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Tuesday, April 10, 2018

Changing the Fred Design

My trike (Fred) as it is now was used to estimate the weight supported by the front wheel of some variations. Measured weight on front wheel is 38.5 kg. 

This one has the bottom bracket placed in front of the fork inside the frame and the seat moved to maintain seat to pedal distance, but no other frame changes. Moving the bottom bracket might mean a V-brake would no longer fit behind the front fork.  A disk brake would make a good alternative. Calculated weight on front wheel is 44.7kg.

This version is the same as 2 but the rear frame is foreshortened. Calculated weight on front wheel is 38kg.

Changing the trike to a bike by swapping the 2 back wheels for 1 back wheel which increases the effective wheelbase.  The dotted (lower) back wheel is in a folded position which reduces the length. I based the rear wheel frame sketch on the recent, nicely built Python bike by Ingo Kollibay shown here. Calculated weight on front wheel is 42.9kg

A version of 4 with the bottom bracket and seat moved forward. Calculated weight on front wheel is 49.2kg, the highest weight recorded here.  If the rear wheel could fold, the stored bike becomes could become smaller than the trike versions. 

Forks reversed on current trike. Calculated weight on front wheel is 38.4 kg.

Spreadsheet used for calculation.


Its been a few weeks now since I attempted the Green Wedge Hills Audax ride which seriously challenged the hill climbing abilities of my trike because the front wheel slips on very steep hills.  So I got to thinking, what would improve the hill climbing of the Fred Trikes?  A chat with Simon Watt at the OzHPV Ford Proving Ground Records weekend spurred me on, he discussed moving the bottom bracket forward of the steering fork on the frame, something I hadn't contemplated recently but was in some early direct drive fwd bikes I made.

So what would help the trike climb hills? More weight on the front wheel would help, but thinking about it doesn't really solve anything, a little bit of research and calculation and putting things down on paper might shed a bit more light.

To start with, I weighed myself, then weighed myself and my trike, then weighed the front wheel when I was sitting on the trike with my feet on the pedals. Then the trike was drawn to scale and potential changes put into drawings. Its fairly easy to draw the changes as most of them involve sliding one or other feature up and down the frame - which only consists of Aluminium RHS - so implementing the changes would not be out of reach either. And my earliest working Fred trikes like this one used 2 x 4 timber as the frame, so altering or experimenting with that wouldn't be super-hard either.

  The drawings estimated the position of the centre of gravity (CG) from the front wheel centre by assuming the CG remains 850mm behind the pedals as per my initial weigh-in and calcs.  This treats the bike and rider CG as being in the same spot which is an approximation, but a reasonable one as I weigh 5 times as much as the trike. And yes, I know, all the forces on the bike wheels should be written down in Newton, not kg, but most of us understand kg, and by putting things in Newton would be more correct but less comprehensible.

And the results? Converting the trike to a bike by using a rear wheel suspension frame like this one built by Ingo Kollibay, and moving the bottom bracket forward would put the most weight on the front wheel (Pic5) .  If the rear wheel frame could be folded, then the size of the cycle would be reduced.  There would be some secondary effects I haven't mentioned here.  For example moving the BB forward of the fork would shorten the steerer, making for more turning for a given sideways tiller handlebar movement.

Regards Steve Nurse

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