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Sweep bars - Danger!

1778 Views 11 Replies 10 Participants Last post by rockcrusher, Nov 2, 2021

telemike

Discussion Starter · Nov 1, 2021

Two weeks ago, I posted a thread about losing focus on an easy trail. I was mounting a boardwalk with a 10" or so step up. I preloaded and pulled the bars up to put the front wheel on the boardwalk and let the rear tire do it's thing. The front wheel stopped dead and I went over the bars suffering a relatively serious injury to my hip, knee, and shoulder that are all clearing up admirably. Abrasions are mostly still in bandaid mode. Getting back on the bike after the fall, I noticed that my bars had be rotated upwards about 40 degrees in the crash.

Lesson: Keep the focus!

Wrong, at least as the cause of this crash. Thinking about the rotated bars, I went to the shop and yanked them hard and they rotated again about 40 degrees. The stem bolts were torqued to the required 5nm but the bar still rotated. It rotated because the swept back grips gave me lots of leverage to rotate the bars that is not there in a straighter bar. So, when I pulled up on the bars to lift the front wheel, the bars rotated and the front wheel stayed pretty much where it was leading to over the bars and injury.

The bars are a Surly Terminal bar which has a lot of sweep back and was not a good choice for the bike but was free. 35 degrees of sweep is too much and I am looking for a bar with about a 14 - 18 degree sweep which seems to be my natural angle.

So, if you are going to use swept back bars, make sure the bolts are torqued to the stem manufacturer's specs and use friction paste. I still believe that the correct amount of sweep is much better on wrists, hands, and elbows and will use a lesser swept bar.

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PhillipJ

· #2 · Nov 1, 2021 (Edited)

Lesson: Don't yank on the bars to lift your front wheel.

There must be more going on here. Look at the rise on BMX bars. Those riders are putting huge forces on the bike and not rotating the bars.

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telemike

Discussion Starter · #3 · Nov 1, 2021

PhillipJ said:
Lesson: Don't yank on the bars to lift your front wheel.

Well, that's a hell of a way to learn that lesson! No permanent damage.

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mikesee

· #4 · Nov 1, 2021

I've been riding 20-30* swept bars a few days a week for more than a decade. Haven't had this happen.

Instead of laying a blanket blame on the bars, perhaps y'ought check the stem to see if it's in spec, and then do the same with the torque wrench.

And then measure the bar clamp diameter.

Lots of possibilities. But not using swept bars, is -- for me -- not one.

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steadite

· #5 · Nov 2, 2021 (Edited)

One thing about bolted joints is that, except in extremely well-controlled situations, the relationship between torque and clampload on bolted fasteners is TERRIBLE; there’s a huge amount of variation in clampload for a specified torque. The variation is especially bad when the threads/head are unlubricated and dirty. Pre-applied thread locker is another source of variation. The threads in your stem might not be well-formed and would increase friction…there are a number of possibilities that may explain low clampload.

I guess all I’m saying is to take the torque specifications with a grain of salt. You may want to go past it a bit, and you may need carbon paste to add friction if you have an especially long lever arm.

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justin70

· #6 · Nov 2, 2021

carbon bars or aluminum?

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749800

· #7 · Nov 2, 2021

I've got Jones bars on a couple of bikes, so the lever arms are exactly where you wouldn't want them in this kind of scenario.

Thanks for the heads-up. Fortunately, they are aluminum so I can torque the bajeezus out of them.

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749800

· #8 · Nov 2, 2021

Next up, adjustable stems...

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kapusta

· #9 · Nov 2, 2021

justin70 said:
carbon bars or aluminum?

The bar is by Surly, so…. definitely not CF.

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Reactions: 1

Hodo

· #10 · Nov 2, 2021

mikesee said:
I've been riding 20-30* swept bars a few days a week for more than a decade. Haven't had this happen.

Instead of laying a blanket blame on the bars, perhaps y'ought check the stem to see if it's in spec, and then do the same with the torque wrench.

And then measure the bar clamp diameter.

Lots of possibilities. But not using swept bars, is -- for me -- not one.

Have to agree with Mike. It's not the bars. Mine get pulled on yanked on, and tumble down hills, they have never rotated. I've bent the sh*t out of them and had to replace, but never rotated.

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jeremy3220

· #11 · Nov 2, 2021

PhillipJ said:
There must be more going on here. Look at the rise on BMX bars. Those riders are putting huge forces on the bike and not rotating the bars.

They do rotate though. That's why the torque spec in the bmx world is usually "as hard as you can" with a normal size allen key.

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rockcrusher

· #12 · Nov 2, 2021

I weigh a lot, my bike is rigid and has been rigid for at least a decade now, I rode rocky AZ trails and now ride PNW stunt and jump filled trails and have used high sweep (22° and higher) all that time and have never had a bar rotate. I have crashed, jumped, dropped, descended, ascended, stopped quick, nose wheelied, been trialsy, and rested my bike upside down on my bars.

All with a standard 4 bolt stem that I tighten to spec. My current bar, a jones even uses an adapter to get it to the size of that stem adding another surface to the joint.

This sounds like a case of too loose bars vs. high sweep. I second the recommendation of torqueing your stem with the bolts lubed. Also ensuring that your stem bar interface has been cleaned with alcohol and making sure that your stem/bar interface is round on both components, as a last resort a friction paste could help if all else doesn't.

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