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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?

Ted Hohl
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segfault
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    Sounds like a perfect application of the hiterite dropper post from yesteryear. https://mbaction.com/sept-hardtales/ Clamps around seat post limits lower height, set the spring to the correct height for the taller rider. Just need to find a very rare 1980's bicycle part... – mattnz Jul 07 '23 at 07:03
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    easier than I thought.... https://www.ebay.com/itm/183231146016 – mattnz Jul 07 '23 at 07:04

8 Answers8

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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.

Nathan Knutson
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    Might be worth adding that a quality quick release seat clamp could help with this. It does have to be a good one, imo, otherwise it might not have enough clamping force. – Weiwen Ng Jul 06 '23 at 17:33
  • Carbon paste on the post helps a lot as you can use quite low pressure on the QR and still have a stable seat post. – mattnz Jul 07 '23 at 06:54
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    Getting the saddle perfectly straight is always a hassle. For me that’s the strongest argument against just marking the two positions and calling it good enough. There has to be a better way to at least prevent sideways twist of the seatpost. – Michael Jul 07 '23 at 07:21
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    @Michael Just a matter of getting it perfectly straight just once and then making a vertical mark on the seat post and tube. Aligning those two lines every time you adjust the height won't take you more than a second. – TooTea Jul 07 '23 at 10:37
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    @TooTea: I’ve tried that but it wasn’t precise enough. – Michael Jul 07 '23 at 11:34
  • Height adjustment depends on the dropper. Mine has intermediate steps along its travel and others have infinite adjust. Not all droppers are binary up/down – Warren Burton Jul 07 '23 at 15:04
  • @WarrenBurton The point is that even with an infinite adjust post, it can be difficult to reliably achieve the same height (by eyeballing the amount of extension) unless that height is full extension/compression. Same goes for a stepped dropper with many steps. I imagine it may be easier for stepped droppers with like <5 steps total, but at that point you might as well just get a shorter dropper and use the full extension/compression positions. – MaplePanda Jul 07 '23 at 20:03
  • @MaplePanda I don’t think it’s necessary with a dropper to adjust the saddle height by eye while off the bike. One of the beauties of a dropper is that you can adjust it based on feel while riding. – RLH Jul 09 '23 at 17:42
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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.

Rеnаud
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    The question doesn't give any argument, so how is this answer "the opposite argument"? Did you mean to oppose one of the other answers? If so, best to say which one (perhaps using a link). – Toby Speight Jul 07 '23 at 13:37
  • You can get a consistent stopping spot for a dropper post with something like the Wolf Tooth Components Valais 25: https://www.wolftoothcomponents.com/products/valais-25?_pos=2&_sid=3d3a1b250&_ss=r without worrying about damaging the post. – Soupy Jul 07 '23 at 20:23
  • @Soupy I didn't know about this product, that may indeed be a better alternative to tape (looks like it's designed to avoid the interface between the dust cap and the post). – Rеnаud Jul 08 '23 at 05:18
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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.

MaplePanda
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  • As I mention in a comment above, I don’t think marking heights on a dropper is necessary — they can be adjusted on the fly to match a good feel, (unlike a quick-release where resetting it requires getting off and fiddling). – RLH Jul 09 '23 at 17:49
  • @RLH I guess it depends on how accurate you want your saddle height to be, and how quickly you want to make each height change between riders. If OP is particularly picky for example, it might be annoying to have to adjust up and down multiple times trying to hit the sweet spot. Also, my comments and answer were somewhat intended as a response to Nathan's answer, where he describes the lack of off-the-bike precision adjustment as a flaw. – MaplePanda Jul 09 '23 at 23:16
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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.

Mauro
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  • This is a very ingenious idea. As for what clamps to use: I suspect a hose clamp might be the cheapest clamp here since it's not a load-bearing component. – juhist Jul 15 '23 at 11:43
  • Even a few wraps of electrical tape may be enough – Mauro Jul 16 '23 at 12:07
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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.

Weiwen Ng
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    Let's value your free time at 50$ per hour: the 2$ you spend each time to accurately move the seatpost makes the 300$ investment in a dropper a no-brainer: absoultely worhthwhile, paid back in less than a month.

    I am doing this trade-off because adjusting the seat post is an absolutely menial task, no one can find any pleasure in doing that.

