Did I pick the wrong bike?

[Davidmnz]

Did a bunch of measurements on a 2015 Giant Talon 27.5 4 to see if things would fit before buying. Looked OK and was on sale so I bought one. However the one thing I neglected to measure was the rear dropout slot depth. After dropping the motor (Smart Pie 3) into the new frame the depth of these slots has me a bit worried. See the photos below. The slots are about 10 mm deep which means a 14 mm axle doesn't fit fully within the slots. It's not so bad on the right side because the derailleur hanger helps and the tabbed washer actually butts up against it. But on the right side the tabbed washer sits nowhere near the frame so it's effectively just a flat washer. And only about 2/3 of the washer surface is actually bearing on the frame.

I did measure the dropout slot depth on another alloy bike today and this was also only 10 mm. I eyeballed some other alloy bikes and they all seem to have ~10 mm dropout depth. The depth on my old steel bike is 18 mm.

So did I pick the wrong bike? Or will it be OK with low power/torque? Should I fit torque arms ASAP or will I only need them if/when I go to higher power/torque? I have read this thread http://endless-sphere.com/forums/viewtopic.php?f=2&t=14195 which seemed to conclude that nut torque contributes a lot to axle spinout resistance. Anyone know what the safe maximum torque is for the GM axles before they strip?

[Bikemad]

Hi David,

I couldn't tell from your photos whether the axle is fully seated in the dropouts. Usually the 5mm radius of the dropout needs to filed slightly to allow the larger radius of the 14mm axle to go in fully. You may also be able to file the dropout slightly deeper to provide more support around the axle, but you need to be careful not to weaken the frame too much.

You may be able to get away without torque arms if you keep the maximum current at or below 18 Amps, but it would be a lot safer to fit torque arms anyway.

I would be tempted apply a fine layer of coarse valve grinding paste between the mating surfaces of the frame and the outer tabbed washer/torque arm as this will provide a lot more grip to prevent the washer from moving against the frame. The only drawback with this technique is that if the nut is not done up tight enough to prevent movement, then the abrasive nature of the grinding paste will cause is to wear very quickly if it is does begin to move.

Incidentally, grinding paste is also very useful for removing tight screws with worn heads. Simply apply some grinding past to the end of the screwdriver and the abrasive particles produce a lot of grip which helps to prevent the screwdriver blade from slipping out of the worn screw head.
 
Check out this post for more information regarding the torque of the axle nut.

Alan
 

[Morgen 3Eman]

Hi David,


My bet is that a MP3 at full torque will spread those dropouts within a short usage period. 

I have a suggestion for you to consider…

Your photos show the dropout plates to be surrounded by chassis tubes, forming a recess.  I would try shaping some steel? to fit the exact shape of the recess, but long enough to form a properly deep enough dropout for the GM axle and washer to fully support the nut.  I would epoxy the plates in position to make them effectively act as a torque arm. 

TTFN,
Dennis


I put the question mark behind the steel, as I don't know how much thread you have left on the axle.  If you had lots of open thread, you could use Alloy in a thicker section. 

[Morgen 3Eman]

I scribbled in some green to show the shape I suggested.  Alternatively, If you have access to welding equipment, the yellow section could be bolted to the chassis for torque reaction.

TTFN,
Dennis

[Davidmnz]

I couldn't tell from your photos whether the axle is fully seated in the dropouts. Usually the 5mm radius of the dropout needs to filed slightly to allow the larger radius of the 14mm axle to go in fully. You may also be able to file the dropout slightly deeper to provide more support around the axle, but you need to be careful not to weaken the frame too much.

Actually I figured it would be better to file down the axle threads instead of the dropout so I don't weaken the frame at all. I don't think filing the threads will weaken the axle. I have done this a bit but haven't got a perfect fit yet. Ultimately it won't make a lot of difference as the axle will only sit less than 1 mm deeper in the dropout.

The axle material seems very soft. It was very easy to file down the threads with light pressure on the file.

You may be able to get away without torque arms if you keep the maximum current at or below 18 Amps, but it would be a lot safer to fit torque arms anyway.

I would love to fit torque arms if I could find some that I think would actually contribute much. It seems to me the Grin Tech rear torque arm is designed so the motor torque force will slide the hose clamps along the seat stay. I have read a few threads (can't recall if they were about Grin Tech arms) which said that the hose clamps do slide. I would think a design that clamped to the chain stay might be better although I guess any design using hose clamps is a bit dodgy.

