Bike size and cyclist hight

[briansnail]

When I was a nipper the golden rule was saddle should come up to the top of the hip bone.I am 5 ft 6 and would normally select a medium size advertised bike as I am getting another one.
However I have seen large riders riding small bikes with a tall seat post.I prefer this as i find (just personal) small bikes handle better and are easier to ride.I am lightweight and small.

The question I would like to ask.Is there any good or bad points getting a smaller bike with a raised seat-post?

Thanks in advance to any replies.
************************************************************************
I ride Brompton and a 100% British Vintage

[Nearholmer]

My experience of riding a “one size fits all” Pashley Parabike for a few years with the seat-post extended almost to the limit, is that the relationship between seat height, seat-post angle, and wheelbase is crucial to good, or in the case of me on that bike, bad, handling.

So, have a look at the overall geometry of the bike, as set-up for the individual rider, and my suggestion would be that if the seat position ends-up too far back in relation to the rear axle, even if everything else, seat height, seat setback, reach etc, is right, then handling will be horrible.

What precisely “too far back” is is presumably a matter of personal taste!

Likewise the converse, having everything else right, but the seat ending up a long way ahead of the rear axle because of the geometry of the bike can be “not to personal taste”. I found a “proper tourer” with long chain stays didn’t suit me either. “Too far forward” is equally a matter of personal taste, I guess!

I think what I’m saying in a long and roundabout way is that you have to find a bike size and shape , a.k.a. geometry, that you like, nominal size alone isn’t much of a clue .

[AndyK]

I'm a shade over 5'6"/169cm, which usually puts me on a boundary between bike sizes. I tend to go for the larger size because I have big feet and - given consistent geometry between sizes - the larger frame means less toe overlap. Something to think about if you too are blessed with size 46 flippers.

[531colin]

These are the rules:
A”small” bike is OK provided you can get the bars high enough and you don’t kick the front wheel.
A “big” bike is OK provided the reach to the bars isn’t too long, with ( for example) a shorter stem fitted.
Saddle position is set fore and aft so you have the “right amount” of weight on your hands when everything like seat height and reach is set up properly ….. saddle position is more directly related to the bottom bracket axle than the wheel axle.
Most bikes have the same chainstay length for all sizes; seat tube angle and crank length may change slightly between different sizes.
Read my DIY bike fit linked.

[Nearholmer]

saddle position is more directly related to the bottom bracket axle than the wheel axle.

That’s certainly where you measure it from, and set it from, but the point I was (clumsily) trying to get across is that if the frame is on the small size for an individual, so that setting the seat position results in it being far back in relation to the back axle, the handling can become peculiar.

[531colin]

saddle position is more directly related to the bottom bracket axle than the wheel axle.

That’s certainly where you measure it from, and set it from, but the point I was (clumsily) trying to get across is that if the frame is on the small size for an individual, so that setting the seat position results in it being far back in relation to the back axle, the handling can become peculiar.

But chainstay length is usually constant across all sizes: so the only difference “size” makes is perhaps half a degree on seat tube angle. (It doesn’t matter if the height is made up of seat tube with more or less seatpost exposed, as seat tube and seatpost are in a straight line)
I think what you are saying is you prefer brighter handling bikes, and that’s fine, and bikes with brighter handling usually come with shorter chainstay than relaxed handling tourers

[drossall]

Convention now says that it's the reach (from saddle to bars) that matters. A small bike with a raised saddle will tend to be shorter as well, so you'll be cramped. Of course, there's latitude to adjust with a longer stem etc, except on a Brompton, noting your signature, and only within limits.

[slowster]

Is there any good or bad points getting a smaller bike with a raised seat-post?

Toe overlap is more likely with a smaller frame, especially with wider tyres and mudguards. Comparing the front centre measurement of any potential new bike with your existing bike and shoes should reveal if there will be toe overlap, and if so how much..

A smaller frame will have a shorter head tube, which will necessitate either a corresponding increase in the height of spacers to achieve the same bar height, or a stem with more rise. The height of spacers is largely a matter of aesthetic preference, except for forks with carbon steerers for which most manufacturers stipulate no more than 30mm of spacers.

To give the same reach a smaller frame (with a shorter top tube) will require a longer stem. The effect of a longer stem on steering may or may not be something you find significant. Similarly smaller sized bikes are sometimes supplied with slightly narrower drop bars, which you might want to replace if you find them too narrow.

