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Old 08-07-06, 06:05 AM   #1
Jacob J
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Shearing strength?

I've modified a baby seat and changed the vetical supports to use the screws on the back of my Cannondale T800 that were for the rack. The baby seat is rated for a max 40 pounds, so my question is am I putting to much weight on those srews? I am clueless as to what those racks are rated for weight wise.

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Jacob
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Old 08-07-06, 11:33 AM   #2
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Originally Posted by Jacob J
I've modified a baby seat and changed the vetical supports to use the screws on the back of my Cannondale T800 that were for the rack. The baby seat is rated for a max 40 pounds, so my question is am I putting to much weight on those srews? I am clueless as to what those racks are rated for weight wise.

Thanks,
Jacob
Typical rear racks are weighted to carry 25-40 pounds, however most of the weight is supported by the dropouts rather than by the screws near the seat post. I am slightly confused about how you have this hooked up... is the baby seat ENTIRELY supported by the screws near the seat post?
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Old 08-07-06, 11:49 AM   #3
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Quote:
Originally Posted by moxfyre
Typical rear racks are weighted to carry 25-40 pounds, however most of the weight is supported by the dropouts rather than by the screws near the seat post. I am slightly confused about how you have this hooked up... is the baby seat ENTIRELY supported by the screws near the seat post?

moxfyre, The baby seat clamps around the seat post and originally had two "legs" that clamped on to the rear of the frame. http://us.st11.yimg.com/us.st.yimg.c..._1909_22405039 The problem with this arangement was the frame tubing being aluminun is to large in diameter for the "leg" clamps to work. So I fashioned a new set of "legs" that attaches to the scews down by the dropouts. I hope this isn't too confusing!

Thanks for the info Mothra and moxfyre,
Jacob
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Old 08-07-06, 11:52 AM   #4
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Baby seats are very dangerous. I used to work at an LBS where the owner would not sell them. Trailer's are much safer.
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Old 08-07-06, 12:49 PM   #5
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I would be concerned about side to side wobble with such, and as Rensho says the baby is at more risk with this setup than with a trailer in the event of a spill. As to the screw I assume a standard 5mmx0.8
screw? The fail weight in shear for such a screw, typical cheap type is going to be well above 400# so you are likely safe from that point of view. Subbing a grade 8.8 socket head screw would nearly double the shear strength.
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Old 08-08-06, 06:08 AM   #6
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Thanks everyone!
Jacob
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Old 08-08-06, 07:20 AM   #7
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One other thought: aluminum dropouts may not allow you tighten the screws enough before they strip out the threads in the hole.

One solution is to use longer screws and thread them through the dropouts from the inside, effectly creating a stud sticking out. Slide the babyseat arm onto the stud (screw) and secure with a washer and nut on the outside.

This way you have all-steel fastening, eliminating the weak link (aluminum).
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Old 08-08-06, 10:54 AM   #8
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One other thought: aluminum dropouts may not allow you tighten the screws enough before they strip out the threads in the hole.

One solution is to use longer screws and thread them through the dropouts from the inside, effectly creating a stud sticking out. Slide the babyseat arm onto the stud (screw) and secure with a washer and nut on the outside.

This way you have all-steel fastening, eliminating the weak link (aluminum).

Nice thought, I'll take a look at that posability.

thanks,
Jacob
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