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Is this safe?

Matt.KMatt.K Posts: 105
edited September 2009 in Commuting chat
After getting fed up with sweaty back from wearing a rucksack for the commute I decided to invest in one of these to use with my rack pack from the old hybrid:
http://www.wiggle.co.uk/p/cycle/7/LifeLine_Alloy_Seat_Post_Rack/5360033789/

But when attached to the seatpost it is ridiculously high up and the rack has to be set far back to avoid the saddle.

So to make it look more normal I fed it through the stays and secured it on the frame instead of the seatpost.
But I'm worried that this could be bad for the frame in the long run and don't want it to snap.
The bag weighs about 4kg plus 1kg for the rack, and the max stated load is 10kg so I'm well within the limits that way.

Can anyone reassure me, or tell me I'm stupid for doing this? Either way.

Pics to explain what I mean:

2009-09-29%2017.50.03.jpg

2009-09-29%2017.50.11.jpg


Oh, and I know I need to move my reflector!

Thanks,
Matt

Posts

  • Is it butted tubing, and does the seat post extend that far down the seat tube?

    The only problem I could imagine is that the frame tubing is going to be much thinner than that of the seat post and there is quite a bit of leverage around the rack clamp.

    If its reinforced by the seatpost at that point I would have thought it would be okay.
  • redveeredvee Posts: 11,921
    I've seen it on the other bikes. The only issue I could see is crushing of the seattube through too much pressure being applied to keep the rack clamp tight. The only solution is to use a loooong seatpost that extends down as far as the clamp.
    I've added a signature to prove it is still possible.
  • +1 on the seatpost, its be most dangerous if the rack fixing was at the bottom point of the seatpost inside the frame - it'd flex unevenly it could be worth measuring how far the post actually is in the frame.

    You could also put a bit of a brace around the rack central bar and up round the seat rails to take some of the weight and bounce off the frame tube.
  • bolbol Posts: 138
    Lots of kids' seats attach like this, and they're often a lot heavier.
  • elcanielcani Posts: 280
    I asked the exact same question about the same product a while ago and the general view seemed to be that it wasn't a good idea.

    http://www.bikeradar.com/forum/viewtopi ... +seat+tube

    Cheers
  • That's terrible, I expect the tube to crumple with the first big bump.

    You could try mounting the rack on the (much stronger) seat post, and suspend the bag beneath the rack with bungees. This would keep your bag low and protect it against shock.
  • snailracer wrote:
    That's terrible, I expect the tube to crumple with the first big bump.

    You could try mounting the rack on the (much stronger) seat post, and suspend the bag beneath the rack with bungees. This would keep your bag low and protect it against shock.

    Pessimistically, your bag looks a bit too big to try what I suggested :(
  • FlasheartFlasheart Posts: 1,278
    Nap D will be after you...no white garage door behind the bike dammit :roll:
    The universal aptitude for ineptitude makes any human accomplishment an incredible miracle. ...Stapp’s Ironical Paradox Law
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  • bol wrote:
    Lots of kids' seats attach like this, and they're often a lot heavier.

    Non-one has yet responded to this point; personally I've used child seats in this manner on a few bikes on bumpy roads and I've never yet encountered a problem.

    Have I just been lucky? I suspect not since this is how the seats are designed but I'd be interested in opinions.
  • bol wrote:
    Lots of kids' seats attach like this, and they're often a lot heavier.

    Non-one has yet responded to this point; personally I've used child seats in this manner on a few bikes on bumpy roads and I've never yet encountered a problem.

    Have I just been lucky? I suspect not since this is how the seats are designed but I'd be interested in opinions.

    Child seats are usually mounted on a very springy arm, which helps dissipate the shock of hitting bumps before it reaches the clamp. A rack is much less springy.
  • PBoPBo Posts: 2,493
    bol wrote:
    Lots of kids' seats attach like this, and they're often a lot heavier.

    Non-one has yet responded to this point; personally I've used child seats in this manner on a few bikes on bumpy roads and I've never yet encountered a problem.

    Have I just been lucky? I suspect not since this is how the seats are designed but I'd be interested in opinions.

