Motors dimension for wheel building

[Deleted member 4366]

Sorry, I made a mistake above - too late at night!

I''ve used two calculators, which give:
121.4 and 120.6
121.1 and 122.5

The hub is 108mm and the ERD is 340. That means the radial gap between the two is (340 - 108)/2 = 116mm, which would be the 0-cross size
The length of the spokes will be a bit longer than that because they'll be at a slight angle, so 121mm seems about right. Your 250mm is more than twice as long as the gap! In fact I can see that all those spoke lengths are exactly double what they should be, but I can't see why. Try changing the number of spokes to 36 to see what happens.

 

[catsnapper]

Sorry, I made a mistake above - too late at night!

121.1 and 122.5
The hub is 108mm and the ERD is 340. That means the radial gap between the two is (340 - 108)/2 = 116mm, which would be the 0-cross size
. Your 250mm is more than twice as long as the gap! In fact I can see that all those spoke lengths are exactly double what they should be, but I can't see why. Try changing the number of spokes to 36 to see what happens.

Hmmmm.......

 

[cwah]

i tried to change the measures but I have the same results... I'm asking seller for confirmation

 

[Joneser]

Hubs
front
Left flange 108⌀
Right flange 108⌀
Center to left flange 20
Center to right flange 20
Spoke hole 4.5⌀
Rims Sun CR18
ERD 340
OSB 0
Spokes 36
Crosses 1
Spoke length left 120.3 mm
Spoke length right 120.3 mm

I hope this is right!

I note the Q85 spoke holes are about 3mm and the Sun CR18 rim holes are 4 or 4.5mm. Does this matter?

 

[dingrpdl]

I don't know if the Q85 manufacturer keeps their hub dimensions the same over time. But I know that Keyde doesn't. The first front hub motor I ordered last summer for my Brompton had a 90mm PCD. I ordered another one six months later and had spokes cut assuming that the hub would be identical to the first one. Wrong. The second motor had a redesigned, slightly beefier flange with a 93mm PCD. Fortunately, that only made a ~1mm difference in the required spoke length with the 1-cross pattern I was going to lace.

 

[jerrysimon]

I have now laced quite a few 16" and 20" wheels. I always use the spoke pro calc on line after I purchased his book as well.

http://www.wheelpro.co.uk/spokecalc/

All my small wheels with front motor hubs are now laced radial which is sufficient, easy and strong. They also look great

I would not use radial on larger front wheels. In practice for my small front wheel radial builds I have to take about 1-2mm off the measurement given in the online spoke calc.

As stated Keyde motor dimensions do vary. I have a black and a silver one which measure both 92mm and 90mm flange diameters. The early versions of these hub also had different measurements. All my narrow Tongxin motor measurements have been consistent. The safest bet is to never order your spokes until after you have the motor in your hand!

Rim wise you should be ok but again for rims you have not used before its better to have it in you hand before you order the spokes.

Wheel building skill takes a while to master. Its not hard and I would always encourage people to have a go if they feel like minded. However, it does take a while and a few builds to develop the skills to get consistent results and your spoke measurements spot on.

Regards

Jerry

 

[jerrysimon]

I should add that I by no means consider myself an all round wheel builder, having only ever built two larger front wheels and only a few smaller rear hub wheels. I do however feel confident that I can consistently build a good front motor hub wheel in the smaller 16/20" size.

 

[cwah]

Jerry, does the book worth it? Isn't it the same to read sheldon brown articles?

 

[jerrysimon]

I used both when I was first learning to build my own wheels.

There are lots of resources on the web including loads of good videos though.

Regards

Jerry

 

[Joneser]

Strange. Just trying to move this on an re-did the calculator and got a slightly different result:
Complete wheel data
Hub: q85, front
Left flange ⌀: 108 mm
Right flange ⌀: 108 mm
Centre to left: 20 mm
Centre to right: 20 mm
Spoke hole ⌀: 3 mm
Rim: Sun CR18
ERD ⌀: 340 mm
OSB: 0 mm
Size:
Tension for 36 spokes
CrossesSpoke length leftSpoke lenght rightTension ratio L/R
0116.2116.2100%
1120.8120.8100%
2133.2133.2100%
3150.3150.3100%
4168.9168.9100%

Jerry so if I go for radial would you recommend knocking off 1.5mm (from the 116.2)?

 

[cwah]

The nipples from the wheel laced by bmsbattery always get loose. I tighten them and 5mn after they get loose again.

Any idea why?

 

[Deleted member 4366]

You haven't done them tight enough.

 

[cwah]

Really? I was thinking to maybe put some loctite

 

[cwah]

Ok, I contacter the spokes seller, he gave me another site to test the spokes length:
http://www.wheelpro.co.uk/spokecalc/

I have new measure now.

It does look like the measure you gave me d8veh. How many cross pattern should I choose?

 

[Deleted member 4366]

I'd use one-cross on short spokes like that.

 

[cwah]

Thanks D8veh. You're the most helpful chap I've met

 

[cwah]

I've finally found back this initial thread, but now I'm trying to lace a rear BPM2 on a 26" spare bike.

My rim ERD calculation shows about 517mm hole to hole:

By adding 10mm, I can expect the ERD to be about 530mm??


So calculation would be the following:

With 108mm flange diameter and 19mm left flange distance and 15mm right flange distance.

So at 1 cross pattern, I should buy 182.6mm left spokes and 182.2mm right spokes.

Is that alright as calculation? And what's the best place to buy these spokes?

Thanks

 

[Deleted member 4366]

10mm added to the inside diameter is too much. Add 5mm to get the approximate ERD.

 

[cwah]

so about 525 erd? and other data from the bpm should be correct too? I need to get some spokes now

 

[Deleted member 4366]

There's a couple of mistakes in your calculations.
I'd use a ERD of 517 +5mm = 522mm
The circle diameter of the spoke holes is 168mm, not 180mm. 180mm is the diameter of the edge of the flange.
For a high-torque motor like the BPM, I'd use a 2-cross pattern.

When I put these results into my spoke calculator, it gives the length as 203.5mm for 2-cross and 184mm for one-cross. 203.3 and 203.8 if I add in the 4.75mm off-set to the centr of the hub.