My life with Python trikes Mk1 , Mk1.5 , Mk2 ?

Joined
Feb 20, 2013
Messages
465
Location
Axedale, Victoria, Australia
Website
axerail.coffeecup.com
The Panhard Rod system I described is what I view as semi-independent. Movement of only one wheel causes only slight movement to the other. I never gave it a thought when I was fiddling with my discarded beam axle trike. If I ever resurrect it, or build a rear suspension delta, I will be trying a Panhard. A solid and/or beam axle sufficed for a hundred years for many cars, trucks, and buses. Leaf springs provided sideways axle location but coil springs require a trailing arm on each. The Toyota Crown I described had coil springs. The trailing arms, one on each side, must have had rubber bushes to allow for a slight twisting during single wheel movement - the same as leaf springs. Trikes would not need much weight in the required steelwork.

Plenty of videos and images on the Internet.
 
Joined
Feb 7, 2008
Messages
2,237
Location
Nottinghamshire England
So I have recently been riding in the appalling rain we have been having , well between the showers if possible !

I have been getting VERY muddy/dirty much worse than any of the last 4 years I have ridden the route due to a huge Amazon warehouse being built along side my path.

Then I came across this :-



he calls it a modular velomobile ? is can be ridden as trike - rear fairing only - rear & front fairing and maybe front fairing only ?



This is his spec :-

About 4 km / h faster than unclad. If it gets warmer now, I take off the front fairing and the bottom plate and ride only with the rear fairing.

The front fairing weighs 6.5kg the rear fairing 5kg and the trike a GTI 13.5kg. So a very impressive 25kg and 100 litres of storage.
If I tried to carry the same with Ortleib Back Roller City panniers
Capacity 5 x 20 litre = 100 litres
Cost 5 x £45 = £225
Weight 5 x 0.76kg = 3.8 kg

So lets say the Python is my base can I do the same ? as wheel arches are much preferable to mud guards and his rear fairing is much easier to clean than panniers & more water proof etc ...



Now the Python is considerable shorter behind the seat than his trike so can you see that a mudguard about 1" bigger than the tyre will over lap the seat , shame as I had a use for that curved area on his fairing.

However the rear fairing needs to be removable to get the trike in the car so it can actually be a little longer than the existing trike frame and I will get more luggage volume by it being considerably higher that current pannier rails ?

The outer rails will have to come off to allow access for new rear fairing ....



So I will make to new rails and weld them inside the frame [ white ] that will allow me to still carry 3 panniers 2 outside the rails and one in-between them.
Then the outer rails will be trimmed off at the red cut marks , an added bonus is it will actually leave the rear outer rail long enough to carry a 4th pannier if needed.[ over reflectors ]
One the rails are sorted I will build the wheel arches from some cheap/scrap plywood join them with a small floor and see if they work.

If the mudguards pass muster I need to plan the rest of the fairing and when nearing completion the whole pannier frame/water bottle holders/tent load area will have to be remove to allow it to slide in.

Exciting stuff......
 
Joined
Feb 7, 2008
Messages
2,237
Location
Nottinghamshire England
Ok I am getting a little confused with the suspension jargon and not completely sure what influences what ?

So can I start by talking about Idea A ?



In my head this is probably the simplest arrangement to visualise ?

So the swing arm has a wheel at one end and a pivot attaching to the frame at the other ?

The suspension device , my cut down fork leg or a spring/elastomer is attached by a bracket at X and another at it's other end.

Do both the brackets have to allow the suspension device to pivot ?
Does position X give maximum wheel travel and minimum force ?
Does position Y give minimum wheel travel and maximum force ?

or have I got this all wrong ?
 
Joined
Feb 20, 2013
Messages
465
Location
Axedale, Victoria, Australia
Website
axerail.coffeecup.com
Do both the brackets have to allow the suspension device to pivot? Yes. There will be a slight rotation at each end of the suspension unit.

Does position X give maximum wheel travel and minimum force? This will be the position that most resists wheel movement or, to put it another way, the stiffest suspension result.

Does position Y give minimum wheel travel and maximum force? Mounting the bottom of the suspension unit at this place, and leaving the top where it is, will give a softer ride as there is much more leverage applied to the suspension unit by the swing arm.

