Bicycle Mechanics - How much weight can a seatpost support?
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The Big Wheel
05-20-09, 03:52 PM
I already own a rear rack, however I have read that the seat post can only support 25lbs. The rack that I have now will be used for a battery for my bike that I am turning into an e-bike. The battery weighs 16 pounds, so I still have about 9 pounds to play with.
However, I want to mount two rear racks on my seatpost.
My plan is to buy this rear rack: http://www.amazon.com/Delta-Post-Porter-Pannier-Support/dp/B000ACAMEW/ref=cm_cr_pr_product_top
and these bags:
Since my bike is a soft tail I can't buy the regular racks that have the two tubes for support that go down to the wheels. I also can't mount the battery anywhere else.
So right now it looks like my only option is to take off the rear rack I have on right now and mount the above rack and bags.
1. Would be possible to mount two racks on the seatpost?
2. Can I buy a stronger seatpost that will carry more than 25 pounds?
3. I know delta claims 25 pounds, but has anyone actually put more than 25 pounds on their rack?
4. The 25 pound limit, is that per rack?
5. Or for the seatpost?
6. Is it possible to have 2 rear racks attached to the seatpost with 25 pounds each? (The most important question of all)
Here is the bike:
it all has to do with leverage.
those seat posts can carry a 200lbs rider no problem, but the weight is not sitting too far out on the 'arm'
with a rack, the 'arm' is extended very far off the body and has much more leverage to break the seatpost or body.
it's probably the rack, more specifically, the clamp, that's rated to 25lbs and takes into account hitting a curb or large pot hole which can put a lot of G-force on arms outside the body.
05-20-09, 04:15 PM
I think the weight limit is for the seat post and assumes the weight is central on the rack. Therefore, if you could mount the battery close up to the seatpost it would put a much smaller bending moment (weight X distance) into the post. That might allow you to carry 15 lb of baggage on the rack, so you should look for much smaller panniers. I have recently seen a link to baggage carriers that mount either side of the frame below the handle bars - do a search on these forums.
05-20-09, 05:30 PM
I suspect it's a restriction as much, if not more so, of the rack rather than the seat post. The strength of the seat post, however, is dependent on the material of the post, and the material thickness. The same holds true of the rack which sticks out horizontally and is a cantilever. In general heavier material is stronger. You might try tying a helium filled baloon to the far end of the rack.
I really doubt the weight limit has anything to do with the seat post. Rack limit is almost certainly imposed by the manufacturer and relates to the rack itself and/or its clamp. Fully circumferential clamp requiring
seat post removal to install, would be considerably stronger than one mounted with a separate plate, but
both would be more than strong enough for a 25# limit rack which applies to the rack itself. Likely you could exceed this by 25-50% before any real concern about breakage, which would not be the seat post.
ATB posts are rated at least 2x body weight, dynamically and the essentially static load of the rack will
not stress them at all. A bit puzzled how you would mount two racks- 45-60D angle between the racks?
Have to get used to mount/dismount by stepping through and much weight discrepancy on one side might
futz with ride dynamics.
05-20-09, 09:12 PM
One frequently reported problem with seatpost clamping type racks is that the rack tends to rotate to one side or the other when it carries a heavy load. This may be part of the weight restrictions listed for such racks. Also why they are considered a last resort solutionj to adding a rear rack by many.
I have never seen front pannier racks for suspension front forks.
BTW why choose a full suspension MTB for a motor conversion? IMO about like trying to convert a motocross motorcycle to a luggage hauler for street use, difficult to do and not too practical when finished.
The Big Wheel
05-21-09, 09:31 AM
Thanks for the information. I have decided to buy the rack listed in my original post and remove the old one. I will mount the battery on top of it, giving me about 6 pounds for storage (9 pounds really but minus the weight of the top rear bag and two pannies, actually I don't think the three bags are 3 pounds total).
6 pounds may not seem like much but I will mostly have clothes, gym shoes, etc. in those bags, plus like someone mentioned the 25lb limit probably can carry 20-25% more so that's 11-12 pounds of storage. I will be using this bike for commuting, not cross country trips.
BUT, if I was going cross country, I would get this for $80 at walmart, has a 100lb capacity. Good for another one or two batteries plus equipment. :)
05-21-09, 09:46 AM
are you planning to go off any really big jumps?????
you should be fine...
05-21-09, 01:01 PM
If I were building an e-bike, I'd do it with a rigid frame and put that battery in the front triangle. No way I want that much weight out back there.
05-21-09, 05:14 PM
I've got the same or similar Delta clamp-on rack, and I think it uses aluminum bolts. If so, that's what I'd worry about as a failure point. Replacing them with steel bolts can cause galvanic corrosion issues, so at a minimum use plenty of anti-seize.
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