    Yes, kids seats go up to 25kg, and are attached like this....however, my fixing bracket is much bigger - maybe spreads the load somewhat? (plus springy arms)

    but, I have to be brutally honest and say that my gut instinct about 5kgs causing serious damage when going over a bump is ---bollox....the frame's made of metal not paper ffs
  • snailracer wrote:
    bol wrote:
    Lots of kids' seats attach like this, and they're often a lot heavier.

    Non-one has yet responded to this point; personally I've used child seats in this manner on a few bikes on bumpy roads and I've never yet encountered a problem.

    Have I just been lucky? I suspect not since this is how the seats are designed but I'd be interested in opinions.

    Child seats are usually mounted on a very springy arm, which helps dissipate the shock of hitting bumps before it reaches the clamp. A rack is much less springy.

    True but a child is much heavier so that would be balanced out wouldn't it? I think PBo's comment about spreading the load is also valid but tbh I think bikes are stronger than you're giving them credit for.
  • AndyMancAndyManc Posts: 1,393
    Is there any reason why you don't want a 'proper' rear rack ?

    I was reluctant to get a traditional rack and , like you, bought a couple of seat post racks, whilst they do the job (but often destabilise your bike) they are very limiting.

    Rear carriers are very lightweight and offer far more options, you’ll end up keeping it fixed on all the time and you won't notice it's there.

    .
    Specialized Hardrock Pro/Trek FX 7.3 Hybrid/Specialized Enduro/Specialized Tri-Cross Sport
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  • Matt.KMatt.K Posts: 105
    Interesting range of responses.

    Snail racer - I think I may try hanging it upside down as the main post can still be angled up and I can flip the rack on it. But then I get scared that I could be riding along with all my stuff falling out behind me!

    AndyManc - The bike doesn't have any mounts for racks. I could try some p-clips and see if I can get one to attach that way, but I like the ability to flip a lever and take the whole lot in to the office with me instead of fannying on with the velcro straps. I also like the idea that when it's raining I can slip a plastic bag over the whole lot to waterproof it.

    Oh, and I'm not on the bike today, but I'll check the seatpost length when I get home.
  • PBo wrote:
    bol wrote:
    Lots of kids' seats attach like this, and they're often a lot heavier.

    Non-one has yet responded to this point; personally I've used child seats in this manner on a few bikes on bumpy roads and I've never yet encountered a problem.

    Have I just been lucky? I suspect not since this is how the seats are designed but I'd be interested in opinions.

    Yes, kids seats go up to 25kg, and are attached like this....however, my fixing bracket is much bigger - maybe spreads the load somewhat? (plus springy arms)

    but, I have to be brutally honest and say that my gut instinct about 5kgs causing serious damage when going over a bump is ---bollox....the frame's made of metal not paper ffs

    I agree - compared to a child seat the forces on that bracket will be tiny (unless you're in the habit of carrying half a dozen bricks to work) even if you take a bouncy route.

    I might be nervous about doing it on a carbon frame but a steel or alu frame will have no trouble at all with this.
  • El Gordo wrote:
    PBo wrote:
    bol wrote:
    Lots of kids' seats attach like this, and they're often a lot heavier.

    Non-one has yet responded to this point; personally I've used child seats in this manner on a few bikes on bumpy roads and I've never yet encountered a problem.

    Have I just been lucky? I suspect not since this is how the seats are designed but I'd be interested in opinions.

    Yes, kids seats go up to 25kg, and are attached like this....however, my fixing bracket is much bigger - maybe spreads the load somewhat? (plus springy arms)

    but, I have to be brutally honest and say that my gut instinct about 5kgs causing serious damage when going over a bump is ---bollox....the frame's made of metal not paper ffs

    I agree - compared to a child seat the forces on that bracket will be tiny (unless you're in the habit of carrying half a dozen bricks to work) even if you take a bouncy route.

    I might be nervous about doing it on a carbon frame but a steel or alu frame will have no trouble at all with this.

    "Quality" steel seat tubes are as thin as 0.6mm. Ironically, you might be better off with a cheap drain-pipe steel bike frame for this application.

    UPDATE for the 21st century:

    "Today road bikes are commonly 1.125 top tube & seat tube, 1.25" down tubes. Wall thickness of .65x.4mm, .7x.4mm, .8x.5mm, .9x.6mm are in use. Tensile strengths of 170000 to over 200000 are now available. I doubt steel tubing will ever get thinner because of the "tin can effect", denting and buckling."

    Source: Tom Teesdale Custom Bicycles
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