Steel dropout spread from over tightening

[Propofolboy]

Hi everyone.
I'm pretty new to this and a bit disappointed this morning.

I installed a 1000W hubmotor on my front steel fork of my fatbike a couple weeks ago, and thought I'd done a good job. The spacing was tight, so I wasn't able to put any washers between the inside of the dropout and the axel shoulder of the hub motor (it has quite a small shoulder I now realize). On the outside of the dropout I installed a c washer, then a 1/4" stainless steel torque arm bolted to fender eyelet, and an axel nut on BOTH sides. On the non wire side of the hub motor I used a torque wrench to 60 N.m. On the wire side I just used a crescent wrench, and probably only 30 N.m.

Things had been going well until yesterday I checked the axel nut and they were loose. I decided to take the whole setup apart to make sure nothing was wrong. I hadn't had any issues at all riding the bike (no spinout or anything). To my horror I discovered that the inside of the dropout on the side I torqued to 60 N.m had become damaged. It looked the the shoulder of the axel of the hub motor had dug into the bottom of the dropout, spread it out about 2 mm and distorted the metal. The top 60% of the dropout however was fine, only the bottom had spread out. The other side dropout was perfect, no sign of spinout or anything. The two torque arms were also perfect, and very snug on the axel (no sign of spinout).

My impression was then that the torque of the hub motor had not created any spinout despite having only hand tightened nuts for at least a few of my rides, but the process of torqueing the axel nut had created a wedging effect and spread out the one dropout and slightly damaged it.

My solution which I spent last night doing was to slightly cold set my forks about 4 mm wider so I could now place two heavy 2 mm thick washers inside the dropouts between the hub motor axel and the (slightly damaged) dropout. I filed the dropouts about 0.5 mm deeper. This created a pretty tight fit. I then put the c washers, torque arms and axel nuts on the outside. I then put on some Loctite and torqued it up to about 40 N.m. Everything now seems very solid, and I don't think the inside of the dropout is being spread by the axel shoulder with that heavy washer protecting it and providing more surface area against the non-damaged part of the dropout.

So I guess my question is -- is my slightly spread out single dropout going to be a problem if I now have everything tight. I read Justin Le's 8 page thread on spinout and forks with great interest. The takeaway from it seems to be that if there is slight play between the axel and the dropout its not a big deal as long as you have torque arms and tight axel nuts to provide friction. He seemed to indicate that really the dropouts are not that important to prevent spinout and talked about even rounding out the axel on the dropout so it would spin freely to test how well the torque arms and friction protect the system. Indeed just the dropout alone with no friction or torque arms really provides very limited spinout protection (like less than 20 N.m at best). It is a bit reassuring to me as well that despite the damaged dropout, and only hand tight nuts there was absolutely no signs of spinout to the two torque arms or the other dropout that I hadn't tightened as much...

Thanks!

 

[teklektik]

Sounds fine.

As long as your TAs are well-affixed and doing their job to counter shaft rotation, the dropouts are only left with the task of supporting the bike and keeping the shaft in general neighborhood of the bike. A little dropout spread isn't what you'd shoot for in the installation, but isn't really an issue if you're saddled with it...

This 'small axle shoulder' issue often gets ignored with exactly this result so it's good you got after it. It's particularly nasty with aluminum forks which go to fracturing instead of just deforming...

 

[Propofolboy]

Awesome thanks!

I think if I would have retightened the axel nuts instead of taking it all down I would have further spread and deformed it, and then it would have loosened again, and eventually it would be beyond saving. I just hope I haven't gotten to that point yet!

 

[2old]

It's a steel frame therefore it's possible to cut the old dropout off and weld on a replacement. I did that with an MTB where the dropout had cracked after about 20 years of use. Big plus (for me) was the new dropouts were sliding and the bike was used as a single speed.

 

[Propofolboy]

Awesome that's good to know. Hopefully I have now limited the damage to these forks and they will be ok.
Did you do the welding yourself?

 

[2old]

Had a friend do it since he had the equipment and some dropouts that he had hacked off a "donor" frame. Dropouts are sold by Paragon Machine Works, among other frame building suppliers.

 

[Voltron]

The old Amped kits had a warning in the manual about not overtightened the axle nuts or it could force the dropout over the axle shoulder.. which of course begs the question why not just make the shoulder bigger. I always hesitate not to use the steel washers with the axle shaped hole to spread the point loading if there's any possible way to squeeze one in.

 

[Chalo]

The old Amped kits had a warning in the manual about not overtightened the axle nuts or it could force the dropout over the axle shoulder.. which of course begs the question why not just make the shoulder bigger.

It also begs the question why use a flatted axle to transmit torque, when there are much better ways?

The answer to both questions is China.

 

[Voltron]

While not perfect of course, the flatted axle has been shown to be functional in millions of bikes, and reverse compatible with virtually every bike. Not ideal, but adequate for low power systems for bikes with straight steel tubes, no suspension, and rim brakes...

Maybe we should be thanking China for shouldering the burden of manufacturing the future of low cost, practical lower impact transportation for the whole rest of the world to resell to itself.

 

[Chalo]

While not perfect of course, the flatted axle has been shown to be functional in millions of bikes, and reverse compatible with virtually every bike. Not ideal, but adequate for low power systems for bikes with straight steel tubes, no suspension, and rim brakes...

Coaster brakes could do it that way, but they don't. Heinzmann hub motors could do it that way, but they don't. They're both just as compatible as flatted axles (actually more so), but also more functional and fit for purpose. And both these things have been around much longer than Chinese crap hubmotors.

The Chinese crapsters chose to do it wrong after having been shown how to do it right. Coaster brake hubs come from China, after all.

 

[dogman dan]

Couldn't agree with Chalo more on this one. They need to imitate the heinzmann design. The TA should be built into the motor, then a round 10mm axle will work fine.

At the very least, a larger shoulder on the axle itself so it does not dig in. But noooooo, it would cost em a buck more per motor to use a larger diameter bearing. My former employer used to supply a very thin spring steel washer to go on the inside, but it did mean stretching the fork 1 mm per side to fit them.

Your fix is perfect, but shame on them for making you have to do it. I find it unbelievable that china won't copy the heinzmann patent. They shamelessly copy everybody else. But that design would cost em 2 bucks more per unit.