A friend and I have been operating a bike rental shop for about a year, and just recently we've started to notice more and more customers extending the seatpost well beyond the maximum line. So far we haven't had any frame issues, just bent seatposts, but we're trying to get ahead of the problem before it costs us too much.

We've discussed assessing fines for this, as we do stress ahead of time not to overextend the seatpost, but we would like to explore some preventative measures. Are there any devices or solutions out there that would prevent someone from extending a seatpost too far?

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    I'll get right to what's alluded to in the one answer so far: why are they renting bikes that are too small? Commented Mar 6, 2021 at 15:40
  • 16
    send out the correct size bikes ffs!
    – Noise
    Commented Mar 6, 2021 at 16:30
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    Your bikes are way too small.
    – Carel
    Commented Mar 6, 2021 at 20:20
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    If I rocked up and asked for a 62+ cm frame, could you accommodate me? If not, what's the largest frame size you can supply, hypothetically?
    – Criggie
    Commented Mar 6, 2021 at 21:16
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    This question is missing the big point: WHY are the customers doing this? Once you can answer that question, you can change the business model to match the needs of your customers. Commented Mar 7, 2021 at 17:40

4 Answers 4


I assume they are extending the seat post that far because they need it for an appropriate saddle height.

So the solution is simple: Offer bikes with larger frame sizes or at the very least equip the bikes with long seatposts which can be extended far enough.

Any other solution would force your customers to ride with a too low saddle.

If you still want to make sure that they can’t be extended too far, just in case a giant with long legs comes along, there is a solution I can think of: If you have seatposts where you can see through the tube (common on cheap steel seatposts) you could try to attach a string (or steel cable) somewhere at the bottom bracket, thread it through the seat tube and seatpost and tie it to the saddle rails. Could lead to some noise though if the cable rattles inside the seat tube.

  • The rental scheme near work has a stop on the seat post, but those bikes are custom built
    – Chris H
    Commented Mar 6, 2021 at 14:48
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    Could possibly work with an external steel cable connecting the seat cradle to (e.g.) crossbar or rear stays. Something like electrical cable might be a good choice to minimise abrasion - but use proper crimped swages to close the loops, not electric terminal blocks! Commented Mar 7, 2021 at 15:03

If these are beach cruisers or similar, they'll typically have 22.2 or 25.4mm seatposts (7/8" or 1"). If so, putting longer seatposts in all of them would be cheap. This isn't scummy if what you're dealing with is a bunch of seatposts that are just too short for a one-size bike.

If you haven't already, make sure you're hooked up with one of the wholesalers that cater to your type of business. In the US, the behemoth is JBI, but there are others.

Note this only applies if the kind of bikes you're renting are some kind of casual, one- or two-size thing. If we're talking about more serious bikes then yes, the answer is stick to appropriate sized bikes.


I think the best solution would be to use a seat collar with a security bolt and then pre-adjust the seat height when they check the bike out. Perhaps have some seats on the ready with extra long seat posts for people who need a higher saddle height.

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    I think that would be a terrible solution, I'd hate to have a wrong saddle height and not being able to fix it.
    – Pelle
    Commented Mar 8, 2021 at 8:35

Nothing on a rental bike should be removable or adjustable by the renter. Everything including wheels and seat posts should at the very least need a wrench to remove/adjust, and ideally should need a special tool with a security fastener.

Yes, that means you'll need to adjust the seat post height when you send the bike out. That shouldn't take more than a couple of minutes per bike, and if you can maintain a bike then you can surely manage that. And if you don't have a bike that fits them, you don't (can't) rent them a bike.

This also protects you against theft of parts. With quick releases on wheels and seat posts, thefts of bike parts is common almost everywhere, and if you've not yet lost anything then you've been extremely lucky.

Of course preventing renters from fixing a puncture might be inconvenient for them. But you can't trust your renters to know how to fix a puncture without causing damage; and for cycling around town, having to take the bike back to be fixed is only a minor inconvenience at most. This would be different if you're renting mountain bikes for long treks, of course, but then you're talking a very different target market, and usually a trek like that has a guide/leader who you work with anyway.

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    I don't think it's viable to stop people fixing punctures if they're going to be more than a few minutes walk from the bike shop. A typical tourist use is surely to see the sights outside a town, or the far side of town. I've rented basic hybrids from shops that have offered a puncture kit, for example. BTW the shop had a decent range of sizes - I'm XL or even 2XL and the bike I rented fitted better than the one I owned at the time.
    – Chris H
    Commented Mar 7, 2021 at 13:01
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    Taking the bike back to be fixed may be more than just an inconvenience. For example, I traversed the Outer Hebrides with a couple of friends who both rented tourers near home and took them on the train to Oban. When one of them got a puncture, it was 5-10 minutes to fix, but would have taken at least a day in each direction to return to the shop, not to mention the cost of the train and ferry tickets and a taxi to get from the puncture site to the ferry slip. Commented Mar 7, 2021 at 14:52
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    Typical cycle rentals in the Netherlands do allow the people who rent to adjust the seat height. (And we have a lot of those shops.) As it is almost impossible to set a seat to the right height before you have ridden on the bike and you may need to adjust after a short while. (And too often when the seat is adjusted the post is not secured right to the seat drops in use, which is an other good reason to let the renter adjust.) Also our typical renters do not ride around in town, unless they are foreign tourists, but quite long day trips. For patching tubes you do not need to take out wheels.
    – Willeke
    Commented Mar 7, 2021 at 19:26
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    This wasn't off the beaten track - mostly on A-roads. But anyway, nothing in the question says what kind of market they are in. So it seems you've made an unwarranted assumption. Commented Mar 7, 2021 at 20:53
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    @Graham maybe so, I've seen idiots too. If I was planning to rent I'd probably have my own tools anyway. But I'd refuse to rent from somewhere that had a policy of risking stranding their riders, unless they have a 24hr mechanic on call at no cost - and even that might be no good in many areas where you might rent a hybrid to go into the countryside for a picnic; the bike paths can be a long way from the road.
    – Chris H
    Commented Mar 7, 2021 at 21:08

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