How much it varies between one place and another can be calculated but dimensions must be known in order to do it. You should be able to get a closer idea of how it works from Mr. Google. Leaving the top of the suspension unit where it is, it is all to do with the difference in the distance between the wheel axle, the front of the swing arm, the location of the suspension unit, and the angle at which the suspension unit meets the swing arm. Perpendicular requires a greater force to compress the spring. Alternatively, you could just move the top of the suspension unit forward for a variation. If you don't have the required engineering skills, just try relocating the suspension unit top or bottom, and see what happens. It will end up like the three bears - too soft, too hard or, just right.

You could have a few mounting points and be able to adjust it for a variety of loads. You need to make sure that the swing arm does not allow the wheel to move laterally (sideways).
 
Last edited:
Joined
Oct 19, 2012
Messages
543
Location
Wakefield, UK
x gives the least suspension travel and the shock is at it's stiffest.
y is the opposite assuming you move the top mount accordingly to keep it vertical.
If you used the top mount in the picture and y you'd have no functioning suspension as y never moves towards the top mount pictured.

I suggest you want to keep travel down to a couple of inch. Any more and it's going to lean out in a corner much more than you'll likely want it to.

Also you can angle the suspension unit. If you angle it 45 degrees it will move half as much for the same wheel movement or in other words be only half as resistant as the full 90 degrees. A large cheap regular rear suspension unit might move 2" tops. If you have the unit at 90 degrees and close to the wheel your wheel will only move that same 2". Move it half way to the chassis but keep it vertical and you get 4" of wheel movement before that same unit bottoms. The suspension is also half as stiff. Keep it at the wheel but angle it forward at the top at 45 degrees and again you get 4" of wheel movement with it feeling half as stiff as vertical. Your fork leg is certain to have softer springing than a rear unit. A rear unit will be too stiff to put vertical at the wheel. If you used a short rear unit with only 1" of travel, mounted it at the wheel end and angled it forward to 45 degrees that 1" travel unit would supply 2" of wheel movement before bottoming and would be half as stiff as if it were used vertical. Keeping it vertical and moving that same unit half way to the chassis has exactly the same effect. Angling it or keeping vertical will depend on chassis rail positions and is a matter of which provides the best way to fit things in in accordance with the chassis.
 
Last edited:
Joined
May 31, 2013
Messages
2,374
Location
South Benfleet, Essex, England, UK
Law of levers, "X" provides the most resistance.
However, I think the "Shock" will just droop to its maximum of travel immediately it is placed under load.
I suspect bicycle front-shocks are not man enough for the job. In their natural habitat only 30% of rider weight is on the forks. In this config you will have 70%.
Personally I hate the front-suspension forks because of incessant brake-dive and the feeling that I am see-sawing along. I will be very interested to see how/IF this works. Keep going, someone has to take one for the team Paul. :D
I would reduce the ironwork and have the suspension at a variable angle more like a "Y" on its side. Oh, and "Gussets" need to be added for added safety. IMHO.
 
Joined
Oct 19, 2012
Messages
543
Location
Wakefield, UK
A lot of forks only run a spring in one side. If there's a spring both sides then they're half as soft again. The designed weight on the unit is a very valid point:-

A cheap long rear unit from 26" wheeled bike, designed for a 12 stone rider and has a typical leverage of 2:1 from it's triangle = Designed to cope with 16.8 stone if it were vertical with no leverage (ie at X)

A cheap short rear unit from a kids 20" bike, designed for a 6 stone rider again with a 2:1 ratio = Designed to cope with 8.4 stone if used vertical at X

A cheap fork spring from a 20" kids bike designed for a 6 stone rider with zero leverage = designed for just 1.8 stone if used at X (this assumes there was just one spring in the forks - half it if there were two)

If you weigh 12 stone and assuming the weight bias of your trike is 70:30 to the rear then the rear is holding 8.4 stone (4.2 stone each side) so you want a unit designed for 4.2 stone to sit at X

The guestimated candidate is a cheap short 1" travel rear shock mounted at X and angle 45 degrees forward or mounted vertical halfway from the wheel to the chassis. The latter is technically better than the former but not by much. This should "in theory" provide 2" of travel to a 12 stone rider. The assumptions are:-
that a diamond frame bike has a weight bias of 70:30 to the rear.
that your trike is the same (hard to tell as looking back, most piccies of your trike are close-ups on whatever part you've been working on)
that a typical triangle at the rear of a typical bike has a 2:1 leverage ratio.
The maths will need altering to suit different assumptions or actual measured ratios.
 
