Python touring tandem trike

[pegasus]

So I started another project.
A python touring tandem trike, a spin off from my python suitcase bike.
The concept is: a covered storage box over the rear wheels which can hold 100+ liters of camping gear, the stoker on top/inside the storage box and with a python front end.
The trike is made of tree parts; rear storage box, in between boom and a python front, so it can be transported easily.
The python has 2 x 8 speed transmission, the stoker has a midmotor and a 6 x 6 speed transmission.
I want to ride the "way of the roses", a 170 miles (270 km) route across northern England, next year.
My original plan was to do it with the suitcase python unaccompanied,
but instead the plan is to ride the route both ways in 2 stages/weekends instead, I have 45 miles to the start and the route goes 15 miles north of where i live, so 1st weekend we ride to the start in Morecombe and ride the route back to our house, a 2 days ride, and then a second 4 days ride for the rest of the route and back home.
But first get it built.

 

[Popshot]

Presumably the stoker is driving the rear wheels?

 

[stormbird]

Yikes that's going to be a load on the pivot ?

or have I miss understood the concept ?

Paul

 

[DannyC]

I would recommend a "PPP" utilising Angular contact bearings for the pivot. But that's just me.

 

[pegasus]

Presumably the stoker is driving the rear wheels?

Yes, but only one wheel.

 

[pegasus]

Yikes that's going to be a load on the pivot ?
Paul

I have contemplated the reason why my python is twitchy and yours isn't, having very similar angels and setup.
The main difference is that I made the distance between the rod ends longer than you, if i remember right, yours was about 8cm and mine about 12cm, a difference of 1,5. So according to the formula on openbikes webpage http://en.openbike.org/wiki/Calculations my rod ends has 33% less radial force each, which, I presume, makes the front end easier to move and the python twitchy on higher speed.
So I'm going to keep the same distance, or slightly longer, than my python and the higher load on the rod ends, hopefully, makes it less twitchy.
The formula above, if I use 12cm between the rod ends, gives a 7.8kn load on each rod end, if the bike is fully loaded (200kg), the dynamic load rating for a 12mm rod end is 7kn, the static load rating is 17.5kn. I think a python pivot is in between static and dynamic, so it's doable.
I might keep a spare rod end with me when I'm touring.

 

[stormbird]

Forgotten all about that , you may well be right.

Don't forget to beef up the mountings I broke some 3mm on mine built to the same design as yours !

failure

Paul

 

[Popshot]

If using rod ends I'd think the quality of them may play a part in how easily they rotate under load too. The weight of the rider(s) will also heavily determine the load on them. Even if the stoker is 100% over the rear axle, the pilot is likely further from the rear axle meaning a greater % of their weight will be on the front. I agree with Danny in that angular bearings are the better option for this pivot or indeed any python pivot. Both bicycles and motorcycles use similar bearings but rod ends are simple shell type bearings. Cheap angular bearings will work well here but cheap rod ends will work differently to expensive ones under load with likely a high inertia or "sticktion" before slipping. It's possibly that sticktion that causes twitchyness. Paul's rod ends obviously work well but the window for working well would appear to be narrow. I would expect angular bearings to have a wider window with close to zero sticktion.

 

[pegasus]

Don't forget to beef up the mountings I broke some 3mm on mine built to the same design as yours !

I will design the to the pivot as my first python, using a 80 x 40 x 3 rectangular tube cut into 2 U pieces, the different on my first design compared to yours is that I sandwiched each
rod end.

(picture from the archive)

This time I'm going square, using 40 x 40 x 1.5 for the main boom.

 

[pegasus]

It's possibly that sticktion that causes twitchyness.

I think the opposite, having a longer distance between the rod ends makes my front end move easier and therefore be more likely to "twitch".
Mine is harder to learn to ride than Paul's but ones you are use to it is no problem, you only feel the twitchyness when you go past 20mph.
This python will be built for touring not racing.
I have already spent my cash on cheap rod ends so that's what I'm going to use.

 

[pegasus]

The frame is slowly coming together. I started with the rear box and boom all from 40 x 40 x 1.5.

it's in two pieces

Inside the holes on the boom is welded m8 nuts so a bearing box can be bolted on top of the frame, which will also hold the frame parts together.
The rear frame is 900mm wide which is the total width, the wheels will be mounted inside the rear box.
The leftover box section was cut in halve and into bits, which will be part of the wheel frame.