Torque arm on a suspension fork?

[vmcroller]

Hi folks,

First post here! I've done a bit of digging in previous threads but can't find the answer, so here's my question: is there anything inherently wrong with using a torque arm on a suspension fork?

Backstory: I converted my Priority Onyx into an e-bike via Grin's ready-to-roll (front hub) kit, and recently took the bike to a shop with the hope that they could swap out my rigid fork for a suspension fork. I'd previously spoken to a mechanic that worked at the shop, but by the time I arrived with my bike, the owner himself was *absolutely aghast* that I'd even consider this. He looked at me like I'd told him I was going to replace my aluminum frame with one made of cardboard.

Am I missing something? I don't really understand what I'm missing here. Dude straight-up was scoffing at me while I was trying to process *why* exactly he was so terrified of this combination.

Anyone got a thought or idea here?

 

[AGS]

Yeah, he should have explained the reasons why a front hub motor won’t work well with suspension forks instead of scoffing in disgust.

 

[richj8990]

Yeah, he should have explained the reasons why a front hub motor won’t work well with suspension forks instead of scoffing in disgust.

Maybe if the front hub motor is like 3000W then it may not work well. Something that's 250-1000W with two (not one) torque arms is going to be perfectly fine. I have no idea who made up the story that a mountain bike suspension fork, that is designed to take a ton of abuse for thousands and thousands of miles, can't handle some stress. They were BUILT to handle stress. Maybe not built for front hub motors, but built to handle a lot of stress. Again, with torque arms.

Bike shops are very, very conservative about what they do and do not work on. Just put on a couple of torque arms and ride. You may need to file down the dropouts a little to get the axle to fit. The horror stories you hear are from people who put on a 2000+W front motor with no torque arms, and eventually the dropouts snap off, motor and wheel goes away from the bike, front of bike slams down on the pavement, rider goes over the bars. I've never, ever heard that happening with torque arms.

 

[Chalo]

Flatted axles like those on almost all Chinese made hub motors exert a large prying force on the axle slot. Any suspension forks that aren't complete garbage are probably made of cast aluminum or magnesium at the axle slot. These are relatively weak and brittle materials that respond to prying by snapping off. Even using torque arms may not prevent this failure if the axle has any ability to move slightly in the torque arm hole. It's a dangerous failure because it can result in complete ejection of the front wheel.

Suspension forks almost all have deep recesses for the axle QR that must be either filled in or shaved away to accommodate a hub motor axle that sits lower in the fork tip and has nuts that are too large to fit into the recess. When such a fork and wheel combination is used with a disc brake, hard braking can cause the wheel to eject from the dropout using the disc caliper as a fulcrum. This happened to me recently, but not with a suspension fork. The other end of the axle held on, barely, and the fact that the fork was made of strong ductile chromoly steel allowed me to bring the bike to a kinda-controlled stop without getting hurt. If it had been a suspension fork, both fork tips would have broken off instantly and I would have been thrown headfirst.

A third concern, less safety critical, is that when a hub motor pulls the bike along by the fork tips, it binds the sliding action of a telescopic fork and can in effect lock out the suspension while power is applied. It also prematurely wears out and loosens the bushings that the fork sliders run on, making the fork loose and sloppy.

If none of these things matters to you, then knock yourself out. Go for it. Figure out for yourself why it's a bad idea.

Also maybe respect that there are sound reasons why someone with incalculably more relevant experience than you have might scoff at something that you don't fully understand.

 

[vmcroller]

Not reading your message, Chalo. Will read others that aren't weirdly hostile or suggest *anyone* lean into being scoffed at, though! Happy to see this forum has an "ignore" function

 

[vmcroller]

Rich,

Yeah, I hadn't considered a second torque arm. It's a good idea. Suspension fork I purchased isn't necessarily a mountain bike fork (realize there isn't a hard-and-fast definition), but I imagine still decently robust (Amazon.com).

I saw two things from the snide goofball's post above before deciding to not read further: the bit about me being some smarmy lil' jerk who deserves to be scoffed at for my insolence of not understanding things (??), and something about the fork dropouts being cast aluminum, which I'm sure they are. The cast part *does* worry me a bit, though I was planning on doing some progressive stress testing with motor output capped at sequentially higher levels -- 100w, 200w, 300w, etc., maybe up to 1200-1500w. There are some decent hills here where I can hit peak power without actually moving very fast. Pretty much ride exclusively on harrowing DC streets, so will have to dial it down and keep output capped at 750w for normal use.

