Why Dropout Lugs Break

[rbscebu]

I am working on fitting an ebike kit to my MTB. The kit is based on a 48V 1000W non-geared rear wheel hub motor. The MTB has an aluminium alloy frame (including dropouts).

During my research on this project, many articles recommended that the axel of the hub motor be fitted with a torque arm, particularly on aluminium alloy framed bikes. This is to help prevent the dropout lugs from breaking due to the torque applied to the lugs by the hub-motor axel.

Being a mechanical engineer, this got my inquisitive juices flowing. I decided to calculate how much torque my existing aluminium alloy dropout lug could take before failure. The result surprised me. My dropout lugs could theoretically take about 150N.m of torque each side (or 300N.m total) before failure. This is well above the maximum torque that most hub motors can apply.

This then got me thinking as to why dropout lugs were failing at much lower torque loads. The answer lay in the design of the existing dropout lugs and the hub-motor axel.

In the picture below, figure 1 shows a typical dropout log arrangement and the cross-section of a typical hub-motor axel. Due to the shape the axel, it cannot fully seat into the shape of the lug cut-out. The sharp shoulder on the axel between the flat and the threaded portion sits at the top of the half-round cut-out. This is shown in figure 2.

Any vertical load on the axel (and there is plenty when riding) will tend to move the steel axel further up into the aluminium alloy lug out-out forcing the sharp axel shoulder into the aluminium alloy. See figure 3.

These notches create a stress raiser in the aluminium allow that greatly reduces the strength of the dropout lugs against a torque load. See figure 4. Thus the cause of dropout lugs breaking well below their design limit.

 

[AHicks]

As one that has done a few conversions now, I can tell you your concern is easily handled on alum. frames with just a few minutes and a mill file or something similar. You do want to eliminate that gap between the axle and the mounting lug for good support. The area of concern is as you have noted. You want to use the file to carefully extend the flat down further into the radius - without increasing the width of the slot the 2 flat sides describe. Increasing that slot width just increases the potential for the axle to twist under the power and regen. torque loads.

This deepening/dressing of those corners will likely be necessary for correct wheel alignment as well as setting the disc brake up to work properly, with the pads running full width on the disc. Bottom line, it's worth taking/alloting time to spend a few extra minutes here doing this right - but it's nothing that might be described as a show stopper.

 

[rich c]

I'd suggest the design is appropriate for the original design parameters. It's when you far exceed the original parameters that you have the failures. The single point doesn't reduce the strength, it allows the stresses to concentrate at a single point to initiate failure.

 

[indianajo]

Bike frames come from *****. The tensile strength of the alloys used have nothing to do with the tensile strength of real steel or 3130 aluminum. Steel is not supposed to include lead or copper, and aluminum is not supposed to include silicon or zinc. As one who worked with people who used Harbor Freight "tools", I've seen plenty of raggedy fractures in the metal. And my preferred supplier of screws & bolts doesn't use ***** for their generic line, they use ******. Cost three times what the lumberyard charges.
The blacksmith demonstrater at Biltmore, whose day job was making custom leaf springs, said he couldn't even get consistent spring quality steel from the suppliers anymore. All kinds of QA certificates were shipped with the product, but the ****ese source steel would fracture in places due to inconsistency. The low volume user is not worth shipping a quality product.
Some suppliers are able to get decent quality out of that country. I was shocked my yubabike with the African name was sourced from there, but so far the initial quality and performance have been superb. Diligent QA procedures and honest QA people are difference, I presume.

 

[AHicks]

So what's your point? Don't buy a bike built in China? That's not going to leave you with many options, especially if affordability is a factor....

 

[indianajo]

US bike vendors pay a lot of attention to a frame not falling apart and stabbing the customer's child in the belly, as my sears bike did in 1957. the quality of the front welds are visually very impressive these days.
vendors see no liability suits involved in the dropout lug breaking. especially if the customer performed an unauthorized conversion to an electric drive bike. so you get what you pay for -junk.
I electric converted a 197? huffy frame built in ohio. You can find a similar bike on any scrap truck driving around on garbage day. If you want real steel today, buy from an integrated electric bike vendor that has his reputation on the line, or buy a $$$$ custom frame made of real steel (probably also ****ese imitation steel unless the welder is very sophisticated).
It is very difficult to buy anything but instant garbage these days, when people want the "best price".
Another point is that engineering calculations like in post 1 on this garbage are IMHO rather worthless. Ordinary steel can be "assumed" to have tensile strength of 60000 psi. this garbage we bought in toy bicycles might have spots in the dropout lug that won't withstand 10000 psi.

 

[harryS]

They break because the majority of users don't know much. They go for a front hub because they're afraid of setting up a derailleur. If you can't do that, you probably cannot be trusted to tighten a nut.

 

[rbscebu]

As one that has done a few conversions now, I can tell you your concern is easily handled on alum. frames with just a few minutes and a mill file or something similar. You do want to eliminate that gap between the axle and the mounting lug for good support. The area of concern is as you have noted. You want to use the file to carefully extend the flat down further into the radius - without increasing the width of the slot the 2 flat sides describe. Increasing that slot width just increases the potential for the axle to twist under the power and regen. torque loads.

