Newby Questions

Hi all. I'm a complete newby to the ebike side of cycling - and am in need of advice of those wiser than i (thats not hard lol!). Be gentle...

I've got a 245kv 63-64 Exceed RC motor which i'm trying to work out what its actual loaded max rpm will be when running on 37v - is there any way to work it out in order to make more accurate gearing choices? Is there a way to work out torque outputs of motors?

The use of a 15mm HTD 5M belt as a secondary reduction as well as primary off the motor with around 2000-2500w? Would the secondary belt cope or shred?
Was thinking could use 16/72 for both prim and sec reductions but i wonder if the 16t pulleys could be a touch small and induce skip. Any thoughts?

How do 9 speed (say Shimano XT level) chains, cassettes and rear mechs hold up to approx 2000-2500w? I expect they might wear rather quickly but would the chain snap without careful throttle application? Especially in the higher gears - 6th - 9th?

Any help would be appreciated!

Paul

Hello and welcome. I'm new-ish here myself.

Kv is rpm per volt so a 245Kv motor at 37v (245*37) = 9065 rpm no load.

Now to the real world. Manufacturing tolarence of Kv varies from place to place with cheaper motors being usually more likely to be off. Reading reviews of only a few motors on Hobbycity will prove that. The actual Kv of your motor though is probably somewhere near 245 so its probably fine for guesstimating speed and such.

Max loaded rpm is something else. With no idea of your load (hopefully not stall) I'll assume usual medium to light ebiking abuse with a normal sized pack with a healthy C rating.

±9K rpm no load, i'd say expect 8K loaded.

Something else to check would be the absolute max rpm your ESC can handle.

Where are you getting 37 volts from? Is that the nominal voltage of your pack? The voltage it reads fresh off the charger?

With the information supplied I don't know how to work out torque but if you gear your wheel to be around 20 - 30 mph it should (with plenty of anecdotal evidence around here) work just fine. PMDC motors are much simpler to work out torque.

Two stages of 16/72 is about a 20:1 which puts your theoretical max wheel rpm at 450. On a 26" (I doubt this is the exact dia) wheel that puts you at 35mph. Real world max speed would likely be a bit over 30mph.

While not perfectly applicable here is a torque calculator commonly used by us Battlebot guys:

http://www.architeuthis-dux.org/torquecalc.asp

This calc was designed for two or more motor skid steer applications but for one motor inputting 0.5 for "motors per side" will yield results for us normally one motor guys. There is a wide selection of PMDC motors there but if you have the stats for your own BLDC you can input them. The help button is good for everything else.

You do not need to incorporate your motor into your bike chain. Take a look around and some of the clever ways this has been worked around. Grinhill's snap on pulley is probably the easiest and best off the shelf solution but if your landlord has a HAAS cnc downstairs then make friends with recumpence and buy some white industries products.

Pictures would help in giving aid along with a more extensive parts list including esc, batteries... and all that juicy junk.

- Adam

Playing around with my simulator might answer a few questions.

For straightforward calculations, no-load-rpm = v*Kv of the motor. If you don't have that information, then you're out of luck unless you have the appropriate testing equipment.

Yeah, better yet use his simulator.

Hi Paul,

As you probably realise, belt drives have a torque limit (for a given number of teeth engaged on the driver pulley), rather than a power limit. Your second stage will have to be able to handle 4.5 times the torque of the first stage (72/16), or it will slip. So, you need to know the maximum torque you'll use from the motor.

Easier to use the Gates program to work out the belt spec., (though the ratings it gives are quite conservative) :
http://www.gates.com/designflex/index.cfm?location_id=809

You can also use the tables in the attached doc.


If you know the voltage constant, you also know the torque constant. Using SI units, they are the same.

Kv (Volts/radians per sec) = Kt (Nm/amps)

360 degrees (1 revolution) is 2 pi radians (roughly 6.28).

If you just want an approx speed to work out the gearing, use 85% of no-load speed.

Kv is radians per volt? I could have sworn it was rpm.

RemoteContact said:

Kv is radians per volt? I could have sworn it was rpm.

Click to expand...

You can use vernacular units if you want

Yes, rpm per volt is used in RC motor specifications.

mmm ok. Just hoping that The sprocket I just ordered won't be wrong through gearing calcs. 10Kw!! yeahh !! 5mph!! Nooo!!!

Hi,

RemoteContact said:

You do not need to incorporate your motor into your bike chain. Take a look around and some of the clever ways this has been worked around. Grinhill's snap on pulley is probably the easiest and best off the shelf solution but if your landlord has a HAAS cnc downstairs then make friends with recumpence and buy some white industries products.

- Adam

Click to expand...

Not necessary but it might be the easiest way and better performance with about 1/2 (72V x 65A = 4,875)the power:
http://www.endless-sphere.com/forums/viewtopic.php?f=28&t=8035&p=122860&hilit=+dahon#p122860

Anyway, with a 55A controller limit, this setup peaks out at about 2600W, but because of the gearing benefit of going through the 3-speed hub, this combo actually outperforms the 24s4p 72V 5303-based setup with a 4110-modded controller set to about 65A.

This surprising performance "boost" by taking advantage of the bike's gearing has started me on this quest now, to convert all my bikes to similar weight-saving configurations.

Click to expand...

In addition to better performance, using half the power, you are probably less likely to blow a motor or esc.

BTW "white industries products" use bicycle chain.

Hi guys - thankyou for your responses and its nice to meet you all

Adam - yeah sorry mate i should have given a build description to give a better idea of what i'm trying to achieve!

