Belt RC drive on a Full Suspension Cannondale Prophet.

[jag]

I'm upgrading to full suspension Ebikes for the wife and me. See http://www.endless-sphere.com/forums/viewtopic.php?f=6&t=17242&p=251772#p251772

How about this new plan for doing an RC bike on the Prophet:
Make a 1 stage 15:1 or 20:1 reduction using a very long timing belt.

1. Mount a 1 or 1.5" fine pitch toothed pulley on a Astro 3220
2. Use a 20" bike rim as a large smooth pulley. Mount this to spokes of rear wheel.
3. Somehow find a matching very long fine pitch toothed belt.

Plan is to run this with 36 to 48V 15Ah A123 pack with a 100A+ controller. Using a 3 turn 3220 with 20:1 reduction speed (under full load) at 48V should be about 60km/h with 10,000rpm on motor. This is for offroading.
To go faster I can just swap in a larger front pulley.

A conclusion from the thread http://www.endless-sphere.com/forums/viewtopic.php?f=2&t=17401&start=15#p254804 was that it is good to build in some slip/clutching into the drive train to reduce stress on touchdowns or other sudden acceleration/de-acceleration during aggressive riding.

Here I was thinking to set the belt tension just tight enough so it doesn't slip under full motor acceleration, but it if forced by a sudden jerks / large forces on the rear wheel the belt can slip instead of back driving the motor. With a toothed front pulley the slip will happen on the very large rear pulley/rim. That should be large enough to dissipate any friction heat.

There are plenty of BMX 20" rims I could try for the rear pulley. Just have to find one with a reasonably flat bottom.

Questions:
Any tips on where to find the long belt and matching front pulley? I'm hoping for a lighteweight belt with a small tooth pitch. Maybe 13mm or so wide belt, and with lengths over 2m available.

 

[Miles]

Martin,

Have you seen this system? http://www.motoredbikes.com/showthread.php?t=5938

For us, the limiting factor is usually the amount of torque the drive pulley can handle, before ratcheting.

 

[spinningmagnets]

Eric Peltzer started with a rim-pulley and belt, and he listed all the parts and sources of his various versions. He might have some useful performance data and links...

http://www.electricycle.com/

 

[spinningmagnets]

Grinhill used the same pulleys from the link Miles provides above. His posts may have useful info too.

http://endless-sphere.com/forums/viewtopic.php?f=28&t=9172&start=0

 

[jag]

Martin,

Have you seen this system? http://www.motoredbikes.com/showthread.php?t=5938

For us, the limiting factor is usually the amount of torque the drive pulley can handle, before ratcheting.

Thanks to Miles and Spinningmagnets for the useful links.

I'm thinking to try the GEBE wheel, seen here in an image copied from the link above. I read through Grinhill's build thread.
I'm going to need a longer belt than GEBE offers. The c-c distance of my motor mount and rear wheel is 21".
Miles and Grinhill identified the belt ast a Gates 5mm pitch 9mm wide (XXX-5M-9 HTD). Not as beefy as the 13mm belt Matt/recumperence uses, but hopefully enough.
Also have to find some motor pulleys that fit the Astro 3220 shaft. Maybe an 11 and a 13 tooth.

 

[Miles]

Also have to find some motor pulleys that fit the Astro 3220 shaft. Maybe an 11 and a 13 tooth.

Such a low tooth count wouldn't be able to handle the torque, I'm afraid. At the very least, you'd need an 18t drive pulley.

You could get an 11mm wide belt, too - this should still work in the GEBE pulley.

 

[jag]

Also have to find some motor pulleys that fit the Astro 3220 shaft. Maybe an 11 and a 13 tooth.

Such a low tooth count wouldn't be able to handle the torque, I'm afraid. At the very least, you'd need an 18t drive pulley.

You could get an 11mm wide belt, too - this should still work in the GEBE pulley.

I guess my tradeoff is:
More tooth on drive pulley => less stress on belt, but more torque/stress on motor => more I^2R losses in motor and controller.
Few teeth on drive pulley => more stress on belt (more force per tooth), but less torque required from motor, so more efficient.

GEBE offers 11,12,13,14 tooth pulleys with their setups and mentions the belt lasts about 1 year in regular commuting with the 25 to 35cc gas engines they recommend (which may have less torque than an Astro). Grinhill mentioned that he was careful not to blip the throttle, but this was mainly because he used a small 1-way bearing. The one way bearing was also the first part that failed.

I was planning to use a higher tooth count pulley for quiet city riding, and switch to a lower tooth count for trail riding, and make sure I have plenty of spare belts. Does it matter to match odd tooth count on one pulley and even on the other (for even wear)? Or is that just for chains?

 

[Miles]

How much torque do they get from those gas engines, I wonder? Less than 2 Nm, I'd guess........

Bear in mind, the torque handling, at low tooth counts, is in proportion to the teeth in mesh on the driver pulley, not the drive ratio........