[Nearholmer]

But chainstay length is usually constant across all sizes: so the only difference “size” makes is perhaps half a degree on seat tube angle. (It doesn’t matter if the height is made up of seat tube with more or less seatpost exposed, as seat tube and seatpost are in a straight line)

Perhaps my experience had been overly coloured by the Pashley Parabike, which has a seat tube a very long way from the vertical. With the seat at near maximum height, and positioned on the rails to accommodate thigh length that goes with longish legs, it was so far back that the whole experience was a bit “clown bike”.

But, yes, on a more normal bike, with a more normal seat tube angle, it seems that I do like short chain stays, not that I knew that until I tried the tourer with long chain stays, which to me felt really cumbersome.

[Cadence]

Nobody has mentioned "standover" height yet, As someone who is short in the legs I have always had a problem with top tubes. My vintage touring bike is a 21"frame (700c wheels), which just about provides the necessary "clearance". When my son left home he left behind his 90's budget MTB (steel frame, rigid forks, 26" wheels). During lockdown I put it to use until I had an emergency stop - both feet off the pedals and painful contact with the "crown jewels" on the top tube! It's a 20" frame, but of course, being an MTB the bottom bracket is significantly higher than my old Claud Butler.
When I came to buy a hybrid I opted for an 18" frame (700c wheels) with sloping top tube and straight handlebars- should be OK? Well yes, but only just due to the even higher bottom bracket. I've had to push the saddle right back and fit a stem riser and a 120mm stem to get anywhere near to a decent riding position due to the short length of the top tube.
If you are even slightly "unusual" trying to find a good-sized bike is a nightmare, particularly as there is so much variation of BB heights, top tube and seat tube lengths and different ways that manufacturers determine what their sizes are.

[531colin]

But chainstay length is usually constant across all sizes: so the only difference “size” makes is perhaps half a degree on seat tube angle. (It doesn’t matter if the height is made up of seat tube with more or less seatpost exposed, as seat tube and seatpost are in a straight line)

Perhaps my experience had been overly coloured by the Pashley Parabike, which has a seat tube a very long way from the vertical. With the seat at near maximum height, and positioned on the rails to accommodate thigh length that goes with longish legs, it was so far back that the whole experience was a bit “clown bike”.

But, yes, on a more normal bike, with a more normal seat tube angle, it seems that I do like short chain stays, not that I knew that until I tried the tourer with long chain stays, which to me felt really cumbersome.

Apologies for not quoting selectively but I’m on the phone. ..and I may have completely misunderstood anyway!
Para bike seat tube “a long way from vertical” presumably means laid back or a slack seat tube angle . If you are then sliding saddle back on rails, that sounds to me like you are setting saddle position to get a long enough reach, rather than swapping the stem, which may not even be possible on a para bike.
On a bike with too short a reach I’ve been reduced to sitting on the very back of the saddle and it doesn’t really work for more than a few miles.

[Manc33]

When I was a nipper the golden rule was saddle should come up to the top of the hip bone.

Different heights of bottom bracket and different crank lengths rule this out nowadays.

[ed.lazda]

I go up a size compared to the advice given on most bike buying web sites. There are two reasons for this:

1. The current tendency to have yards of seat post showing. I don't know if this is just fashion or if there is a good reason, but I don't like it. This is a purely aesthetic judgment.

2. If buying something remotely sporty, it means I can have the handlebars high enough relative to the saddle to avoid neck issues, even with a carbon steerer which only allows for 30mm of spacers.

Standover height is a potential issue with this approach, but it hasn't been a problem yet.

[Nearholmer]

rather than swapping the stem, which may not even be possible on a para bike

The Parabike does indeed have a very slack seat-tube angle, but it also comes with a mega-adjustable stem and handlebars. The reason for pushing the seat back as well as high was that my leg-length is close to the theoretical limit for that bike, right on it in fact, so to get anything approaching KOPS-ish I had to go a long way back.

Anyway, it’s such an unusual bike that I probably shouldn’t have dragged it into the conversation. It’s not an inherently bad design either, I’ve spoken to people who’ve done incredible mileages on them, and it’s a cracking concept for family utility use, but I reckon it really tops out at, say, 30” or 31” ‘bike inside leg’, whereas they cite 33”.

[Mick F]

Another rule, is to put your elbow against the saddle-nose, and your finger-tips should just touch the handlebars.

And another one, is to sit on the bike and lean against a wall or something, and pedal backwards using your heels on the pedals. Your legs should be straight on the bottom of each pedal-stroke.

Rules of thumb perhaps, but they work for me.