My normal ride is an EPX TerraShark (pic below) carbonfibre framed lightweight mtb with 155mm rear suspension travel. So when deciding on an ebike design, the brief was to keep power up around 2kw+ and the weight down to a minimum.

For my first build i will keep it simple and do a rigid mtb frame then incorporate the drive into a full susser design similar to an Orange Alpine frame (pic below).

I bought a cheap mtb dirt jumping frame from Chain Reaction Cycles for $143AUD and used my fork, wheels etc from the EPX to build the ebike. I still havn't bought a another headset yet so its unridable at the moment plus the chain is slightly too short for this frame.
The frame will be chemically dipped to remove paint then the down tube will be cut out from just above the botton bracket to about 150mm before the head tube. The seat tube will be cut around 120mm up from the bottom bracket. I will TIG weld on machined out 10mm plates to these tubes.

I was hoping to encase the motor, batt, esc, servo tester etc within an aluminium / carbon fibre bolt together module. 3mm carbon fibre sheets, 1 sitting vertical on each side, 80mm apart to encase the 80mm long planetary 15:1 gearbox i plan to use and also use as a stressed member as it is quite strong externally. 10mm machined alu plates would bolt together top and bottom creating a water resistant enclosure which when used with 10mm machined alu bulkheads to seal off the batt from the esc/ motor etc, would create a stiff structure in place of the down tube.

Ok the build is planned as follows:

Battery: 37v LiPo 12ah 10c from True RC.net with a Hyperion charger and balancer

Motor: i've bought an Exceed RC Monster Power 160 245kv 60A cons 78A burst

Gearbox: Rino Mechanical Planetary PGC23 15:1 Output torque: 45NM nominal, 90NM max. Body size: 65mm x 65mm x 80mm. Input bore 10mm, custom output shaft 14mm.

Esc: C.C. HV110 and will add caps as a precaution - just gotta find out what caps and where to put 'em! - as you can tell i'm not i'm more into the fabrication side rather than electrical - i leave that to the brainyacs i see on these forums! :lol:

Ok so the drive i'd like to do goes as follows: HTD 5M 15mm belt from motor to gearbox input - 44T/28T= 1:1.57

Then 15:1 reduction through the planetary gearbox (Rino reckon 90% efficiency at "full load")

An 18T Trials ENO freewheel on an adapter on the gearbox output with a chain going to the cranks (2 x 34T chainrings on an ENO fw flange type i bought from sickbike parts) 34T/18T= 1:1.888

Then 34T chainring to the 11T- 34T cassette on the back wheel.

So.. assuming a safe 7500rpm at the motor: 7500rpm / 1.57 = 4777rpm / 15 = 318.46rpm / 1.888 = 168.68rpm at the cranks which should be about 20kph in firtst gear with 34T/34T and hopefully around 60kph in 8th or 9th gear (13T, 11T).

Does that all make sense lol? I could and quite possibly an wrong lol!

I've had trouble uploading a few pics, will try again later.

Cheers,

Paul

The EPX TerraShark now disassembled to build up the ebike. Still have to get either Echo or TryAll cranks to mount the freewheel...

Paul

Welcome Paul.

Your second stage will mos tlikely not handle that much power. I shredded a 5mm pitch, 15mm wide belt on the second stage at 3kw. Much depends on hte RPM you will be running the second stage at when you draw 3kw.

I would highly recommend running a small pitch (maybe #25) chain for the second stage reduction.

Anyway, both bikes look nice for your project.

Also, you have made a very good start. It looks like you have already done some research on this.

Lastly, I would look at Hobby City Zippy lipo packs. They are MUCH less costly from what I have seen. But, prices do change rapidly in this hobby.............

Matt

Hi Matt, cheers mate

I stumbled across this site a few months ago and have been watching among others, your buildup of the recumbent and now the PK Ripper - very nice build quality and execution two thumbs up! The PK will be so much fun to ride - torque induced wheelies 8)

The drive belt idea was an alternative idea to what i wanted to do all along - a planetary gearbox to do the majority of reduction (15:1). So if i could use a HTD 5M belt primary drive off the motor with 28T/44T pulleys going to the gearbox. Then run an 18T Eno Trials fw on the gearbox output (using an adapter with twin grub screws) to the trials cranks with a pair of 34T chainrings on the above flanged Eno i bought. Using 7500rpm motor spd, 34T/11T final drive, 650mm tire OD, this puts the bike at about 62kph in 9th gear (if my calcs are correct lol).

Matt, do you still have any of those adapters to suit an Eno fw? Being the output from the gearbox i would probably need a steel one although aluminium would be nice for driveline weight reduction! The gearbox output dia will be 14mm.

Although i shouldn't overlook using a secondary chain reduction i guess. Which leads me to another question: Is the torque on a chain decreased / increased much by how you gear it?

For example, using the 9sp mtb transmission for motor and pedaling drive. Is the torque on the chain increased by much when shifting from 1:1 (34T/34T) to 1:3.09 (11T/34T 9th gear)?

There are so many ways to go!


Cheers,

Paul

Yes, I would say you need a chain for your secondary stage. The chain can take all you could throw at it.

Also, yes, the torque on the chain will increase when you shift to a taller gear (under most riding scinerios).

Matt

Hi Paul,

Timma2500 said:

Using 7500rpm motor spd, 34T/11T final drive, 650mm tire OD, this puts the bike at about 62kph in 9th gear (if my calcs are correct lol).

Cheers,

Paul

Click to expand...

I think you just need to multiply voltage x kv x .8 or .85 to get the rpm. Then to xlate that into kph or mph use this calculator (change the "gear units pulldown to either MPH or kph):

Sheldon Brown's Gear Calculator

Click to expand...

http://www.sheldonbrown.com/gears/