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

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Topic: advice on springing a python trike ? long post !!!
Posted By: stormbird
Subject: advice on springing a python trike ? long post !!!
Date 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




Replies:
Posted By: GeoffBird
Date 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.


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Right Time - Right Place - Wrong Speed


Posted By: stormbird
Date 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
  


Posted By: GeoffBird
Date 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. 

http://uk.rs-online.com/web/p/dampers-stops/3270426/" rel="nofollow - http://uk.rs-online.com/web/p/dampers-stops/3270426/


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Right Time - Right Place - Wrong Speed


Posted By: GeoffBird
Date 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.


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Right Time - Right Place - Wrong Speed


Posted By: GeoffBird
Date 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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Right Time - Right Place - Wrong Speed


Posted By: stormbird
Date 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



Posted By: GeoffBird
Date 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.


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Right Time - Right Place - Wrong Speed


Posted By: GeoffBird
Date Posted: 22 October 2017 at 10:39pm
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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Right Time - Right Place - Wrong Speed


Posted By: stormbird
Date 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

 


Posted By: GeoffBird
Date Posted: 23 October 2017 at 1:57pm
Spring preload: The damper will have something to limit its extension (or sometimes on cars there is a separate droop stop). Preload compresses the spring against this stop. The purpose of this is to limit the amount of damper travel required, as the static deflection (the amount the spring compresses when the weight of rider and trike is placed on it) will be reduced.

Imagine a spring/damper requires 1" of travel to absorb a 1g bump. This will mean you will need a spring with the capacity to absorb at least twice that (2"), because the static deflection will be equal to the travel required for a 1g hit. This assumes the spring starts at 'free length', in other words, its uncompressed length. If we were to apply 1/2" of preload to this spring then the static deflection will only be 1/2" and the total damper travel required will only be 1 1/2", not 2".

Preload becomes more necessary the softer the suspension. A saloon car will have a suspension frequency of about 1.5 Hz, so will need about 4 1/2" of travel at the wheel to absorb a 1g bump, so accommodating an equal amount of droop travel becomes impractical. Most cars require spring compressors to fit or remove the springs for this reason.

In the case of your first picture, you can give the spring preload simply by tightening the nut and bolt.


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Right Time - Right Place - Wrong Speed


Posted By: GeoffBird
Date Posted: 23 October 2017 at 2:03pm
Here is a graph I made for and article on suspension I wrote which might be useful:




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Right Time - Right Place - Wrong Speed


Posted By: GeoffBird
Date Posted: 23 October 2017 at 2:14pm
The reason I have advised using a higher spring frequency for your trike than for a saloon car is that it is more difficult to have a system that doesn't absorb your pedal power if the suspension is very soft. 2.5 Hz should give you a noticeable suspension effect while avoiding these problems. A traditional racing car would have suspension frequencies in the 2-2.5 Hz range. Modern race cars use harder suspension because they have to resist aerodynamic downforce.

Mountain bikes have hard springs and lots of travel because they are designed to absorb 2g (or more) bumps. I find MTB suspension fairly useless on the road as it is too stiff.


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Right Time - Right Place - Wrong Speed


Posted By: stormbird
Date Posted: 24 October 2017 at 6:05pm
Geoff

So looking at your figures and my scrap pile I have :-

Distance Shock to Pivot Distance Axle to Pivot B/A sq Spring rate camping load





2.5 8.5 11.6 1156 1541
3.0 8.5 8.0 803 1070
3.5 8.5 5.9 590 786
4.0 8.5 4.5 452 602
4.5 8.5 3.6 357 476
5.0 8.5 2.9 289 385





Weight on rear wheels 150 1.5 travel in inches
wheel rate = 100 B10 / c20






+ camping load 200 1.5 travel in inches
wheel rate = 133 B13 / c13






Springs available 550 – 650 - 750 – 850



So I am thinking Option 1 :-



Looks as though if I select 3.5" I can use a 650 lb spring for normal riding and have the option of a 750lb for the camping load if needed ?

Need to weight existing trike next and get a feel for the weighs involved ?

The central bolt shown is 12mm 6" long is that over kill ? can you buy threaded rod that is 8.8 hardness ?

all for now Paul




Posted By: GeoffBird
Date Posted: 25 October 2017 at 4:09pm
Hi Paul, Threaded rod is called studding. Try a search. The load on the bolt is purely from the preload. If you know the amount of preload and the spring weight, you can work out the load.

