Crank length or frame size to put saddle in right place?

bails87

If you've got short legs and a long body then you should be on a bigger frame than someone with the same height but 'average' proportions.

If I sit on the saddle my feet are far too far out in front of me

By what measure? A feeling? The plumbline thing? If the latter then surely the same frame(s) would be 'wrong' for everybody. You've got the saddle as far forward as it'll go, and it's still not far enough forward, on all three bikes of different sizes. If it was just the frame with the huge seatpost then I could understand it (although a bigger frame with a shorter seatpost may put you in the same position i any case)

- if I just pedal I find I have moved forward on my saddle till I am sitting more on the nose - even on the flat

Is the seat angled forwards? Are the bars too low? These could be pitching your weight forwards and making you slide to the front of the saddle.

Given bikes have top tubes that can vary in length by only 2cm before you customise it further with cockpit setup

Just picked a bike at random, Boardman MTB HT Team. There's a 5cm difference between large and small.

supersonic

What is the bike and seat angle, out of interest?

YeehaaMcgee

supersonic wrote:

What is the bike and seat angle, out of interest?

dhobiwallah wrote:

One is a mid-90s Proflex in Large (19"), one is a Merlin Malt hardtail in Med (17") and one is an Iron Horse full suss in small (15" I think).

Not sure about seat angle though.

diy

Ok, with those revised measurement, it doesn't sound that bad. I had a similar problem on my road bike which was fixed by raising the seat nose 1cm.

I would consider going back 0.5cm, nose up 0.5cm and drop the seat 0.5cm and see how that feels.

dhobiwallah

supersonic wrote:

What is the bike and seat angle, out of interest?

The Malt has a 73 degree seat angle with a 22.75" ETT


The Iron Horse MKIII is 73.5 SA with a 21.5" ETT

The Proflex is mid-90s all over the shop geometry :shock: Having replaced the original Girvin Scaffolding with a more modern 100mm fork and shorter stem I'm sure it may even be worse than originally planned but I have turned that into a singlespeed winter bridleway sludge hack - its not really an issue, I only really mentioned in passing to make the point that I have the same prob on a large frame (which isn't all that surprising)

I had noticed that the saddle on the med hardtail was quite tilted, and I have the stem set up with negative rise, so altering those may help.

The comedy seatpost also has a little layback, so an inline may solve that one... However I picked up a nearly new Med front triangle for £30 and a short stem/wider bar in a CRC bargain frenzy so will entirely change that - just trying to understand my sizing issues before I change even more!

bails87 wrote:

If I sit on the saddle my feet are far too far out in front of me

By what measure? A feeling? The plumbline thing? .....

Kind of, visually they look so far forward a plumb would go through my ankle rather than the pedal axle. If I have to spin for a bit (and don't think about seat position) I find I am further forward by the end of it. I had assumed this meant I was sitting unaturally far backwards when starting off and the forward movement showed my saddle was in the wrong place; it may however be the bar/saddle angle combo mentioned..

YeehaaMcgee

Saddles are always behind the BB, you realise that, don't you? Just checking. The more you raise the saddle, the further back they go.

cooldad

OCD. And for that reason, I'm out.

dhobiwallah

YeehaaMcgee wrote:

Saddles are always behind the BB, you realise that, don't you? Just checking. The more you raise the saddle, the further back they go.

Yes, but the seat tube on the bike is the same angle regardless of size, so if I go up a size presumably the saddle will be in the same place relative to the bb (just more of it is frame and less of it is post). Just trying to get an idea of how to size the bike for the 20 min linking slogs on road. Once the saddle is dropped it is an irrelevance.

cooldad wrote:

OCD. And for that reason, I'm out.

OCD, really? I just answered the question - as to the seat angles (and gave the comparative TT lengths). Given this thread is about bike fit and saddle position for a given height (albeit via crank length) I would have thought it entirely relevant. The whole 'just go with what feels good' brigade is why I am here in the first place; my 10 mins round a housing estate on my small bike hopping off kerbs felt great; it wasn't till I'd bought it and was an hour into a ride I realised I couldn't raise the saddle high enough to cover any sort of distance comfortably

supersonic

If you couldn't get the saddle high enough then surely the seat post is not long enough? Or is it because you feel too far back when the saddle is up? A layback post is certainly not going to help you here. How does the reach feel when the saddle is at the highest?

73 degrees is about average for an MTB, probably a little on the steep side.

styxd

Honestly, read Steve Hoggs website, if nothing else it explains things clearly unlike this forum where everyone is going round in circles.

supersonic

styxd wrote:

Honestly, read Steve Hoggs website, if nothing else it explains things clearly unlike this forum where everyone is going round in circles.

I just read his bit on seat height. And he says experiment and get what feels 'fluent', are no equations that work it out. Which is pretty much what we are all saying. Plus most his stuff is vey much for road and track racers. An MTBer may set the saddle a little lower than what is optimum efficiency.

dhobiwallah

Sorry, misunderstanding there - the reason I bought the longer seatpost is because I couldn't get the saddle high enough originally with the 350mm post the bike came with.

None of my posts are intentionally layback, they are the 10-20mm layback which seems pretty standard as opposed to inline posts which seem to be the exception rather than the norm (and weren't fitted to any of my bikes initially, so a similar clamp/layback on my longer post seemed sensible at the time).

I reckon I am going to play with my saddle angle and stem height on my Med hardtail to see if I am really just slipping forward rather than naturally moving to where my pedalling wants to take me. On the Small full-suss I think I will swap the frame/stem/bars to see where I am (with a standard length post) and then think about an inline if that still needs tweaking.