Decent plan to ensure safety cushion? Other ideas to validate safety?

 

[bengxe]

That "snide goofball" put a lot more effort into helping you than most will be willing to. A "decent plan to ensure a safety cushion" would be a rear hub motor. You can choose to ignore everyone who doesn't spell things out simply and agree with you, but you won't learn much that way.

 

[vmcroller]

What is the deal with people here? I'm trying to learn. I'm looking for kindness and not being belittled. That's it! Am I on 4chan? Jesus.

Pro tip: people that are requesting help often need things spelled out simply!

 

[Chalo]

You know, you came in bent out of shape after taking honest feedback from a mechanic personally. Now you've done the same a couple times in rapid succession here. I'll leave it to you to draw your own conclusions.

 

[vmcroller]

Not bent out of shape, just really confused. Their conception and my conception really at strong odds. From that it tonally became "you're a smarmy little brat for thinking you know better" from internet strangers I've never met. Weird.

People here don't even know the mechanic! Maybe he's a prick! Maybe not! It's so bizarre that from *that* the read was "this jerk needs to be taken down a peg!". Like I said -- weird.

Am fine being told I'm wrong about my initial question... that's why I posted! Less interested in being told I'm right/wrong, plus that I'm an insolent jerk.

Anyone not interested in doling out punishment for me overstepping my bounds of questioning someone they've never met, and responding to my initial question, I'd really like your feedback.

 

[Chalo]

If you don't want frank advice, don't ask frank people. Go ask someone who's eager to sell you something expensive.

 

[bengxe]

Your question was thoroughly answered in post #4. If you're having trouble understanding it you need to ask specific questions.

 

[vmcroller]

Nah, not reading that, nor engaging with either of you guys any further.

Can you both quit responding so others (like the first two posters) might offer their advice?

 

[nicobie]

If you don't want to read what people take the time to type and I might add that obviously know a lot more about ebikes than you, then why did you even bother to come here and take up our time?

 

[E-HP]

Your question was thoroughly answered in post #4.

Agreed. Sometimes you have to choose between the doctor that will save your life, and the one with good bedside manners.

Actually reminded me of stuff I forgot about after deciding not to go that way. Having the front wheel come off at 40 mph would suck. Those lawyer lips make the actual dropout even thinner. Dual torque arms aren't as much for the dropout as they are for holding the wheel on when a dropout snaps or the axle spin out. I think you need to have them anchored to the back of the legs so the twisting motion of the axle tends to move the axle upwards.

There's a lot of pics of them failing. The bike shop guy probably has seen some examples too.

 

[harrisonpatm]

Nah, not reading that, nor engaging with either of you guys any further.

Can you both quit responding so others (like the first two posters) might offer their advice?

In #4, Chalo wrote you 3 neatly organized and spaced sections in which he objectively described and explained the possible outcomes to your idea. In order to notice anything resembling anything crass, you would have had to read past that, through his technical response and into the 4th and 5th sections, before getting to anything subjective. In your response #5, you said you're not reading it because you don't like anything resembling potential hostility. Suggesting that you didn't actually read his informative response, but rather just skimmed through a chunky block of text in order to first find something that might be offensive to your sensibilities. Leading other posters to believe that your priority here isn't to get new information, but to receive affirmation. Which is fine. Just don't be surprised at the level of response you get when you look to be offended first and foremost.

 

[riding_on]

Not reading your message, Chalo. Will read others that aren't weirdly hostile or suggest *anyone* lean into being scoffed at, though! Happy to see this forum has an "ignore" function

I may be completely wrong here, but part of what I got from his statement is that the suspension absorbs/dampens some of the energy from pedaling. A lot less of a hassle when the energy being wasted isn't from your legs, and is instead just wasted energy from your battery; but I can see how a cycling purist would just find the idea distasteful without being able to put into words why.

I still think he should have explained, instead of using admonishment, but then again, my way of doing things only works if people are actually willing to learn why something is bad. I don't admonish people for not wanting to, personally, but maybe I should. Who knows.

 

[glennb]

Maybe you’ll engage with a story and personal experience.

Many years ago I attempted to align the brake tab on a rockshox fork by bending it. It snapped off with very little force applied. Up to that point I’d bent, hammered, twisted, drilled, filed, melted, and otherwise broken just about anything you could imagine, and yet my brain could not reconcile what just happened. Literally stared at in in disbelief for over a minute.