Last edited:
Joined
Feb 7, 2008
Messages
2,237
Location
Nottinghamshire England
Thanks most help full , and I will NOT stick my head in the sand and ignore the rear MTB springs as I have plenty ;)

This is plan B




Why a plan B ? well I want as little ' frame ' as possible so this has much less than plan A ?

However this is where I can't figure out what the deuce affects what and where all the parts need to be ?



The blue yellow is it's current form and the wheel base is likely to remain the same due to limitations getting it into the car.

The black one in the background was the Mk1 and had a slightly shorter w/b

When last weighed the black one , with me on and ready to roll but no camping gear :-

Front 39 kg rear 29kg each wheel so

39 Kg and 58kg gives a 97kg total so that gives a approx 40/60 weight distribution [ I think ]
 
Joined
Oct 19, 2012
Messages
543
Location
Wakefield, UK
Much in that one depends on the dimension x to z. Think of an arc with the swing arm pivot as the centre and start the arc at Z moving clockwise. This is the direction the top of the shock is going to take as the wheel bounces up. As you can see it doesn't move directly towards q but does get nearer so there will be some compression. The larger the distance x to z is the less z moves towards q so you get less compression for the same distance the wheel is going to move (ie a softer ride). The shorter the distance x to z the more that arc travels towards q so the more the shock compresses for the same deflection of the wheel.

If you raise q higher above the swing arm pivot at the same time you raise z to keep the shock horizontal then the higher you go the stiffer the shock feels at it looses effective leverage. If you raise x to z so it is the same as x to y keeping the shock horizontal it has the same effect as a shock at x in idea A. Above is harder. lower is softer.

There is also the point that whilst that design looses chassis weight it adds that same length of metal to the swing arm to raise z in the air. There's also the point of forces not going into the chassis along a tube but into it at a place the could cause stresses to crack welds. In the latter you're stressing the weld at x because x to z is giving the forces leverage. In idea A you are stressing the welds in the chassis between the top suck mount and the pivot. A more weld friendy approach would be to mount the shock halfway along the swing arm and at 45 degrees to join to the top left of the chassis upright. Less chassis and less leverage of forces into chassis and swing arm. Such a position means that the leverage from the wheel into the shock is 4:1 so a 1 inch movement short shock would give a max 4" wheel movement. This is probably too much but could be tamed with an appropriately harder spring. One of the harder springs found on a short shock and some preload may suffice as you have less weight on each rear wheel than expected.
 
Joined
Feb 7, 2008
Messages
2,237
Location
Nottinghamshire England
OK I understand we need some dimensions ?

However so do I to know where to start ?

So I have a rear shock that has what looks like 2" of travel and is 6" between pivot centres and looks to have enough adjustment to take a 350lb spring up to the 750lb it is currently fitted with.

So assuming we start with a variant of Idea A?

How long would be a good starting length for the swing arm ? I shall probably make it a bit longer so the wheel position can be changed if necessary ?

thanks Paul
 
Joined
Oct 19, 2012
Messages
543
Location
Wakefield, UK
Assuming you weigh 12 stone (76 kg) then your trike takes about 40kg across the two rear wheels including the trike weight itself. So just 20 KG per wheel. Even a 350 lb spring is way too hard to place vertical at the wheel. You need some leverage to use that. If you placed the bottom shock mount half way from the wheel to the swing arm pivot and angled the top forwards 45 degrees you would compress a 350LB spring by half an inch. This would give you 1" of movement at the wheel. So with no preload the rear would sag by almost 1" when you sat on it. I'd suggest that would give you about an inch up and an inch down which should be perfect. More and you're rolling too much IMO. It doesn't matter if that 350lb spring is on a long or short shock, it'll move the same. The only difference is the longer one has a much lower probability of bottoming out over the bad bumps.
The longer the swing arm the better. The wheel moves in an arc around the pivot. In practice the wheel isn't moving far around the arc so it's not a major issue. I'd suggest about 10" putting the pivot next to the tyre if making one from scratch but if you want to use a crack arm that ought to be fine.