You don.t have any damping with this arrangement. A crude way of damping is to introduce friction into the system, but it's not ideal.

Not sure I'm qualified to give engineering advice - I've just bent the fork crown on one of my bikes under heavy braking! (It does have a 180 mm disc on a 20" wheel and a 4-pot Hope caliper)


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Right Time - Right Place - Wrong Speed


Posted By: stormbird
Date Posted: 25 October 2017 at 5:09pm
Originally posted by GeoffBird GeoffBird wrote:

Not sure I'm qualified to give engineering advice - I've just bent the fork crown on one of my bikes under heavy braking! (It does have a 180 mm disc on a 20" wheel and a 4-pot Hope caliper)


Geoff

Ah however real world data is worth more than some figures on a spreadsheet Wink

Ok real world figures for my project are as follows, for current trike and it's daily payload + me :-

Each rear wheel 64 lb giving a total weight at the rear of 128lb

Front wheel 86 lb

Giving a total rolling weight of approx 214 lb or  97 kilo

regards Paul


Posted By: Andrew S
Date Posted: 25 October 2017 at 5:16pm
I hope all of this will see the light of day as a magazine article when it's all sorted, or possibly a Special Bumper Suspension Edition to see us through the long dark winter months.


Posted By: GeoffBird
Date Posted: 25 October 2017 at 6:42pm
 Surely for the Spring Edition would be more apt Andrew?

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Right Time - Right Place - Wrong Speed


Posted By: Andrew S
Date Posted: 25 October 2017 at 10:22pm
LOL


Posted By: stormbird
Date Posted: 25 October 2017 at 11:07pm
Ok adding more accurate figures from post above I get :-

Distance Shock to Pivot Distance Axle to Pivot B/A sq Spring rate camping load





2.5 8.5 11.6 986 1372
3.0 8.5 8.0 685 953
3.5 8.5 5.9 503 700
4.0 8.5 4.5 385 536
4.5 8.5 3.6 304 423
5.0 8.5 2.9 247 343





Weight on rear wheels 128 1.5 travel in inches
wheel rate = 85 B10 / c20






50lb + camping load 178 1.5 travel in inches
wheel rate = 119 B13 / c13






Springs available = 550 – 650 – 750 – 850 lb




It looks like 3.5" is the best option using a 550 lb spring , that will be only 10% stiffer than predicted value ?

If fully camping load carried I could change to the 750 lb spring ? only 7% weaker than predicted value and I have a 450 lb and 650 lb to fall back on ?

This article is it meant to be written by me asking for advice ?
Geoff giving advice ?
Or from some other perspective ?

I can't see anyone else on here wanting to know how to spring a python rear end in such a crude way , and hats off to Geoff for persevering with such a lame request Clap

regards Paul



Posted By: GeoffBird
Date Posted: 25 October 2017 at 11:46pm
You are a perfect student and me a less than perfect professor, Paul. Your trike is obviously very usable in the Real WorldTM and I'm sure an article on how you built it, how you've used it and how you plan to improve it would be of great interest.

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Right Time - Right Place - Wrong Speed


Posted By: stormbird
Date Posted: 26 October 2017 at 11:13am
Hmmmm.....

Walked into that one , didn't I LOL

There was an article publish a couple years ago covering my build of the first Python trike.

I can write this up also .....

However I am a bit of a butterfly flitting from project to project with far to many on the go at once, the curse of being retired.

So completely unable to meet editorial deadlines , however it does need building and testing thoroughly before next years Holland jaunt in June.

all for now Paul
 


Posted By: stormbird
Date Posted: 09 February 2018 at 12:46pm
So this is a mockup of the spring mount.
The joint at the top will not be a 45' mitre joint but trying a sort of lap joint where it is easier to cut and there are 6 welds holding the joint instead of 4 of the mitre joint ?



Like this.

Plan 1

then I need some spring seats/locators to stop spring creeping out of position , probably a washer with a tube welded on that fits inside the spring and the whole lot maintained in place with some 8mm threaded rod and lock nuts ?

or

Plan 2

could the springs be held top and bottom with washers with a slot clamping the outer coils to the frame tubes ?

unsure of stress being put on the retaining rod and whether the rod should be 8.8 hardened ?



