Front Fork Dropout Spreading

[diverdon]

Hello , This is my third Magic Pie bike ...I have an Electra Townie 26 inch with the 1000 watt MP5 on the front forks ... I have tried both front and rear motors front seem? to be less trouble ..OK changing a tire tonite and I noticed the motor "axel" was rocking in the dropout a little ...never noticed this before ...I have always had torque arms on both sides ..I'm pretty easy on the bike ride 6 miles most every day at 10-12 MPH pedaling with some motor assist ...The forks are aluminum when I got it all back together and torqued down the "axel" wasn't moving ... So how much should I worry ?? Is there even any way to replace forks with steel ones and still be assured to have the same geometry ?? Ugh !! Sorry this is so long ..any help would be appreciated ...Don J ..

[Bikemad]

Hi Don,

If you've got movement in the axle then the axle was probably not done up tight enough.

If you have recessed dropouts (Lawyer Lips) where the original wheel nut would fit inside, it is important to use suitable washers that fit inside these recesses and apply pressure to the inner recessed surface, not the raised outer surface:

I would think those suspension forks should be OK to use provided a pair of suitable torque arms are fitted, but you might need to modify two of the supplied axle washers to fit properly inside the "C" shaped recesses on the outside of the fork dropouts to provide a nice flat surface for the torque arms to press against when the axle nut is tightened:



The outside diameter of the washers should be small enough to just fit inside the recess and should ideally be chamfered/radiused on the outer edge of the inner surface so that it does not apply any outward force at the outer edge of the washer.
If the washer is too large (or not chamfered or radiused to allow clearance for the fillet inside the recess) just tightening the axle nuts could exert enough pressure to force the dropouts apart causing the casting to crack:

With a pair of correctly fitted torque arms and properly tightened axle nuts, the wheel should be safe and secure and there should be no further spreading of the dropouts. 

Alan
 

[diverdon]

    Hello , thanks for your help ..You were exactly right ..i foumd the washers you showed me in my bike hardware and installed them ....wheel seems tight when all done 2 toruqe arms ..would you replace the forks for a little movement when checked unassembled ?? The fork is $89 not crazy ..I'm a retired car/truck mech ....maybe I could do it ...although it looks like I'm the one who botched the MP5 install ...Thanks for your help ...Don J.

[Bikemad]

Hi Don,
I'm pleased to hear that you now have your front wheel fitted properly.

If the fork dropouts are only spread slightly and are not cracked, I would not replace them, but if there are any signs of cracks starting to appear, I would probably play safe and replace them.
As long as the axle is not moving at all when everything is properly tightened up with two torque arms (and the correct washers) fitted, it is unlikely to cause a problem.

If you regularly check the tightness of the axle nuts and torque arm fixings, you should notice if anything is starting to work loose.
I try to ensure that my axle nuts are always kept nice and tight, so that that everything remains nice and safe when I'm out riding.
Alan
 

[diverdon]

Hi Alan ....Thanks for the reply ...I will let this front fork untill I have nothing better to do than tear down this whole thing ... seems tight ? only when I really slow down like to go over a speed bump then give it a little "Gas" to get going again sometimes I think I feel sort of a "clunk" ...Could be the steering stem ...seems tight thou ...could be in my head "very likely" or could be the dropout clunking Seems tight thou ...I have searched for the torque spec for the 21 mm nut that goes on the axel and cannot find it ...Do you know the torque spec ?? ...It's a large thread but with the cutout on the axel I'm afraid to go too tight still I'll bet I have it over the spec already ...Sorry this is so long ...Thanks Again for all your help ....

[Bikemad]

Check out this post regarding axle nut torque settings.

Alan

[diverdon]

I have just been Googling torque figures, and for a low grade M14 x 1.5 nut the tightening torque is 79Nm (~58ft/lbs).

But this does not take into consideration the fact that the axle thread is incomplete due to the two flats which have no thread whatsoever:

According to my calculations, machining the flats will have actually removed almost 50% of the threaded section of the axle, therefore it would be a bit unfair to expect it to withstand the full recommended torque.

As half the thread has been removed, I would therefore suggest that the recommended torque should also be reduced by 50% too.
This would give a more realistic recommended tightening torque of just 39.5Nm, or 28.76 foot pounds.

As I mentioned in a previous post, a longer (deeper) nut will be able to withstand a much greater torque without stripping because the applied load is spread over more thread.

If a 10mm deep nut strips at just over 36 foot pounds, then a 20mm deep nut should withstand just over 72 foot pounds (97.61Nm) before it strips.