This deepening/dressing of those corners will likely be necessary for correct wheel alignment as well as setting the disc brake up to work properly, with the pads running full width on the disc. Bottom line, it's worth taking/alloting time to spend a few extra minutes here doing this right - but it's nothing that might be described as a show stopper.

AHick, you are missing the point. The problem is not that the hub-motor axel is not seated fully into the lug cut-out. The problem is that the sharp shoulders on the axel cut into the lugs.

Any sharp corners (or cracks) in a stressed member concentrate the stress at that corner. Filing (or milling) the lug cut-out to match the shape of the axel, still produces these unwanted sharp corners.

So what's your point? Don't buy a bike built in China? That's not going to leave you with many options, especially if affordability is a factor....

It is not a "China" bike problem. It is a problem for all ebikes, particularly those that have lug cut-outs closer to the vertical.

The point is that if you want to reduce the chance of your dropout lugs breaking, you need to transfer as much of the torque out of the lugs AND try and prevent the hub-motor axel from forming a notch (stress raiser) in the lugs. My calculations have shown to me that it is not the overall torque being applied to (undamaged) lugs that is causing the failure. It has a lot more to do with the axel forming a notch(es) in the lugs, thus concentrating the stress at the notch(es).

 

[Nova Haibike]

Are dropouts actually failing from cracks developing at the axle cutout itself, or are they failing where the dropout and frame tubing are welded together? I can't speak about e-bikes with hub motors, but I have never seen a dropout break at the axle cutout on a regular bike. I have seen many break at welds or brazed points, though, and also where dropouts are poorly designed to begin with.

 

[rbscebu]

The only broken dropouts that I have seen on ebikes have been in China. Most had broken lugs (although some had broken elsewhere) and all of those broken lugs broke where the axel shoulder had dig into the lug at or near the start of the semicircular part of the lug cut-out. These dropouts where made of both steel and aluminium.

Most of the ebikes with broken lugs did not have a torque arm fitted. The few that did have a torque arm fitted broke because the torque arm had become loose and/or the the torque arm had worn, allowing the axel to move against the dropout lugs.

I am not saying that torque arms are not beneficial. What I am saying is that there are two things that should be addressed with regard to dropouts when converting a peddle-only bicycle into an ebike. You need to distribute the axel torque forces away from the dropout lugs AND you need to prevent the sharp axel shoulders from digging into the dropout lugs.

 

[rbscebu]

Here is a photo of the rear dropouts on my MTB that I am converting into an ebike (axel nut and washer removed for clarity).

​

I have designed a stainless steel torque plate (one each side) that will bolt onto the dropouts. These torque plates will hold the hub-motor axel about 1mm away from the start of the semicircle part of the lug cut-outs and will transfer the axel torque to the dropout plates. All this is done via the two M5 bolts connecting each torque plate to the dropout plate.

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The two M5 bolts should be sufficient to hold the axel in position but I have not yet determined if they are also sufficient to take the axel torque nor if the dropout plate is strong enough to take the axel torque. If not, I will have to fit an additional arm onto the torque plate to transfer the torque load to one of the stays.

 

[indianajo]

I hope you have a lasar cutter to cut an oblong hole in a stainless steel torque plate. That stuff is hard to drill even, takes a titanium coat drill to make normal progress.
I make torque plates out of 1.2 mm or thicker carbon steel plate, drill two 8 mm holes then connect them with a round file and square the edges with a triangle file. Takes about 2 hours, costs nothing. I make them about 10 mm long and wrap the frame strut with a sheet steel strap.
The shafts in the hub motors I've bought have been such *****y steel that I wouldn't feel good about filing them down any. You get more lever arm anyway with the full length flat area. I use elastic stop nuts on the mount of the torque arm to eliminate loose bolts. If those are not available use double nuts and counter rotate to lock them down.
As far a steel quality from modern sources, my 197? Ohio sourced frame I converted weighs about 15 lb more than a modern bike and is very likely to meet 60000 psi tensile strength test. I never had 1970's bolts break with a ragged edge as I have had modern "grade 8" bolts from the farm store do. In the seventies overstressed bolts would strip the thread.

 

[AHicks]

AHick, you are missing the point. The problem is not that the hub-motor axel is not seated fully into the lug cut-out. The problem is that the sharp shoulders on the axel cut into the lugs.

Any sharp corners (or cracks) in a stressed member concentrate the stress at that corner. Filing (or milling) the lug cut-out to match the shape of the axel, still produces these unwanted sharp corners.

My thinking was regarding the idea that with the axle properly fitted, those corners are not as stressed as they would be if there were a gap between the axle and the lug - as there will be if some time and effort to eliminate that gap hasn't been taken. I think you'll see that when you drop the axle into the existing slot on that bike.

I will not disagree on your point regarding stress where the flat transitions to the radiused portion. Let's just say I am suggesting you make the best of a poor situation when doing your conversion.

That plate idea is a good one, but not all bikes are set up to allow something like that, and not all owners are going to be up for fabricating something like that. That, and the fact each bike I've done is just a little different in that area, would suggest that a torque arm should be part of any good plan/install. My thought anyway, FWIW. -Al