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GMAC/MAC hub motor servicing + freewheel modification


1 µW
Aug 8, 2020
I have a special GMAC 10T hub motor with freewheel.
Mounted on a rigid steel frame bike, 700x35C wheels, 110/120 kg (250 lb) total, for everyday use, mostly asphalt roads + backcountry tracks, hilly/low mountain area. Usually 30-40 km trips, 300-400 m of elevation difference, slopes up to 10-12%. 36 V/20 Ah battery + 30/60 A BMS + Baserunner L10 controller. 70 A max phase current, 48 A max battery current. Throttle only. Most of the time I ride full speed which is about 35 km/h (22 mph).
I have this motor stored for about 2 years, first use on August 2022. I rode something like 3000 to 4000 km and opened it to grease the gears... Found dead bearings (severely pitted), cracked seals, worn wires, damaged bearing journal and shat key, and so on, so I ended up making a full servicing... No water inside in spite of the state of the seals, just because this year was very dry, no rain.

The motor is good, but the quality of the components, mechanical design, material selection and assembly is bad on my opinion. I was lucky to open the motor before a major failure that was about to occur.

I won't make many comments on the quality of the gears and freewheel, but although still functional, I had absolutely no confidence in the motor making, so I opened the freewheel to control it. Bad spring design and manufacturing. 1 was already crushed. So I made this simple and quick mod.:
- machine/grind 3 steel stands : about 8x5x3.5 mm + chamfer + 3.8 mm hole;
- cut 3 compression springs in 3.7 mm OD, 0.5 mm wire, 1.05 N/mm (6 lbf/in) coil, about 5 mm long (adjusted to get just a slight pressure on the rollers);
- glue the steel stand with epoxy glue on the freewheel housing;
- assemble & grease everything;
- rivet the side plates with 3 mm steel rivets.
Some pictures below, sorry for the low quality... late and cold in the shop...

It's working good with better engagement and release (but still a bit sticky), about 1000 km since this operation.

FYI, here under the reference and data of the components. Be careful that it may have some differences depending on the MAC/GMAC model (shaft diameter) ?
- left side lip seal, 30x20x5 mm, 2 lips (wiper lip), NBR;
- cover O-ring, ID 145 or 150 mm, 1 mm thickness, NBR 70 sh. A;
- left side ball bearing, 61804-2RS (32x20x7 mm), premium brand price is about 14€;
- rounded shaft key, 6x6 mm lgth 10 mm;
- the 3 planet gears are probably made from PA6 nylon (MC901 ?), 54 teeth, module 1, width 10 mm. 608ZZ bearings. Funny mounting design... Meshing is not so good, the dimension tolerances of the planet carrier (freewheel) are bad. Basic gear calculation gives low acceptable continuous phase current considering the working temperature... I'm thinking in machining better carrier + gears (steel core + reinforced nylon). Sun gear is 27T, ring 135T, steel made, machining is good at first sight. I'll get 3 standard spare gears meanwhile...
- rotor bearing, ?? I tried to remove the rotor with a puller because the bearing was making a little knocking noise but the rotor is bending a lot (2-leg puller)... Not much time plus all the wiring hassle (you need to remove the cable to change the bearing...), I didn't insist. If anyone has some tips to remove the rotor without destroying everything?
- inside freehub bearing, 6902-2RS (28x15x7 mm), premium brand price is about 14€;
- outside freehub bearing, 6901-2RS (24x12x6 mm), premium brand price is about 17€;
- the freehub body is not really serviceable, just greased with bike chain grease. I'll get a spare one.

The axial dimension tolerances are probably a bit loose because in my case, I had trouble putting the last retainer ring on the right side (ground at final...).
The servicing of this motor is really a pain in the ass, cable on the left side + not going through the axle is not a good idea IMO...
My Gmac is from 2018. That bike was 2500w and throttle from the beginning. I bought it in 2022, it was in need of work. I blew the original controller with a shunt mod in the 2nd week. I got an VESC 75100 and build an 52V 72A battery. Fully loaded battery means 4000w. I commuted 5 month and didn`t ride hard. My son rode one day and now i have the first noise from the motor. It still works and i am super careful with the throttle. Just ordered spare pulleys on AliExpress. I hope it will arrive before the pulleys die.
Makes sense to order bearings immediately without checking? Do you have links for me to get a good quality in Europe?