Page 1 of 2

A minimal Q76 offroad build

Posted: Jun 07 2018 10:14pm
by spektrolyte
Image

Frame: Q76
Forks: Rockshox Boxxer RC
Shock: Rockshox Vivid
Brakes: Shimano Saint
Cranks: Raceface Chester
Bars & Stem: Raceface direct mount 35mm
Rear wheel: 19" Holmes MMP rim & Shinko trials tyre
Front wheel: 24" Halo SAS & Vuelta 24x3" tyre
Motor: Cromotor V2 18x4 winding 9.7kV
Controller: Sabvoton 72150 & CA v2.4
Battery: 84v 16AH (5x Multistar 4S LiPo)

I built this to ride trails and handle like a downhill bike. Got the spring rates for the suspension dialled in, geometry sorted and the brakes have great feel.

The rear wheel is far too heavy right now and is killing the handling. Want to go for a lighter motor and a 24" rim. Any suggestions for a motor would be welcome. Needs to top out at around 50km/h with a decent amount of torque. Weight saving rather than all out power is my priority here. Even an MXUS 3k is overkill for this project but there doesn't seem to be much else available.

It's running just a thumb throttle and killswitch on the bars, needs a BMS. The Sabvoton is on loan while my Adaptto gets fixed.

Re: A minimal Q76 offroad build

Posted: Jun 08 2018 12:48am
by motomech
Even an MXUS 3k is overkill for this project.
Really?
Needs to top out at around 50km/h with a decent amount of torque.
I guess the next step down would be the 5 Kg. geared motors: Mac, EZ, BMC and BPM and I don't think they would get you there, especially if by "decent amount of torque" you mean climbing hills that a guy would find hard to walk up. They would be hard pressed to do that and still top out at 30 Mph.
Not much in between other than the big MUXS.
But the real reason I replied is to ask you about the Multistar. How does your pack deal w/ the high discharge rate. A member here, icecube57, did some testing and felt that a 16 Ah pack could only sustain a 60 Amp dis-charge rate. My own experience is, I can see some sag on my 20Ah pack at the lower end of the dis-charge cycle and that is w/a rated controller of only 30 Amps.
You don't think you are working the Multistar too hard?

Re: A minimal Q76 offroad build

Posted: Jun 08 2018 1:04am
by redline2097
I made very similar build on Q76 with exact same Multistar setup than you and Sabvoton 72150, Cycle analyst etc. My build weight exactly 40.0kg (88 lbs).

I have Mxus v3 3000w on 24" 46mm wide DH rim with 12g spokes and Maxxis Holy Roller 24x2.4" weighting 11,5 kg (25lbs). That is about 4kg (9lbs) lighter than Mxus on 17" moto rim with light moto tire.

Image

I have switched red cranks and chainring to black. (Accidentally ordered red ones first) :D

Re: A minimal Q76 offroad build

Posted: Jun 08 2018 1:11am
by redline2097
motomech wrote:
Jun 08 2018 12:48am
Even an MXUS 3k is overkill for this project.
Really?
icecube57, did some testing and felt that a 16 Ah pack could only sustain a 60 Amp dis-charge rate. My own experience is, I can see some sag on my 20Ah pack at the lower end of the dis-charge cycle and that is w/a rated controller of only 30 Amps.
You don't think you are working the Multistar too hard?
Those 4s and 6s packs with 16ah handles twice the amps compared to 20ah packs (icecube57 tested 5c peak ability to them). They actually have two pairs of 8ah packs in parallel inside the pack instead of 1 pack like on 20ah. I have run 20s 16ah pack with heavy use on 100A peaks and they perform very well. I think I will make second wiring for 16s 32ah pack (add 3 more) and try 150A peak with them. Can't fit over 8 pack.

Copied from Hobbyking specs

4s 20ah
Configuration: 4S1P / 14.8V / 4Cell

4s 16 ah
Configuration: 4S2P / 14.8V / 4Cell

So its about twice the performance.

Re: A minimal Q76 offroad build

Posted: Jun 08 2018 5:38am
by spektrolyte
motomech wrote:
Jun 08 2018 12:48am
But the real reason I replied is to ask you about the Multistar. How does your pack deal w/ the high discharge rate.
The multistars can dump insane amounts of amps, thats why people use them.
redline2097 wrote:
Jun 08 2018 1:04am
I made very similar build on Q76 with exact same Multistar setup than you and Sabvoton 72150, Cycle analyst etc. My build weight exactly 40.0kg (88 lbs).
That looks awesome, well done! Does it still feel heavy at the back wheel? I can't even bunnyhop my bike at the moment let alone jump it, back wheel stays planted.

Re: A minimal Q76 offroad build

Posted: Jun 12 2018 12:01pm
by macribs
Bunnyump might be hard, but getting air is just about getting enough speed over a big enough jump and you will get airborne. If you fail on the first try, do it over and approach with higher speed. Repeat until success.

