Is a dropper post a good solution for sharing a bike between two riders?

Question

We have an e-bike with a step-through frame that's shared between two riders with fairly different heights (5'8" and 6'2"). It's used for commuting, kid-toting, and general short-mileage city riding. We both fit comfortably on the bike, but the seat height needs to be adjusted between each rider switch. This switching of riders often happens one or more times per day.

Adjusting the seat height is not difficult, but each time introduces error: the height isn't quite right, the seat ends up not being straight, the quick-release clamp isn't tightened enough and begins to slip.

Would a dropper post be a reasonable solution to this dilemma? Would it be possible to set it to two heights and then easily switch between them? Beyond the expense, are there disadvantages to this solution?

Answer 1

Using a dropper for this purpose is not a good solution. Droppers do not let you easily choose a set height other than all the way up or down, and most of them don't give you a quick visual way of measuring the current extension.

Use a normal seatpost and scribe on two marks to match the two riders' optimal seat height. If the QR isn't able to secure the post easily, get a better one. If there's some kind of poor tolerance situation happening with the frame that makes it difficult to secure the post from rotation, you could have a shop knurl the post such that there are two knurled sections that also mark the two optimal heights, i.e. such that it's right when the knurling for the position chosen is fully hidden.

Answer 2

To take the opposite argument to the most upvoted answer: using a dropper for that purpose is a good solution. It was in fact the most upvoted answer in this question. But it would not be convenient for more than 2 people: the extended dropper is positioned for the tallest rider, and some strong tape (or a proper stop, see comments) can be used as a "limit" for the position of the smaller rider.

It adds some maintenance requirements, but if it's an e-bike with a front suspension, some regular service is also required for the suspension (and the drivetrain), so the dropper can be serviced at the same time.

I would however recommend to take for that application a dropper that is actuated from the saddle and not from the handle bar: it avoids the mess of cable routing, and the temptation to use the dropper at traffic lights (very convenient), which might be not so good for the scotch and the dust cap.

If the bike is parked in grocery stores and other public places (where saddle theft can occur), the dropper post has also another advantage: you can replace the quick release by something else (standard hex key or anti theft collar), and limit this risk. And it would still be more convenient than a quick release.

Answer 3

I normally wouldn't make suggestions which go against Nathan's, but here, I do think a dropper is a good and workable idea. Using some of the ideas in this question about marking different saddle heights, you could devise a setup to reliably set the different saddle heights. And to be frank, for short-distance city riding and whatnot, having some inaccuracy in your saddle height probably won't be too much of an issue.

With a 6" (~150mm) height difference between you two, I'd guess the leg length difference is around half that (70-80mm). If you find a dropper with a similar amount of travel, the bike setup would be easy: fully extended is for rider A and fully compressed is for rider B. It may be possible to shim the post to adjust its travel or install a stop on the stanchion (as Renaud suggests) to accomplish a similar max-min function with otherwise longer droppers.

Of course, the downsides of adding a dropper are weight and complexity, but that's true of any dropper really. Considering that these days the expectation is that MTBs come with droppers, I don't think these two factors are much to worry about. We can also look at how pro road race neutral bikes come with droppers exactly to accommodate varying rider heights, but admittedly that's not entirely comparable to your situation.

Answer 4

Given you have a quick release and the same person uses the bike to and from the destination you could do this instead...

Have two seats and seat posts with a clamp tightened on the seatpost so that when it is placed into the tube the clamp stops the seat at the right height. Then you simply swap out the seat and close the quick release.

Answer 5

I'll offer different reasons than Nathan. You are adding a complex mechanical component to the bike. If you get a cheap dropper, it's possible that the post wears out, and that you can't replace or service the cartridge. Good droppers are expensive, usually in the range of US$250+. You can extend the usual maintenance intervals because you will not be using it in harsh conditions, but they will still benefit from maintenance.

I think that making a light mark on the seatpost and maybe using a good quick release seat clamp could be a better option.

Answer 6

I think a dropper is great for this purpose, as long as you get one with enough travel that it can go from fully extended for the tall rider to at or below pedaling height for the short rider.