So green lines where spring seats will be ?
horizontal cut at blue line
red is where vertical tube will sit [ with notched corner at top ]
purple line show main forces
wonder if I should brace horizontal tube to pivot cheeks light blue lines to stop it bending away from spring caused by vertical forces ?

where the right hand vertical column will be there will be a bracket at the top to take a brace holding the upper seat mount.

regards Paul


Posted By: stormbird
Date Posted: 17 February 2018 at 12:30pm
So got a bit further , to much ' honey do stuff ' !!!



So vertical bit of wood to be replaced by steel and welded in position shown ?

Should give me enough room to get in with me ugh torch and clumsy hands.

The surplus will be cut from the top tube.

On reflection this will be a simple 90' butt joint with a piece of 3mm plate capping off the open end , much easier than the notched corner ...

The 3mm capping will be used as a basis for 2 plates overlapping the joint and having a pair of bolt holes in them to mount the bottom of the upper seat mount ?



So blue line 3mm capping plate

red 2 cheeks that will hold lower seat mount ?

green is seat mount - probably a piece of frame seat tube +clamp so piece bolted to seat can slide inside seat tube and so be adjustable ?

It has been suggested that the top tube may bend ? it is 1.25 x 1.25 x 1.5ish ?

regards paul


Posted By: Yanto
Date Posted: 18 February 2018 at 7:20am
I would say if you keep that set up you will need external gussets on all corners to try and keep everything in shape.  Without doing the maths (moments) you will not know if the tubing is strong enough, never mind the effects of the welding.

Could you slope the top spring mounting tube to get rid of the vertical tube, then have the seat mount directly above (or just in front) of the the spring? This will require fabricating an internal mounting plate for the spring at angle to compensate though.


-------------
Ian, retired.


Posted By: stormbird
Date Posted: 18 February 2018 at 12:16pm
Yanto

Yes,  I think it will have to be gusseted it looks far bigger/clunkier than it actually is as the camera was quite close taking the pictures, the vertical tube is only 3.25" [ 8cm ].

Originally the tube was to be sloping however there are lots of problems for me welding it as there would be an internal angle I could not weld up.
Where the BB shell sits is the best place to weld these parts , however the lips of the pivot bracket prevent me getting in to make a good weld.    

Yes the welding is also suspect , only an amateur working at the limit of my skill/experience Wink

regards Paul


Posted By: Yanto
Date Posted: 18 February 2018 at 3:04pm
The views/scale are deceiving, it does look bigger.

I wasn't commenting on your welding prowess, far better than I could do, I meant the effect of the heating cycle on the steel.


-------------
Ian, retired.


Posted By: stormbird
Date Posted: 18 February 2018 at 4:40pm
Yanto

None taken Big smile

However whilst typing my drivel I had a spanking good idea ?

Realising that if what I was doing looked clunky and crude it probably was .....

I give you the Mk2 rear spring mount ?




As the pink cheeks to the pivot are the strongest steel around why not extend them up to the spring with some 50mm x 3mm steel and add a cross piece at the top to capture the spring ?

This is a mock up from 3mm plywood , the new style mount only needs 3 pieces ?



Plenty of weld area and enough room to get in with the torch :-



For a sense of scale here it is with the seat dropped on :-



and with a wheel also ..





Posted By: stormbird
Date Posted: 21 February 2018 at 6:52pm
Ok hardest welding I have had to do to date Jester



had to tack this while it was jigged and clamped to my workmate , this whilst kneeling down and then had to do 6 tacks whilst standing.

these are not normal TIG positions Smile



Then had to weld 2" using the torch in my left hand , not to be recommended !!!



Needs the welds tidying and a bit of fettle.

Those lips on the top were there to stop me burning the edges whilst welding the spring mounting plate , decided to keep them and cut a slot in each one wide enough to allow my small luggage straps to pass through no extra parts and useful.

Found some of my springs came with domed washers so purloined a couple of those for mounting the spring on.

This seems to be bl**dy heavy ! need to weight it and find out.....

Already thinking about springing both rear wheels and making it lighter ..sigh ...


Posted By: Yanto
Date Posted: 21 February 2018 at 7:00pm
That looks better than MK1, just what i was trying to explain, I would get this version up and running and used a bit before starting another.

-------------
Ian, retired.



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