I have also looked at the Ampedbikes torque arms (probably won't fit very well) and the Dr Bass arms/washers (brilliant solution but won't fit). I suppose I might have to get someone to make some custom arms for me.

I would be tempted apply a fine layer of coarse valve grinding paste between the mating surfaces of the frame and the outer tabbed washer/torque arm as this will provide a lot more grip to prevent the washer from moving against the frame. The only drawback with this technique is that if the nut is not done up tight enough to prevent movement, then the abrasive nature of the grinding paste will cause is to wear very quickly if it is does begin to move.

Incidentally, grinding paste is also very useful for removing tight screws with worn heads. Simply apply some grinding past to the end of the screwdriver and the abrasive particles produce a lot of grip which helps to prevent the screwdriver blade from slipping out of the worn screw head.

Good to know.
 

Check out this post for more information regarding the torque of the axle nut.

Thanks.

[Davidmnz]

Thanks for the suggestions Dennis. I have considered something like you suggest. The axle is long enough that I could add about 10 mm of steel or aluminium to the dropout thickness. I don't have the tools to do this myself so I would have to go to a custom metal shop.

[Morgen 3Eman]

An open end wrench of the appropriate size makes a damn cheap torque arm……

Den is

[Davidmnz]

Yep. I would probably have tried that already if I had horizontal dropouts. Not sure how it would work with vertical dropouts.

[Davidmnz]

Ignoring regen for the moment am I right in thinking the forces on the axle will try to spin it clockwise when viewed from the left side? I'm thinking the stator pushes the rotor anti-clockwise so it must exert a clockwise force on the dropout to avoid spinning. Correct?

[Bikemad]

Yep. I would probably have tried that already if I had horizontal dropouts. Not sure how it would work with vertical dropouts.

You could grind the heads off two spanners and bond them into your dropouts just like Dr Bass does with his custom made plates:



Or perhaps you could do as Denis suggested and weld some plates to the spanner heads that would bolt into the existing holes.

Ignoring regen for the moment am I right in thinking the forces on the axle will try to spin it clockwise when viewed from the left side? I'm thinking the stator pushes the rotor anti-clockwise so it must exert a clockwise force on the dropout to avoid spinning. Correct?

Under power, the torque reaction on the axle will be acting in the opposite direction to the wheel rotation, so it will exert a clockwise force on the dropout which will try to make the dropout open up (or break off) if they are not strong enough to withstand the torsional force being transmitted through the axle.

I once measured the stall torque on my modified Magic Pie and calculated it to be around 93Nm. If this force is exerted on a 10mm deep dropout the lateral force acting on the end of the dropouts is probably around 950kg (475 kg on each end of the axle).

To visualise this amount of torque, if you were to place a 130mm long 10mm spanner onto the flats of the axle, you would have to apply a force of 73kg (160lbs or 11.5 stone) on the opposite end of the spanner to produce 93Nm of torque!



The torque developed by the Smart Pie will obviously be a lot less than my modified Magic Pie, but it still has to be transmitted safely to the frame through the dropouts.

Alan
 

[Morgen 3Eman]

Hi Folks,

I like Alans' idea of bonding the wrench to the dropouts. It is cheap and easy to do.  All you really want to do is keep the dropout from spreading open.   Clean all the paint from the area, give the wrench head and dropout area a rough sanding to improve the bond, and use JB Weld generously.  Don't clamp it so hard that you squeeze all the epoxy out of the joint.  And let it cure for longer than you think is long enough:)
 
TTFN,
Dennis

[Mtbbrad]

Sorry to be off topic but I didn't realize you could get a Smart Pie for a 27.5 wheel.  First I've heard of it!  Keep us posted my friend!

[Davidmnz]

Sorry to mislead you or get your hopes up. Yes, the bike has 27.5 wheels but I will be putting a 26 in the back. May leave the 27.5 in the front or use a 26 from another bike I have.

[Davidmnz]

BTW my plan is to get a metal shop to cut a custom shape in steel which I will epoxy to the dropout. I thought about the wrench head idea but I don't think it will work that well for several reasons: Shiny/smooth surface which will be difficult to roughen for good epoxy bond, not particularly large surface area for epoxy bond, poor fit of axle in jaws. My intention is to get a shape cut which makes optimal use of the dropout surface area for the epoxy bond and has an undersize slot which I will grind for a tight axle fit. The material will likely not be as tough as tool steel but should be easier to work with and easily be as hard as the axle.