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

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GeoffBird View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GeoffBird Quote  Post ReplyReply Direct Link To This Post 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.


Edited by GeoffBird - 23 October 2017 at 2:09pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GeoffBird Quote  Post ReplyReply Direct Link To This Post 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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Post Options Post Options   Thanks (0) Thanks(0)   Quote GeoffBird Quote  Post ReplyReply Direct Link To This Post 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.


Edited by GeoffBird - 23 October 2017 at 2:17pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote stormbird Quote  Post ReplyReply Direct Link To This Post 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


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Post Options Post Options   Thanks (0) Thanks(0)   Quote GeoffBird Quote  Post ReplyReply Direct Link To This Post 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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Post Options Post Options   Thanks (0) Thanks(0)   Quote stormbird Quote  Post ReplyReply Direct Link To This Post 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
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Andrew S Quote  Post ReplyReply Direct Link To This Post 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.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote GeoffBird Quote  Post ReplyReply Direct Link To This Post Posted: 25 October 2017 at 6:42pm
 Surely for the Spring Edition would be more apt Andrew?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Andrew S Quote  Post ReplyReply Direct Link To This Post Posted: 25 October 2017 at 10:22pm
LOL
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Post Options Post Options   Thanks (0) Thanks(0)   Quote stormbird Quote  Post ReplyReply Direct Link To This Post 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

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