In contrast your front kids fork spring, if mounted vertical at the wheel would compress nearer two inch when you sat on it if it had no preload. If you cut that spring down to 60% of it's length and mounted it at the wheel vertically it would be similar to the 350lb spring mounted halfway at 45 degrees. Cutting a coil spring in half effectively doubles it's rate.

It's probably awkward to do but if you could set the kids fork spring up (without preload) , put 20 kg on it and measure the deflection I would be able to give answers that rely less on assumptions and guesses. It only takes a small error in those to equal a larger error in any guestimate. At the moment I'm only guestimating it's rating.
 
Last edited:
Joined
Feb 7, 2008
Messages
2,237
Location
Nottinghamshire England
Assuming you weigh 12 stone (76 kg) then your trike takes about 40kg across the two rear wheels including the trike weight itself. So just 20 KG per wheel.
You been peeking ? about 74kg at present 😢

Even a 350 lb spring is way too hard to place vertical at the wheel.
It is possible I only have 1 x 350lb spring and currently cannot find any for sale on Ebay , half the traders don't even bother putting the spring rate in the advert.

I'd suggest that would give you about an inch up and an inch down which should be perfect.
The current Ice trike with front suspension [ elastomers ] say 40mm of travel so around 2" would be excellent.

The longer the swing arm the better. The wheel moves in an arc around the pivot. In practice the wheel isn't moving far around the arc so it's not a major issue. I'd suggest about 10" putting the pivot next to the tyre if making one from scratch but if you want to use a crank arm that ought to be fine.
On it 10" it is ...

In contrast your front kids fork spring, if mounted vertical at the wheel would compress nearer two inch when you sat on it if it had no preload. If you cut that spring down to 60% of it's length and mounted it at the wheel vertically it would be similar to the 350lb spring mounted halfway st 45 degrees.
May have to ?

Other thing is I have a bag full of new Ford Fiesta Zetec valve springs [ 1 engines worth ] , shorter and smaller diameter coils , no idea of their rating thou and hard to find any figures on the net...

Found them , although mine are for the Mk7 & 8

Single Valve Spring 180 Poundage
Heavy duty valve spring to suit Uprated cam profiles. Use with standard Ford spring retainers.
Taper wound design specifications from high grade OTEVA 75 Silicon Chrome steel wire.
Rated to 13mm maximum valve lift and 10,500rpm overspeed.
 
Joined
Oct 19, 2012
Messages
543
Location
Wakefield, UK
Putting two valve springs end to end would rate them at 90LBS / 40kg and give 1" of travel. Mounting them vertically at the wheel would give you a 1/2" deflection when you sat in it. Using 4 end to end would give you 2" overall movement and about an inch drop when you sat in it with no preload. If you can cobble a way to use 4 end to end you are somewhere near for a vertical mount. If you can mount 2 end to end you can mount the shock at the wheel and tilt the top 45 degrees for the same effect.
 
Joined
Feb 7, 2008
Messages
2,237
Location
Nottinghamshire England
If you have a rush of blood to the wallet there's always these. Pump up to whatever feels right.
Hey not a bad price if I though the idea can work and I can feel a ride improvement ....
 
Joined
Feb 7, 2008
Messages
2,237
Location
Nottinghamshire England
Putting two valve springs end to end would rate them at 90LBS / 40kg and give 1" of travel. Mounting them vertically at the wheel would give you a 1/2" deflection when you sat in it. Using 4 end to end would give you 2" overall movement and about an inch drop when you sat in it with no preload. If you can cobble a way to use 4 end to end you are somewhere near for a vertical mount. If you can mount 2 end to end you can mount the shock at the wheel and tilt the top 45 degrees for the same effect.
Can I just hit them with the TIG and spot them in 3 or 4 places each to make 1 longer spring ?
 
Joined
Feb 7, 2008
Messages
2,237
Location
Nottinghamshire England
If undampened suspension is what you aiming for.
What about seat springs.

Advantage is they are adjustable.
Just a thought.
Never seen those before , which also means I don't have any :(
 
Top