Re: A minimal Q76 offroad build

Posted: Jun 13 2018 7:54pm
by spektrolyte
No you don't understand. The back wheel weighs 19kg, Thats almost 50% of the bike and is causing problems with traction and manouvreability - the rear wheel has so much rotating mass that the brake can't lock the tyre making it very hard to ride offroad. Instead of rolling over things the back wheel slams into everything and getting it off the ground is just dodgy because its so unbalanced.

Re: A minimal Q76 offroad build

Posted: Jun 13 2018 8:29pm
by tolkaNo
9c clone? Leaf?

Re: A minimal Q76 offroad build

Posted: Jun 13 2018 10:23pm
by markz
2wd geared perhaps

Re: A minimal Q76 offroad build

Posted: Jun 14 2018 6:52am
by litespeed
spektrolyte wrote:
Jun 13 2018 7:54pm
No you don't understand. The back wheel weighs 19kg, Thats almost 50% of the bike and is causing problems with traction and manouvreability - the rear wheel has so much rotating mass that the brake can't lock the tyre making it very hard to ride offroad. Instead of rolling over things the back wheel slams into everything and getting it off the ground is just dodgy because its so unbalanced.
While I agree on the “weighing too much part” it sounds like your brake sucks. I have a QS 205 in a 17 moto rim with Shinzo 3” tire on it (read- way more weight!) and can easily lock it up at any speed on any surface.....on one of the grippiest tire you can buy. I run Hayes Stroker Ace but any good 4 piston with an 8” rotor will do. Also, going to a bigger motorcycle tire is a necessity in my opinion. A bicycle tire just doesn’t have the size nor strength to handle heavy hub motors. They add weight but make the motor weight disappear on smooth and lightly rough surfaces. By the time the terrain is rough enough that the tire is leaving the ground over 1-2 inches the weight can’t be masked. I run about 12 psi (I think....been awhile!) and adds a lot of cushion to the ride and really masked the weight well on curb jumps and the like.

Tom

Re: A minimal Q76 offroad build

Posted: Jun 14 2018 12:04pm
by spektrolyte
Thanks Tom,

Something is definitely amiss then. It's got a Shinko trials tyre on a 19" Holmes rim so the weight is going to be about the same as yours, and the rear brake is Shimano Saint with XTR lever, a common mod that once used by downhill racers to get more power than any stock lever/caliper combo available at the time. Maybe my pad/rotor alignment is out. I have wondered how so many people seem to get by with shitty Tektro brakes on these 100lb machines.

Re: A minimal Q76 offroad build

Posted: Jun 14 2018 7:19pm
by spektrolyte
Someone has suggested a MAC 12T motor which has high torque and should give good speed at 84v.

Can anyone comment on how the power may compare to an MXUS 3K?

Re: A minimal Q76 offroad build

Posted: Jun 15 2018 4:17am
by brumbrum
Geared motors usually have more torque than DD motors. The Mac 12t will have lots of initial acceleration, but be careful how many volts you feed it as this could potentially kill the clutch. The only problem with geared motors and off roading will be the strain on the small gears/cogs inside at low speed and lots of amps. The Mac does I believe have stronger composite cogs rather than the usual nylon ones.
I toyed with this idea myself some time back. A high turn Mac will not disappoint and it will need few amps to accelerate. I think Allex may have had one for sale recently for a good price, though I am not sure what size wheel it was laced too. Once again,the smaller dia the better to help spin up the wheel easily and not create masses of heat

Re: A minimal Q76 offroad build

Posted: Jun 15 2018 12:09pm
by Antenor
Maybe first try to dial the rear suspension rate and the pressure of the tyre...i think that will help a lot
If doesnt work, then try a new motor.

I think that frame cam acommodate a BBSH, tilted back. I saw once a 76R frame with one, but dont know if is a special type of frame

https://www.youtube.com/watch?v=mM_K0ZWgVYw&t=57s

Re: A minimal Q76 offroad build

Posted: Jun 15 2018 2:58pm
by brumbrum
Thom, if you want a light weight high torque bike you need to go mid-drive. The 'tangent ascent' will tick all your boxes :wink: Though there will be some noise... Buzzzzzzzzzzz.

Re: A minimal Q76 offroad build

Posted: Jun 16 2018 2:43am
by brumbrum
Antenor wrote:
Jun 15 2018 12:09pm
Maybe first try to dial the rear suspension rate and the pressure of the tyre...i think that will help a lot
If doesnt work, then try a new motor.

I think that frame cam acommodate a BBSH, tilted back. I saw once a 76R frame with one, but dont know if is a special type of frame

https://www.youtube.com/watch?v=mM_K0ZWgVYw&t=57s
Yes, unfortunately that is the nwer q76r which has a curved bottom on the frame to purposely accommodate a bafang mid drive unit.