     – EarlGrey Jul 07 '23 at 08:24
  • @EarlGrey And you really don’t need a $300 dropper for this. It just needs to hold position, which most do well—having perfectly smooth actuation and stuff isn’t that important when you only ever need to use the thing once at the start of each ride. – MaplePanda Jul 07 '23 at 20:07
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    @EarlGrey How do you spend 2 minutes and 24 seconds to accurately move the seatpost? From personal experience (see also my answer) it takes between 2 and 3 seconds, including fitting the saddle into the frame. Taking your calculation, that's about $0.03-$0.04 each time. Then a $300 dropper becomes worth it after 7500-10000 seat adjustments, which would be definitely more than 5 years, and that's not even accounting for the maintenance your dropper needs. –  Jul 09 '23 at 16:55
  • TBH, I have no personal experience with droppers. My understanding is that they can potentially wear out, and I'd assume lower quality droppers might be vulnerable to this. Especially if not maintained. Otherwise, Servaes' line of thought is also consistent with mine. – Weiwen Ng Jul 09 '23 at 20:13
  • @Servaes Beyond the expense, are there disadvantages to this solution? so you are confirming OP evaluation. Thanks! – EarlGrey Jul 10 '23 at 07:36
  • @EarlGrey Why do you put words in my mouth? I'm simply replying to your particular argument, which I find weak. Of course there are other disadvantages, but that's off-topic in a reply to a comment about initial costs versus time spent manually adjusting. –  Jul 10 '23 at 16:38
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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).

RLH
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  • dropper can be adjusted on the fly easily ...if you can control it from the handle bar. Adding such dropper to a bike that does not have one is not so straightforward because of cable routing, and that is even more true on an ebike (internal may be impossible without unmounting the whole drive unit and/or battery), and external cabling might be an issue as well - how to attach a cable outside the "downtube" of a step through, especially if a side needs to be kept open to remove the battery. Hence my suggestion of using a dropper operated from the saddle. Adjustments on the fly are not easy. – Rеnаud Jul 09 '23 at 23:42
  • It’s straightforward to run the cable from an externally actuated dropper along a convenient frame path to the headtube — zip ties to the frame or any existing housing. I’ve got a couple (not on a step through, but I don’t see anything to preclude it). My point is that the OP can solve their problem with a dropper (can I use x to do y? Is generally best answered by “there are types of x that solve your problem” rather than “there are types of x that don’t”), and that the discussion of stops on the travel is adding complications that will be obviated by growing familiarity with the system. – RLH Jul 10 '23 at 00:05
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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.

Criggie
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    This doesn't really address the question of if a dropper post could be used to switch between heights. – DavidW Jul 10 '23 at 00:18
  • @DavidW It addresses the problem by explaining why a dropper is very unnecessary. I'll make this more explicit. –  Jul 10 '23 at 05:30
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    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. OP states: "It's used for commuting, kid-toting, and general short-mileage city riding. " In the Q it is never mentioned about theft risk. Kid-toting means there is a child seating in the seatchild. Enjoy stopping in a traffic light with a kid in the seat to fiddle with the seat clamp. You are very familiar with a different situation, apparently. – EarlGrey Jul 10 '23 at 12:01
  • @EarlGrey Here "situation" doesn't refer to the risk of theft, it refers to the subject of the question. Which is manually adjusting the seat on a very regular basis. –  Jul 10 '23 at 16:39
  • @EarlGrey Also, I'm not sure how a kid on the back (or the front, for that matter) makes any difference. The adjustments 'on the fly' are done with a single hand, as I clearly describe in my answer. I often do it while biking (rolling freely), but I understand that's a bit tricky. Doing it while standing on the ground is a trivial matter. –  Jul 10 '23 at 16:40
  • For the risk of theft, there are two points of view: it may increase the risk of theft for the whole bike (but the OP has one most selling category of bikes anyway) but it also allows to fasten the seat post with an anti theft collar (pitlock for example - I used those the wheels and saddle of the city bikes, it’s more simple than attaching the wheels…), making a trivial theft of the saddle impossible. – Rеnаud Jul 15 '23 at 05:45
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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.

juhist
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  • A small note: Brompton bikes are designed to have frequent "seat post adjustments", and have a wear part for that purpose (seat sleeve). https://www.vlerickfietsen.be/download-asset/158a1670-1731-11ea-9eb5-005056985c04 – Rеnаud Jul 15 '23 at 11:56