I must admit that I usually tighten my axle nuts by feel, because after 40 plus years of tightening nuts and bolts, I now have a pretty good "feel" for the correct tightness of wheel nuts etc. and often have more confidence in my own judgement than the accuracy of some torque wrenches that I have used in the past.

I previously used a torque wrench to check exactly how much I had tightened my axle nuts up, in order to answer a similar question.
I positioned the torque wrench so it was just about level under its own weight and then slackened the nut. I then carefully retightened the nut until the released torque wrench was in the same position as it was before I slackened  the axle nut.
The torque required to tighten the nut back to the same position as before was ~80Nm (59 ft/lbs).

Unfortunately, I am unable to explain why your axle threads appear to be unable to withstand the higher torque that mine are tightened to, unless they had been weakened by over-tightening them initially.

Here is my preferred tool for tightening the axle nuts, a 7/16 Whitworth ring spanner:


I would expect a standard Magic Pie to exert a maximum torque of around 50Nm through the axle, and most of this torsional force would be counteracted by the dropouts and the tabbed torque washers rather than the axle nuts.

Theoretically, the two axle nuts should be able to withstand this amount of torque without the assistance of the axle flats and torque washers etc., provided they were both tightened to at least 25Nm.
I therefore feel that the 39.5Nm, or 28.76 ft/lbs tightening torque calculated above should be more than adequate, especially when combined with the axle flats in the dropouts, torque washers and an additional torque arm.

Alan

     Thanks for the Reply ...It still isn't clear to me ...40 or 60 ft pounds ...40 isn't very much 60 sounds like it might strip out I'll have to borrow a torque wrench as I don't even have one anymore ...I'm an old mechanic and go by the grunts method one grunt is snug two grunts is tight ... One thing that concerns me a little is that when I tighten the axel nut to a "felt tightness" when I go to take that nut off it seems to break loose very easily ...like it isn't tightened enough .. So what torque spec do you use on your bikes ??
                 Thanks again for all of your help ...DJ

[Bikemad]

Hi Don,

If you read that post again, you will see that I actually recommended 39.5Nm, or 28.76 foot pounds, so if you are happy to tighten yours to 40ft/lbs it should be plenty tight enough for the standard axle nuts.

As I've previously mentioned, I always tighten the axle nuts by feel, as I can usually "feel" when a nut is approaching its yield point.

I will admit that I probably tighten mine a bit more than they need to be because I have steel frames and I run a modified Pie at up to 50 Amps with no torque arms fitted.

I have previously measured the torque output of my modified Magic Pie, which worked out to be ~93Nm @ ~66Amps. Therefore a standard Magic Pie will probably produce ~42Nm of torque @ 30Amps.

My modified controller was eventually pushed to it's limit, but before it finally died, it delivered a whopping 97.75 Amps to the motor:


As torque is proportional to the current, this should theoretically have produced almost 138Nm (101ft/lbs) of torque, which my overtightened axle nuts were able to withstand without allowing the axle to move within the frame dropouts (and that was also without any torque arms fitted).

The most important thing to consider is that the torque on the axle is not one directional, as each time you engage regen, the torque on the axle is applied in the opposite direction to when the motor is being powered.
Any movement at the axle fixings whatsoever should be avoided at all costs, as it will inevitably result in the loosening of the axle nuts.

I regularly put a spanner on the axle nuts just to check they are still nice and tight and I haven't needed to retighten them yet, so presumably nothing is moving where it shouldn't move. 

Alan
 

[diverdon]

.. Thanks Alan ...I was confused as it looked like there were 2 opinions .. You really helped me a LOT with the info on the "lawyer lips" and the special washers ...it was probably in some instructions I ignored ...
        97 Amps Woaaah ...What does that do ...Extreme tire burning torque ....and top speed ? I can't imagine ..is there a thread on here about that bike ??
 
                            Thanks Again So Much ..Don 

[Bikemad]

Hi Don,

Here is a picture of "that bike" with my original Magic Pie with the internal controller modified to draw the high current (97.75A) but this bike has subsequently been running on a slightly tweaked MPII for the last eight years and still draws over 45 Amps from my Hailong bottle mount battery:



The high current provided amazing torque and very quick acceleration, but the top speed was still about the same as before, as the maximum speed is ultimately governed by the voltage not the current.

I experienced plenty of wheelspin on loose or muddy surfaces, but the bike would instantly wheelie when full throttle was applied from a standstill on hard surfaces. 

Further details of the shunt modification can be found here, here and here.

Alan