I don’t think the various suggestions for stops in the other answers are necessary, as you can adjust dropper height easily for comfort and efficiency while in motion. I would even consider the addition of a low-limit stop detrimental, as it would preclude the short rider from using the dropper when stopped at intersections or mounting/dismounting (features I find highly useful on my dropper-equipped kid+cargo e-bike).

Answer 7

It's a situation I'm very familiar with; I frequently park my bike in not-so-great neighborhoods with a non-negligible risk of your saddle getting stolen. (And a guarantee of wheels getting stolen if you don't lock everything). So I take my saddle off multiple times per week, sometimes multiple times per day. I've never taken more than 3 seconds to put it back, and very rarely another 2-3 seconds at the next traffic light if I didn't get it right the first time. Here's how:

1. You can mark the height, either with a marker or just a cut with a knife. Even quicker is to measure the height by your body; my saddle height is just below my hipbone.

2. As for the straightness; you can get it roughly right by aligning the tip of the saddle with the frame, standing behind it with one eye closed if necessary. After you lock the quick-release clamp you can give it a few thumps with the palm of your hand for fine adjustments. Also works well at your next traffic light if you didn't eyeball it that well the first time.

3. As for the quick-release clamp; set it to the correct tightness and never change it again.

Answer: No, a dropper post is not a reasonable solution to this dilemma. A reasonable solution is to set your clamp to the correct tightness, and to learn how to adjust your saddle quickly and precisely in practice.

Beyond the expense, another disadvantage to a dropper is the added maintenance and possibly greater risk of theft. On the other hand, there are no clear advantages.

Answer 8

Every single good seatpost QR clamp has a tightness adjustment you can make with a torque wrench. I used to have a slipping seatpost on my Brompton when I bought it and weighed 110 kg.

Do this: clamp the QR tight, set the torque using a torque wrench to the correct value with the QR clamped. Once you have done this, you will note the QR lever is much firmer, requiring a lot of force to tighten the seatpost. Yet it should be possible to tighten it without superhuman force.

What is the correct torque, then? Depends on the seatpost clamp and the bolt size. On Bromptons it's 4-7 Nm, and I put it to 7 Nm to stop the slipping seatpost. At the factory, it was set to 4 Nm which was definitely too loose for a 110 kg person.

On this Brompton, the bolt is a M6 bolt. Usually M6 seatpost collar bolts are tightened to around 9 Nm so I suspect the Brompton could take even more torque on that bolt.

Using Futek bolt torque calculator, selecting metric, 6mm and custom material with 640 Nm proof strength (corresponding to 8.8 grade bolt which is probably the most common bolt grade you see on bikes), and mating thread as 9mm 6061T6 aluminum, it gives 9 Nm recommended torque.

If your bolt is M5, put it to 5 Nm (determine bolt size by using a vernier caliper). While this may sound too little if M6 can take 9 Nm torque, consider this: bolt torque does not hold the seatpost still, bolt tension does. And torque is proportional to tension and bolt size. So 9 Nm M6 bolt is the same tension as 5/6*9 Nm = 7.5 Nm M5 bolt. So 5 Nm is only 33% less tension than the tension in the M6 bolt.

Also if you find M5 bolt inadequate and your clamp has a M5 bolt, maybe you could find somewhere a QR clamp with M6 bolt. Just make sure the clamp diameter is the correct one for your frame.

I find that I can adjust the seat height on my Brompton by eye without any markings and get the seat straight enough not to bother me and at height near enough the correct one so the height error doesn't bother me either, without the seatpost height collar (which would prevent raising it above the set height, easily allowing setting one height for one rider only). Of course Bromptons are for short distances, if riding long distances then correct seat height is very important.

You may consider buying a seatpost with etched markings on it. Some seatposts have a marking scale on the side. If you can find a seatpost with a marking scale, long enough length, correct diameter then consider other properties of a good quality seatpost: the saddle should remain on the seatpost even if one bolt fails, so the saddle attachment should be a two-bolt attachment made in a way that one bolt failure does not cause your ass to drop on the spinning rear wheel.

Usually I recommend seatposts that are polished aluminum since they are less likely to fail due to fatigue than anodized aluminum. However, if the seatpost is slipping such as due to using a QR clamp, it may be useful to consider black anodized aluminum as it has a rougher surface. Yet that might not be necessary: my Brompton seat slips no more after torquing the bolt to the correct torque, and there the seatpost is polished aluminum.