Weight on a Brompton seatpost

[Notsototalnewbie]

Is significant weight on a Brompton seatpost a really bad idea considering the length of it?

The dog has a ‘Buddy Rider’ seat that clamps to a seatpost under the saddle. Combined weight 12kg.

I have other standard bikes to use it with but the Brompton is most useful where trains are involved (many of my trains go via Gatwick and you know what that means).

I’m prepared to risk a bent seatpost for the sake of the experiment - the Brom is not new, and if that happened I’d replace it and accept it was a no go. I’m less prepared to risk a catastrophic failure or frame damage though. There’s photos and videos of people using the combo online but it doesn’t necessarily mean it was a good idea…

Those of you who know me will know I don’t do rough stuff.

[jiberjaber]

I think other components would be at risk before you managed to bend a standard Brompton seat post!

You don't say what the all up load will be.  I think the Brompton has an all up weight limit of 110kg https://us.brompton.com/support/knowledge/what-is-the-recommended-maximum-rider-weight-and-the-recommended-maximum-load

So if you weigh less than 98kg then happy days

You might want to consider the wear on the pivot joint, seat post sleave, seat post rails, and damper in that order before seat post

Have you got the Firm damper on it already? If not you might want it with the extra load.

Others with longer Brompton service may have more of an idea...   

[Notsototalnewbie]

Apologies I should have stuck a link in to show the setup

https://buddyrider.co/

My weight is hovering around 80kg these days. Occasionally I put the full 10kg on the front if I’m camping.

I’m fairly sure I specced the firm damper when I got it in 2011.

[Jurek]

Would something like this

be useful as a starting point for an adaptation to take your doggy-carrier, whilst putting less stress on your seat post ?
ETA - Looking at that, it looks like you can fold the bike without removing it - albeit you wouldn't be able to drop the seat post .

[LittleWheelsandBig]

The front of the babyseat beam lifts off the B's main tube and sits beside the folded bike.

[drossall]

I would worry less about bending the seat post than about the leverage on the frame's seat tube. That's a long post, giving a lot of leverage. That said, people go touring on Bromptons with some seriously impressive loads attached to the same seat post.

[LittleWheelsandBig]

If you are currently using the standard seatpost, the extended seatpost uses a thicker tube wall, so is harder to bend when not at full extension.

[Jurek]

NSTN is taller than I am, and I use a standard seat post at full extension - actually, it's a Brompification Ti post - would that be the same length as a standard Brommy one?

[Notsototalnewbie]

I’ve got the extended seat post (though don’t use it quite at full extension).

Jurek, I found that babyseat during my research  . If I had the means/skills to bodge something like that together (ie stick such a leg on the Buddy Rider) it would absolutely work, but I wouldn’t know where to begin.

[Jurek]

I’ve got the extended seat post (though don’t use it quite at full extension).

Jurek, I found that babyseat during my research  . If I had the means/skills to bodge something like that together (ie stick such a leg on the Buddy Rider) it would absolutely work, but I wouldn’t know where to begin.

Depending on what it looks like, I could probably sort something out to make it work for you.
The downside is that you'd need to buy both - baby & dog (chair).
ETA - if you decide on this and you buy both, I'm happy to pop down to your's and check it out to see if I can make it work - before you send both back to the vendors in the event that it doesn't.

[LittleWheelsandBig]

If I understand it correctly, the Buddy Rider dog seat is cantilevered forward of the seatpost and tends to bend the seatpost towards the handlebar. The main load on the seatpost is from the rider themselves, cantilevered backwards from the seat tube clamp and bending the seatpost away from the handlebar.

The Buddy Rider dog seat would actually reduce the bending moment at the base of the seat post. If the total weight of you, your dog and the dog's seat is below 110kg, you are probably good to go exactly as is.

[Jurek]

If I understand it, the Buddy Rider dog seat is cantilevered forward of the seatpost and tends to bend the seatpost towards the handlebar. The main load on the seatpost is from the rider themselves, cantilevered backwards from the seat tube clamp and bending the seatpost away from the handlebar.

The Buddy Rider dog seat would actually reduce the bending moment at the base of the seat post. If the total weight of you, your dog and the dog's seat is below 110kg, you are probably good to go exactly as is.

That is sound mechanical reasoning.

[citoyen]

That is sound mechanical reasoning.

Two people on the internet talking sense and agreeing with each other - whatever next?

[Notsototalnewbie]

Jurek that is super kind thank you. Regardless,  we should meet up soon, it has been too long and I’m sure dinner is in order.

I do like the sound of LWAB’s reasoning though.

[Kim]

That is sound mechanical reasoning.

Two people on the internet talking sense and agreeing with each other - whatever next?

They go for a bike ride?

[Notsototalnewbie]

Took her out for brief bike ride before the horrible heat hits. Main issue is it wants to spin off slightly to one side, nothing a bit of old inner tube won’t sort as I had bolted the ‘receiver’ on pretty tight.