Re: A minimal Q76 offroad build

Posted: Jun 16 2018 7:26am
by spektrolyte
If I was going to use a Bafang I'd just put it on a decent mountain bike frame instead of this cromo steel lump with 80s suspension design.

Have been handed a dead MXUS which I am going to try and rewind for torque rather than speed. That could be quite a lot of fun if it works.

Im talking to someone at the moment about acquiring a 3kw Tangent for my MTB, the Cro is renowned for being very heavy so for this bike I just need a lighter hub motor. MAC 12T and MXUS are still options but it seems with MAC I would need another controller.

Re: A minimal Q76 offroad build

Posted: Jun 16 2018 6:15pm
by John in CR
spektrolyte wrote:
Jun 16 2018 7:26am
Have been handed a dead MXUS which I am going to try and rewind for torque rather than speed.
There is no such thing. With a different turn count (if you achieve equal to factory copper fill) you will have exactly the same motor with only different combination of voltage and current required for exactly the same performance and efficiency. Since we are voltage limited at the top end, increasing the turn count of the windings only results in a slower lower power motor.

Re: A minimal Q76 offroad build

Posted: Jun 16 2018 6:18pm
by spektrolyte
A 5T motor has more torque and less speed than a 3T motor, the number of turns affects the RPM per volt so you can rewind a motor with more turns to get more torque and less speed at the same voltage.

Prove me wrong here if you like, I see you are a very experienced user of the forum. Tell me what I'm missing.

Re: A minimal Q76 offroad build

Posted: Jun 16 2018 6:32pm
by sacko

Re: A minimal Q76 offroad build

Posted: Jun 16 2018 6:38pm
by sacko
Something is definitely wrong with your brake setup if you can't lock the wheel up. I have had the same rear wheel setup as yourself with Hope M4's. COuld easily lock it up with one finger.

An alternative is a Crystalyte HS/HT. To be honest I am not sure how much difference you are going to feel.

This is a HT3525 on a Halo rim I had after...

Image

I then changed bikes. MXUS which oringally had a 19"JRH rim, which were then changed to Rodi rims..

Image

I would worry taking anything geared along the trails.

Re: A minimal Q76 offroad build

Posted: Jun 16 2018 6:39pm
by spektrolyte
"But here is why I think it works for me. The low speed motor simply tolerates the slower speeds you get when you climb steep stuff. So the same 1500 watts or so I can get out of that motor can be used at slower speeds. It's not more power, or more torque. It's just less stalling of the motor when going slow. So when I ride up a steep hill with 1500w feeding a 2812 winding at 10 mph, less of that 1500w is making heat than would be the case with a 2807 winding. That means more power that reaches the rubber under that particular condition, and feels more torquey. On the flat, less difference in the power because now the fast winding is not making more heat. On the flat, the fast motor seems to be better. "

So what I want is more acceleration, or better efficiency at lower speeds which feels like more power getting to the wheels. Looks like I need to fix that MXUS.

Re: A minimal Q76 offroad build

Posted: Jun 16 2018 7:45pm
by litespeed
I say you just ride what you have.....loosing a few pounds will only make a difference occasionally in certain circumstances.

Your correct in the efficiency of the higher turn motor at slower speeds but you will still heat the motor up. On a geared motor if the wheel comes off the ground and you have pretty much any throttle on when the tire touches you will shock load the gears and either break those or the clutch.

If you want a light rear end you have to go mid drive which will allow you to run around slow.

Tom

Re: A minimal Q76 offroad build

Posted: Jun 17 2018 3:27am
by spektrolyte
Thanks but I'm going to change the motor and am trying to work out what might be the best option.

Re: A minimal Q76 offroad build

Posted: Jun 17 2018 4:10am
by brumbrum
spektrolyte wrote:
Jun 17 2018 3:27am
Thanks but I'm going to change the motor and am trying to work out what might be the best option.
From speaking to you in the past, I get the impression that you want more of a helping hand at getting up hills and general assist than just all out speed without pedalling. If you are just riding general woodland trails with no jumps or very rough terrain i think a MAC could work for you as long as you keep it within tested limits and dont over volt it. Geared motors are most stressed when full throttle feom a stand still.
A high turn mac will natually consume less amps but will also produce less speed than the lower count versions.

Drunkskunk quote: "Its the copper fill that determins the torque. More windings at the same copper fill % reduces the speed, and allows the motor to opperate more efficently at lower speeds under load, but the ultimate torque stays the same". Trying to rewind your mxus is a big job that most would not attempt, and the end result will remain the same unless you have more experienced hands than the chinese workers that have probably wound hundreds of electric motors. If you manage to squeeze in more copper and correctly wind the whole motor, we will all doff our caps you you dear sir!