If you appreciated just how brittle cast forks lowers are, you wouldn’t consider putting a hub motor in one. The material has zero elasticity, it simply snaps.

If your torque arms rely on nuts and bolts that work loose, like every other nut/bolt on a bike, you are toast when it happens.

FWIW, I’m more reckless than cautious and there’s not many things I wouldn’t do on, or to, a bike, but motor in cast forks is among them.

To put that in perspective, the entirety of rest of the list of things I’ve vowed not to do is as follows, most of which resulted in hospital visits and prolonged recovery periods:
- riding in thongs
- riding with dangling shoelaces
- riding on footpath/sidewalk
- riding without lights on unlit roads at night
- riding home drunk and/or stoned
- riding two abreast with inexperienced rider
- riding in thick fog
- riding on icy valley roads
- unevenly torquing stem faceplate

 

[riding_on]

Uh-oh, I think I may be guilty of the last one on your list, glennb. Why is that bad, and should I get the hospital visit pre-authorized tomorrow, or wait until I can see the accident coming?

To be serious though, riding in thongs? I gotta ask: how did you get hurt doing that?!

 

[vmcroller]

Riding_on, glennb, and E-HP, thanks for the genuine responses. The stuff you've each shared is both illuminating and somewhat alarming.

E-HP, I don't have lawyer lips on the new suspension fork, but the dropouts certainly aren't what I'd call beefy. Your points about thickness are helpful.

Riding_on, yeah, the loss of efficiency doesn't bother me too much, truthfully, even if I were straight peddling. Guess it's easier to say that when riding an e-bike, but still. Concur on your take on admonishment. I'm a flight instructor and also teach woodworking safety classes. Being an "I tell it how it is, and proud of it!" type is not at all conducive to imparting knowledge to new folks. Just bad all around.

Glennb, you kinda hit the nail pretty close to the head for me. I'm a little worried I'm going to have to really flex the fork a bit to get it properly seated with spacers for brake alignment, even if they're only ~1/8" thick. I figured that'd possibly be a doable task, but your comment has me second guessing it.

 

[circuitsmith]

Native Washingtonian here.
This city is not for the thin skinned.
FWD is a bad idea on a bicycle on anything besides dry pavement, IMHO.

 

[vmcroller]

Yeah, I generally try to avoid the wet & icy. My bike has an internal gear hub on the rear wheel & is a belt drive, so I'm kinda locked into FWD unfortunately.

 

[glennb]

I tend to skim read and thought I’d missed the part where you stated why you’re intending to use a front motor.

It’s worth pointing out that the only two failures that guarantee a crash are fork and stem.

Everything else is potentially recoverable from, meaning you can mess with everything else to varying degrees, but it’s totally inadvisable to compromise the integrity of these two structural components unless they’re over-engineered and you really know what you’re doing.

It’s easy to get carried away in the pursuit of solutions to problems, and lose sight of the safety implications of your modifications in the process.

Examples from someone who didn’t know better …

Example 1: I installed rivnuts in lightweight steel fork blades to mount bottle cages on. They held up fine for the journey across Europe, but it was still unwise to create stress risers in 1mm wall thickness steel tubing in a stressed structural member. (Luckily the right chainstay proved the weak link on that frame, cracking clean through in the bike’s next trip to New Zealand, before the fork had a chance to fail.)

Example 2: I drilled vertically through top and bottom of a lightweight aluminium stem, to create a cable housing stop for a cantilever brake. The reasoning was sound - it solved the two problems every other cable stop are guilty of, they being unacceptable flex, and high friction cable routing, particularly when used with a negative angled stem (for low bar position).

 

[gromike]

- riding home drunk and/or stoned

Ah, riding stoned isn't all that bad, but riding drunk is deadly.
Riding on LSD, though, can be quite interesting--if you manage to not get yourself crushed.

 

[vmcroller]

Yeahhhh, appreciate the perspective here.

"It’s easy to get carried away in the pursuit of solutions to problems,"... haha oooof, feeling seen here

I initially posted because I'm admittedly not knowledgeable enough to know what I don't know on the subject of swapping forks. Thanks for the explanation plus examples. So much of expertise is knowing where you can make modifications and/or cut corners vs. where *you definitively cannot*. Don't think I fully grasped how much my suspension fork idea would have been playing with fire.