Replacing 26" MP4 Rim

[Marlz]

Hi all,

So I cracked my rear MP4 rim.  Anyone have any experience replacing this?  Do I have to go with a Golden Motors rim or can I use a rim which is more readily available to me?  Anything I should be keeping in mind?

Thank you!

[Bikemad]

Ideally you want to find a 36 hole deep "V" rim with a similar Effective Rim Diameter (ERD) or you will have to replace all of the spokes as well.

If you cannot find a rim with the the correct sized ERD, you will need to calculate the required spoke length using the new rim's ERD to order the correct length spokes.

If you are unable to measure the ERD for the GM rim, you should be able to get a rough measurement by measuring the exact spoke length and altering the ERD measurement for a custom rim on the ebikes.ca spoke calculator until you obtain a similar spoke length to the measured spoke.
The MP III hub shown in the dropdown list is almost identical to the MP4/5 apart from the MP III has a very slightly wider flange spacing.

Using some very rough measurements, I "guestimate" that the GM 26" deep "V" rim is likely to have an ERD of 516~518mm.

Because the spoke flanges on the Magic Pie hubs are very close together, I suggest that you do not use a rim with really offset holes like this:



How did you manage to crack your rim? Were the spokes very loose before it failed?

Alan
 

[Marlz]

Thank you for the detailed information and for always being so helpful.

I was not riding the bike when the accident happened but I was told that the rider rode off the road and there was a concrete drainage channel on the side.  The rear wheel basically got a little air and slammed into the channel.

[Bikemad]

Ah, that explains it then, concrete is not very forgiving!

After a bit more research, I have just revised my original guestimate of the GM 26" ERD to 516~518mm.

26" rims with 516~518mm ERD should fit fine with the existing length spokes (110mm), but I have had a quick look on Google, and I'm unable to find any that would be suitable (apart from very expensive carbon fiber ones). 

Do you have a local GM dealer who may be able to provide you with the correct rim?

Alan
 

[Socalbiker]

Yes, the Golden Motor rim is not robust enough for the weight of ebike and roads with potholes or crooked concrete on trails. Since I needed custom spokes I spent the money to have a good wheel builder set me up with a tougher hoop and spokes. 

[StadsImker]

I don't think spoke should be crossed like this. I a normal bike wheel spokes are crossed even tree times, but not these very short spokes form the left and right side from the rim. I a normal bike wheel spokes are much longer and cross. I am afraid this will not result in a strong and not flexing wheel. All your spokes are bend, because left and right spokes are bended almost around each other... This will result in rubbing, flexing and problems. Only straight spokes have maximum strenght But I hope your rim will preform good for a longtime! This is by the way my old wheel which is replaced with a GM 26"MP5. It has very heavy spokes Gage 12 or even G11. When my GM rims is totally destroyed and not a better rim is availeble.. I will buy again a GM wheel and a new set of spokes, even I broke it allready the first time. Custom rims and spokes are much to expensive or I need to make them myself. But than I need a special tool to roll the treads on spokes whichs are originally much to long for a MP5. This spelcial tool is form Cyclo and called a spoke tread roller, but it's expensive more than 150 USD... and difficult to get.

Greetings,
Philip

[StadsImker]

Looking closely to your photo.... I would not cross spokes over each other... Your spoke elbows are alternating, thats okey, but should not cross over each other. If you don't cross them over each other like you did. They would be MUCH straighter and you still would see them cross. And than your wheel will be much stronger. In my opinion you should re-do your wheel and get a better result.

Greetings,
Philip

[Socalbiker]

I've put down over 900 miles since rebuilding the wheel. No loose, broken spokes or bent rims. Lifetime for this wheel is looking good

Update Aug 2019: I've now put 10,000 miles on this setup with only one broken spoke at 4000 miles.