Frame/rear rack flex with a load

Frame/rear rack flex with a load

Postby Tim » Sat Feb 16, 2013 5:59 pm

Do you guys experience any noticeable flex in the bike frame and/or racks whilst fully loaded?
I've found that with a large load of up to about 25kg's I can feel the frame, or more likely the rear racks, flex from side to side, especially climbing hills, out of the saddle. The rear load seems to "swing", only slightly, but enough to notice. This is most obvious with extra weight mounted up high in a rack-top-bag. If I time the out of saddle swing to the pedaling cadence, I form a sort of rhythm that seems to actually help with climbing. I can also feel a bit of frame twist and wobble, and shakes if I'm pushing the bike (on foot) up really steep hills, god that is hard work on loose ground.
All the rack mounts are correctly tightened so the flex is not caused by loose fittings.
I'm just interested in knowing if there is much difference between different steel compositions and frame construction methods and design in regard to the degree of flexibility.
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by BNA » Sat Feb 16, 2013 7:50 pm

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Re: Frame/rear rack flex with a load

Postby Tandem » Sat Feb 16, 2013 7:50 pm

Yes I had the same problem on my trip in Europe 2 years ago but only when I tried to stand up on some of the steep climbs.

By the time I finished the trip, climbing 55 Cols/Passes I had it worked out how to minimize the swinging/flexibility. When you try to keep your upper body as steady as possible that will minimize it a lot
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Re: Frame/rear rack flex with a load

Postby RonK » Sat Feb 16, 2013 8:07 pm

You are swinging quite a bit of weight, and it's carried high.
I don't find it at all practical to stand. I just pick a low gear, stay seated and grind it out.
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Re: Frame/rear rack flex with a load

Postby Tim » Sat Feb 16, 2013 8:34 pm

Thanks fellas.
It's not that I find the flex to be a problem. I actually enjoy the rhythmic swing.
I just wonder if it is to be expected and whether all heavily loaded bikes do it.
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Re: Frame/rear rack flex with a load

Postby il padrone » Sat Feb 16, 2013 8:45 pm

I sometimes do climb while standing for variety on long climbs, and at the limit for power. But it is a different style to standing on an un-loaded bike. I still spin and avoid a lot of swaying on the bars, just use my weight to turn the cranks harder. I'm only a light guy so this doesn't flex the frame too much.

Re. your frame flex - a few questions.
What sort of frame? Materials? What size?

What sort of pannier racks?

How heavy are you? Do you tend to grind or spin the gears?

Personally, with a Thorn Nomad steel frame and Tubus tubular steel racks I get no frame flex that I notice. I am a lightweight (60kgs) and prefer to spin (90-100 cadence), but I do at times carry a load of 25-30kgs and sometimes even more. A weight of 25kgs should not cause excessive frame flex for a decent quality touring bike.
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Re: Frame/rear rack flex with a load

Postby Tim » Sat Feb 16, 2013 9:01 pm

I only weigh in at 63 kg's and generally prefer high speed spinning rather than grinding big gears. To relieve minor aches and pains though I frequently, if not always, ride out of the saddle up hills, at least for part of the climb. I find it easier and gentler on my knees than remaining seated and grinding.
The Vivente frame is 4130 chro-molly, not top of the line I realise, and a medium size.
The racks are Tubus. Duo on the front and Logo behind.
I wouldn't say that the flex is excessive but is nonetheless noticeable. So far I have had no problems as far as handling goes. There is no high speed wobble or shimmy. Top speed has been 63KPH but I have also ridden at 50KPH on a bumpy dirt rail trail and the bike has felt as solid as a rock.
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Re: Frame/rear rack flex with a load

Postby clackers » Sun Feb 17, 2013 6:05 am

Try redistributing some of your load into front panniers, Tim. Resonance (different for every frame/weight/cadence combination) would seem to be what you're experiencing.

Another example of this phenomenon is the so-called 'Death Wobble' in a descent.
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