 

[jag]

After measuring clearances, it turns out I cannot mount the motor below/near the BB as shown in the previous post. The belt doesn't fit between the swingarm and tire. (This low mounting position will only work with a relatively small rear sprocket or pulley)

I did some more meaurements and mockups (Tip: A Dole pineapple tin can is the same size as the Astro 3220 motor). It looks like I can mount the motor as in the image below.

However now the problem is to find the right size belt. I need a 1860mm long belt, but Gates only has 1720mm and 2100mm in the on-line catalog. Any idea where to find non-standard belt sizes?

(I cannot move the motor much forward, or it would hit the frame during suspension motion. (It is mounted to the swingarm). Taking up the extra 240mm of the 2100mm belt zigzaging the belt around idler pulleys also seems inelegant and would be hard to fit)

Also, Gates lists 9mm and 15mm wide belts. Miles mentioned 11mm. Where can I get that?

B.t.w. The c-c distance between motor axle and rear axle on my bike 21" (motor adjustment could allow for 20.8-21.2"). I calculated the belt length assuming a 17" rear pulley (not sure if this is exact; GEBE doesn't list diam. Found it on another web site.) and 15 tooth front using
c = 21*25.4
d1 = 432 %mm
d2 = 24 % mm for 15 tooth
L = 2*c+(d1-d2)^2/4/c+1.57*(d1+d2)

 

[Miles]

However now the problem is to find the right size belt. I need a 1860mm long belt, but Gates only has 1720mm and 2100mm in the on-line catalog. Any idea where to find non-standard belt sizes?

ContiTech do an 1800mm and 2000mm HTD

You can get belts cut to any width you want.

 

[jag]

Over the week I had some pmail exchange with Grinhill who used the GEBE system, and on Fri I called GEBE and ordered my own 17" 5mm pitch HTD pulley.

Grinhill mentioned that the pulley is only partially toothed. For him it will slip on the rear pulley at about 2kW of power. (Didn't ask speed/rpm, but I assume some regular eBike speed, e.g. 40km/h +-20%. Now it is likely that it was designed to slip on the rear and not the front to limit belt damage (lower per length unit contact pressure). If it had slipped on the front, it might likely tear the teeth in contact. (Remember GEBE uses 11-14tooth front pulleys).

I talked a good hour with a woman at GEBE. She mentioned they had tried various rear pulley designs, with varying number of engaged teeth and smooth sections. Gradually they had increased the number of engaged teeth. (And presumably maximized power transfer capability of the 9mm HTD belt system).

She also mentioned that they has a special Kevlar chord in their belt to deal with the small radi pulleys, and likewise had the pulleys custom made. Among standard belts she recommended ones with poly or fibreglass chord. I'll choose the latter since I wont have a spring tensioning system and I was thinking the poly would stretch too much.

FYI: The GEBE belt is 1635mm-5M-9 HTD. They tension it to about 17lbs. Their front pulleys are threaded 5/16" 24tpi.

Martin

 

[recumpence]

There is actually nothing "Special" about Kevlar cording in belts. I have Kevlar corded belts in my shop right now for RC heli applications. You can get nearly any belt with Kevlar cording. What makes it special is that few people know that.

The only issue with non-Kevlar belts is if you fold the belt to teh point of kinking it. You can break the glass filaments in a fiberglass corded belt that way.

Matt

 

[mdd0127]

The HTD bible has a great section on the advantages of the different belt reinforcements. SDPSI sent me a bunch of great info too. It's pretty easy to understand and takes RPM, power, and tooth count into consideration. I was really impressed with how Gates has provided a huge amount of design information in a very understandable format. I wish other manufacturers followed their lead.

 

[Miles]

http://www.endless-sphere.com/forums/viewtopic.php?p=126539#p126539 See section 6.

 

[jag]

There is actually nothing "Special" about Kevlar cording in belts. I have Kevlar corded belts in my shop right now for RC heli applications. You can get nearly any belt with Kevlar cording. What makes it special is that few people know that.

The only issue with non-Kevlar belts is if you fold the belt to teh point of kinking it. You can break the glass filaments in a fiberglass corded belt that way.

Matt

Just to clarify what GEBE told me: They had tried the standard Kevlar chorded belts (among many alternatives), then after testing decided to special order belts with a different Kevlar chord than what's ordinarily available. Their system runs higher RPM and smaller pulleys than recommended for any of the standard belts.

Gates suggest 20 and up tooth pulleys and speeds up to 3500 rpm. Now this is for a 3000h belt life.

GEBE runs down to 11 tooth pulleys and up to 10k rpm.

Belt failure on an ebike is not catastrophic. Just carry a lightweight spare. So I was thinking to get away with a smaller pulley (17 tooth in my case) and up to 10k rpm even with standard belts. (Likely fibreglass. No kevlar in the 1800mm range unfortunately).

What pulley sizes and rpm do others run?

Martin