motion ratio to the original installation. The middle option above appears to have a very high motion ratio.

Also bear in mind that MTBs tend to have quite stiff suspension to deal with jumps etc. You can go softer for the road.


Edited by GeoffBird - 21 October 2017 at 7:36pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote stormbird Quote  Post ReplyReply Direct Link To This Post Posted: 22 October 2017 at 8:07am
Geoff

Thanks for looking at this.

IIRC I  don't have ' original installation ' just a bunch of bits ? i will look when I can get to the back of the bike shed.

The middle choice does look most like a simple bike implementation , although I doubt many of the cheap Chinese MTB's really get it right the springs are just there for looks and to make a sale ? If I went for this option should I get the distances between the shocker and the frame pivots both equal ? Also I think it needs a brace on the seat side to resist the bending forces?

Is there a way to make the first option work ? looks the neatest and easiest to implement ?

I think I read somewhere these springs allow about 2" of travel ? so I assume I can safely use 1.5"  but how do I calculate the motion ratio for say option 1

I have springs rated between 450 lb and 850 lb IIRC.

regards Paul
  


Edited by stormbird - 22 October 2017 at 8:07am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GeoffBird Quote  Post ReplyReply Direct Link To This Post Posted: 22 October 2017 at 3:10pm
You are right Paul, I'm sure most cheap MTB suspension is largely decorative!

It is the perpendicular distance to the pivot that matters, so the distance from the wheel axle to the pivot divided by the perpendicular distance from the spring centre-line to the pivot gives the motion ratio. I think cheap MTBs usually use a motion ratio of about 2 to 3.

You can measure the travel of the damper. You have to make sure you don't let the spring go 'coilbound' (when the coils touch each other). Just measure the gap between the coils and multiply by the number of coils. You also really need to preload the spring - this is what the threaded rings on dampers are for. This allows you to have a greater proportion of the available travel to be bump travel.

So, you really want at least 40 mm of bump travel, to absorb a 1g  bump (giving a natural frequency of about 2.5 Hz - quite hard). In addition, you will need about 15-20 mm of droop travel (approx. half the bump travel), so a total travel of 55-60 mm at the wheel. Assuming there is about 32kg on each wheel, this gives a wheel rate of 32/40=0.8 kg/mm (8 N/mm). So, if the motion ratio is 2, square that to get 4, so the required spring rate is 3.2 kg/mm (32 N/mm or 180 lb/in). You might find the springs are marked in lb/in or you can load them up with weights and measure the deflection. And you will need a damper with at least 30 mm of travel. Adding a progressive bump stop will also help - you can use softer springs with the same travel. Dampers sometimes have these or you can buy conical rubber stops from RS Components. 



Edited by GeoffBird - 22 October 2017 at 4:18pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GeoffBird Quote  Post ReplyReply Direct Link To This Post Posted: 22 October 2017 at 3:18pm
The above will require about 10 mm of preload BTW. Just noticed your comment about available spring rates. You really want to use a much larger motion ratio for these, if you don't want to have a rock-hard ride. So, if you want a wheel rate of 50 lb/in, you could use a 450 lb spring with a motion ratio of about 3 (3 squared is 9). You can see, as I said before, that a small change in motion ratio has a big effect, so it might be worth making the motion ratio adjustable?

This all assumes one spring per wheel. It is not clear from the photos if you are using one spring unit for both rear wheels or one for each? If the former then double the wheel/spring rates.


Edited by GeoffBird - 22 October 2017 at 4:08pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GeoffBird Quote  Post ReplyReply Direct Link To This Post Posted: 22 October 2017 at 4:04pm
Also, depending on where you lengthen the frame, doing this could have a profound effect on cornering stability. If you just move the rear wheels back, without increasing the width of the back 'axle' then the trike will be much more 'tippy' in corners.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote stormbird Quote  Post ReplyReply Direct Link To This Post Posted: 22 October 2017 at 5:31pm
Geoff

Well we have cruised well past ' out of my depth ' and into ' deep waters ' Big smile



Yes one spring for both wheels , as I wanted it to be simple , just take the bite out of 1" dropped curbs and troughs left by poor road mending etc.

I was originally inspired by this , which looked easily do able ?



Option 1 has a axle to pivot distance of 8.5" and centre spring to pivot distance of 3.5" ?

regards Paul

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Post Options Post Options   Thanks (0) Thanks(0)   Quote GeoffBird Quote  Post ReplyReply Direct Link To This Post Posted: 22 October 2017 at 10:32pm
Okay, lets work in imperial as your springs are imperial. Your motion ratio on the figures you have given is 2.43 (8.5/3.5), which squared, gives 5.9. So, lets say the rear axle supports 150 lb and we want 1.5" to absorb a 1g bump, then the wheel rate will be 100 lb/in (150/1.5), which gives a spring rate of 590 lb/in (100x5.9). The damper will need about 1" of travel if you have about 0.33" of preload, or, if you have 1.25" of travel or more, you could run the spring with no preload, which would just mean the static deflection (the amount the spring sags when you sit on the trike) would be more.

Obviously, if the rear axle is carrying more or less weight, you will have to adjust these figures, and/or adjust your motion ratio. Hope that makes sense.


Edited by GeoffBird - 22 October 2017 at 10:40pm
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I think your motion ratio is about right - it should allow you to try slightly softer or slightly harder springs within the range available to you. I would advise fitting a bump stop, preferably progressive (conical). You should be able to get one to fit your spring/damper unit or you could fit the RS components one separately.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote stormbird Quote  Post ReplyReply Direct Link To This Post Posted: 23 October 2017 at 12:31pm
Originally posted by GeoffBird GeoffBird wrote:

Okay, lets work in imperial as your springs are imperial. Your motion ratio on the figures you have given is 2.43 (8.5/3.5), which squared, gives 5.9. So, lets say the rear axle supports 150 lb and we want 1.5" to absorb a 1g bump, then the wheel rate will be 100 lb/in (150/1.5), which gives a spring rate of 590 lb/in (100x5.9). The damper will need about 1" of travel if you have about 0.33" of preload, or, if you have 1.25" of travel or more, you could run the spring with no preload, which would just mean the static deflection (the amount the spring sags when you sit on the trike) would be more.

Obviously, if the rear axle is carrying more or less weight, you will have to adjust these figures, and/or adjust your motion ratio. Hope that makes sense.

Geoff

Excellent I have caught up !

My  weight approx 154lb and the old trike weighs 60lb [ fully loaded with tools inner tubes maps etc !] and plan to carry about 50lb of camping gear 2 - 3 weeks in a year.

So first plan weigh all 3 corners of old trike with me sat on it , to be a bit more accurate.

Then do a spreadsheet for various spring/pivot distances to determine which spring [ 350,550,650 , 750 or 850 ] would be best to use.

Use option1 and make top spring limiter [ horizontal frame member ] long enough to accommodate all positions spring needs to be in to meet above conditions ?

Make spring adjustable along lower frame member.

use bump stop.

Only thing I don't understand is spring preload ? what does that do ?

Thanks for all your help.
I feel I have enough information to create something that will work as well as possible with the crude plan and crude components.

all the best Paul

 


Edited by stormbird - 23 October 2017 at 12:34pm
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