Author Topic: Fitting a 20" rim to a Magic Pie 5  (Read 6468 times)

Offline DriftTrike

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Fitting a 20" rim to a Magic Pie 5
« on: August 21, 2016, 07:45:07 AM »
Hello!

I am a new member and new to the e-bike world. I have a Magic Pie 5 and I'm trying to figure out how to fit it into a 20" wheel. I did a search and I found this post to be very helpful: http://goldenmotor.com/SMF/index.php?topic=5696.msg32340#msg32340



I wanted to ask to see if anyone has actually attempted that and if there were any pictures. Would anyone know why short spokes would be an issue? Is it because they are hard to source in that short length?

« Last Edit: July 04, 2017, 08:45:58 PM by Bikemad »

Offline Bikemad

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Re: Fitting a 20" rim to a Magic Pie 5
« Reply #1 on: August 21, 2016, 11:08:36 AM »
Hi andto the forum.

There are two main problems with short spokes:
  • Having them specially made to the correct length.
  • Dealing with the large angle that is formed between the spoke and the rim due to the large diameter hub being used with a small diameter rim

There are not many bike shops or spoke suppliers that can produce the required short spokes, and you might end up having to place a long distance order if you cannot find a supply locally.

You don't really want to use a rim with a deep "V" section, as the smaller inner diameter will require even shorter spokes (and even larger spoke angles):



It might also be necessary to file the holes in the rim to accommodate the required spoke angle.
You can make up a simple cardboard template with the correct spoke angles to ensure you file (or drill?) the holes at the correct angle (and also in the correct direction);)

Alan
 
« Last Edit: July 04, 2017, 08:46:20 PM by Bikemad »

Offline DriftTrike

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Re: Fitting a 20" rim to a Magic Pie 5
« Reply #2 on: August 22, 2016, 01:24:32 AM »
Alan, Thank you so much for the info! Looks like getting the right spoke angle is going to be the toughest part. Can you help me understand what's considered a "conservative spoke angle range," a "moderate spoke angle range," and an "extreme spoke angle range"?

It would help to understand what has been tried before. I'm starting to feel like getting this type of fitment to work really requires the right expertise. Since I'm a bit lacking in the tool department, I've looked to outside help to move my project along. I've found a local wheel building expert as well as several competent LBS's and, if this is actually possible, I'll need to decide who the right resource would be to make this work and how much money I want to pour into it to find a solution.

Offline Bikemad

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Re: Fitting a 20" rim to a Magic Pie 5
« Reply #3 on: August 25, 2016, 11:28:40 AM »
Can you help me understand what's considered a "conservative spoke angle range," a "moderate spoke angle range," and an "extreme spoke angle range"?

Some spoke nipples fitted into conventional rims will accommodate up to 12 degrees of spoke angle before the spoke begins to bend, and I'd describe that as conservative:



The original 26" Magic Pie spokes had a very large angle of nearly 30 degrees, which I'd describe as both extreme and totally unacceptable as it results in badly deformed/stressed spokes instead of straight ones:



I would say a moderate spoke angle would be between 12~20 degrees maximum, but sometimes you simply have to make do with larger angles and distorted spokes in preference to radial spokes (0 degree spoke angle means the spokes are perpendicular to the rim and therefore not ideal for transmitting torque).

I suggest you try to find a suitable rim where the holes are all positioned relatively close to the centreline of the rim like this:



But with a Magic Pie and a 20" rim, you will definitely need to avoid any rims that have heavily staggered holes located way off of the centreline like this:



If you can find a suitable rim (and the correct length spokes) it should be possible to lace the rim as shown here:



I had to enter 72 spokes (instead of the Pie's 36) to enable the spoke calculator to produce the required spoke angles and length for this particular spoke pattern (and I then edited out the unused spokes to avoid any confusion).

The Pie's offset "paired" holes in the hub would produce a spoke angle of 4 degrees (90-86), which would be similar to a conventional bike wheel spoke angle and will resist a lot more torque than the sometimes used 0 degree radial lacing.

Alan
 
« Last Edit: July 04, 2017, 10:33:31 PM by Bikemad »

Offline DriftTrike

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Re: Fitting a 20" rim to a Magic Pie 5
« Reply #4 on: September 12, 2016, 04:53:59 PM »
Just an update in case anyone was attempting to fit a big hub motor into a small rim...

I ended up just buying the Golden Motor cast rim.

First of all, finding someone who can cut spokes in a very short length is incredibly difficult. If you can find someone to provide the spokes, then you need to solve the problem of finding a rim that can accommodate your spoke angle. Chances are, you won't unless you go custom. 3d drilling a hole accurately is NOT easy and having someone else build you a rim to your spec is not cheap. If you care about safety and structural integrity, Alan's suggestion to increase the number of spokes and holes in the hub motor is a good one but custom drilling your hub to have more holes requires the right tools. In the end, the cast rim was the stronger, safer, and quicker solution to get me on the road and out of the workshop.