Seat post clamp

[pepecat]

So , my n+1 - though it's actually an n+1/2 an excuse for a bike......
Anyway, it's rather old (80s design?) and some bright spark at peugot decided to make the seat post clamp the same bolt as the one that tightens and keeps the rear cantilever brake in place. Wonderful.
I put a new seat and seat post on the bike, cos the original seat post was about 10cm long (seriously) and not long enough to raise the seat to my height. The last two times i've ridden the bike, I've raised the set to my height, made sure the back brake is in the right place and tightened the bolt as hard as i can. If i don't adjust the back brake properly, it doesn't work....it gets too tight and you can't pull the lever.
And off i go on my bike.......and as i ride, something looses the bolt, and my seat gets gradually lower and lower.....and then moves from side to side with pretty much every push of the legs (left right left right....seat left right left right)....

Is it use of the back brake that is loosening the bolt? And is there any way i can separate the dual action bolt to leave the seat alone?

 

[2Loose]

Do you have a picture?

I don't think I have heard of this, unless you have cantilever brakes and the hanger is attached to the bolt...

 

[Angelfishsolo]

I have seen this setup before but without a picture to refresh my memory can not suggest a solution

So , my n+1 - though it's actually an n+1/2 an excuse for a bike......
Anyway, it's rather old (80s design?) and some bright spark at peugot decided to make the seat post clamp the same bolt as the one that tightens and keeps the rear cantilever brake in place. Wonderful.
I put a new seat and seat post on the bike, cos the original seat post was about 10cm long (seriously) and not long enough to raise the seat to my height. The last two times i've ridden the bike, I've raised the set to my height, made sure the back brake is in the right place and tightened the bolt as hard as i can. If i don't adjust the back brake properly, it doesn't work....it gets too tight and you can't pull the lever.
And off i go on my bike.......and as i ride, something looses the bolt, and my seat gets gradually lower and lower.....and then moves from side to side with pretty much every push of the legs (left right left right....seat left right left right)....

Is it use of the back brake that is loosening the bolt? And is there any way i can separate the dual action bolt to leave the seat alone?

 

[PpPete]

I have this arrangement on several of the families stable of bikes (which have older cantilever brakes). It can indeed be a pain in the a$$. You have to hold the brake cable hanger firmly in the correct alignment whilst tightening the seat post bolt (usually an allen key type)
I've never come across brake action loosening the bolt though. If the hanger alignment is correct then there should be no turning moment on it.

Two possible causes
a) - bolt not tight enough. A regular length allen key is barely long enough to tighten these bolts sufficiently. Get a longer allen key or really really tighten it hard as you can with the short one. I'm assuming it's a steel frame ? Don't get "heavy" with it if it's aluminimum...
2) new seat post is wrong diameter. Even 0.2 diameter difference can be enough to cause this. Because of rust and corrosion and previous abuse it might be difficult to get the correct size seat post in... but going down a size is not the answer. Turning the bike upside down in a workstand and cleaning the inside of the seatpost with a scotchbrite pad taped around a suitable diameter rod is a good starting point if that's the case. Then grease the correct diameter post thoroughly before inserting.

If you are absolutely 100% positive you've got right diameter seat post, and the seat post bolt is OK (not rounded off & thread OK) then one further thing you can try is carbon assembly paste. This is like a grease which increases friction between two components. Clean the post and the tube, and apply sparingly to both before putting seat post back in. Don't get any of it on the bolt threads though.

 

[pepecat]

It is indeed an old style cantilever brake, and yes, I have to hold the brake cable hanger (and brake cable to the lever on the handlebars) in the correct place whilst i tighten the seat post clamp. If the cable drops too much doing this, the back brake wont pull.
It's not an allen key bolt, its a spanner bolt, and i tighten it as best i can, but even so. I can give it some wellie cos it's a steel frame.

I think it may be a seat post issue - i got the seat post from decathlon - its one which has an insert which caters for various sizes of of seat post - mine is 254mm and i can't seem to get them anywhere -guess its not a standard size any more. Nor do i want to spend silly amounts of money on it cos it's my n+ cheap crappy thing.

I can understand the seat going down into the post....but i'd have thought it would have gotten tighter and therefore stuck, not loose and moving from side to side.

Will see if i can get a picture uploaded......

 

[PpPete]

might be worth getting a new seat post bolt though....the ones with the hex bolt are cr@p. usually only a couple of £ at LBS