FWIW it seems the small bike has 170mm cranks anyway; but as I ride that with flats and the others with clips there ain't no way I am going to notice that diffrence!

supersonic

This is what I'd do, justgo out with a set of spanners and allen keys and have a play!

I like quite a steep seat angle for pedalling efficiency.

craigw99

just had a thought what sort of seat is it? if its a wide seat you may be moving forwards on it so it doesn't get in the way i hate wide seats for that reason. Narrow seats and padded shorts for me :-)

steelie600

Instead of buying an inline seatpost, why not twist the laybacks thru 180* and reposition seat. Surely thats gotta work right???

paul.skibum

I have the same length legs as you (give or take I have never stood against a wall and put a book in my crotch to check - weirdo) btu I am only 5'6. I ride a small Santa Cruz, my feet are "ahead" of my when seated on the flat pedalling with seat at the height I use for pedalling - on long climbs I have it a little higher and sit forward and dont notice a problem - the bike is quite short and the only issue I have is banging my knees on the bars in switchbacks.

I run 175mm cranks with no issues - tried a mates 170's a while back and couldnt tell the difference. I have an inline post, saddle in the middle of the rails and the bike has a 71 degree seat tube I believe.

Why not get some pics of your bike on here and your seated position on the bike to help us out - do you have the issue with all your bikes or just one?

cooldad

steelie600 wrote:

Instead of buying an inline seatpost, why not twist the laybacks thru 180* and reposition seat. Surely thats gotta work right???

No

YeehaaMcgee

steelie600 wrote:

Instead of buying an inline seatpost, why not twist the laybacks thru 180* and reposition seat. Surely thats gotta work right???

Duh, he'd end up going backwards then

nicklouse

steelie600 wrote:

Instead of buying an inline seatpost, why not twist the laybacks thru 180* and reposition seat. Surely thats gotta work right???

When rotated It is rare to find a seat post that allows the saddle to be set correctly. They do not have enough rotation to get I close o level.

steelie600

Oh okies! My rsp one will like just assumed they were all like that, Ill get me coat

The Rookie

My Daughter is 5'3", her legs are so short that when she has the saddle set up right (legs not quite straight with pedal at teh bottom) I can sit on her saddle and put both feet FLAT on the floor (and I only have a 31" inside leg) she rode 175 cranks for ages (all she had) but when she went to buy some new, bought 170's (figured she may as well) and she can barely tell the difference, so how 5mm (0.65% of your 30" inside leg) you think will make a difference, I really don't know!

It sounds like your riding position is wrong, you're trying to ride 'sit up and beg' like something out the 1950's (or Holland), on an MTB you need to have some weight on your bars as well (for front end grip), however if you want to sit like that, get a frame with a shorter top tube, fit a shorter stem, fit narrower bars, fit backswept bars, or a combination therof.

YeehaaMcgee

The Beginner wrote:

It sounds like your riding position is wrong, you're trying to ride 'sit up and beg' like something out the 1950's (or Holland), on an MTB you need to have some weight on your bars as well (for front end grip), however if you want to sit like that, get a frame with a shorter top tube, fit a shorter stem, fit narrower bars, fit backswept bars, or a combination therof.

Somewhere in the back of my mind I've been trying to piece this together somehow, and I think you've just nailed it.

dhobiwallah

The Beginner wrote:

It sounds like your riding position is wrong, you're trying to ride 'sit up and beg' like something out the 1950's (or Holland), on an MTB you need to have some weight on your bars as well (for front end grip), however if you want to sit like that, get a frame with a shorter top tube, fit a shorter stem, fit narrower bars, fit backswept bars, or a combination therof.

Possibly, but its just where I end up when I don't think to hard about it - which leads me to think its a natural position to be in; maybe I need to concentrate on shifting my arse back constantly. The whole pedal seems too far forward thing (plumb bob job) might mean that isn't the case. While I accept the whole roadie calc thing doesn't always work for MTB it seems slightly odd that I have to actively overcome this and move my body further back that this recommends and doesn't come naturally. I don't seem to suffer from wandering front end either - if fact considerably less so than my mate.

The Beginner wrote:

My Daughter is 5'3", her legs are so short that when she has the saddle set up right (legs not quite straight with pedal at teh bottom) I can sit on her saddle and put both feet FLAT on the floor (and I only have a 31" inside leg) she rode 175 cranks for ages (all she had) but when she went to buy some new, bought 170's (figured she may as well) and she can barely tell the difference, so how 5mm (0.65% of your 30" inside leg) you think will make a difference, I really don't know!

It wouldn't really be the percentage of your inside leg that was the issue, rather the percentage of your femur which is a little higher (but still tiny). But while saying it is only 0.65% so you won't notice is a little disingenuous - you could say that 20mm is still only 2.5% but I bet people would notice that! I am entirely sure that I would not notice the 5mm change any more than your daughter (as I have pointed out I don't particularly notice subtle changes in any setup on my bike - probably why I have 3 different sized frames).

But with your daughter - just because its what she knows on a bike it doesn't mean it fits her (any more than a sit-up-and-beg rider feels that a stretched out XC position is wrong because it isn't what they know). If you look at her leg bend at the top of her stroke it will be different to yours and much different to a 6'4" guy still running 175mm cranks. One of them is in a mechanically more efficient position than the other - which one I don't know.

When I run with my 6'4" mate I bury him on the uphills and he trounces me on the downs - I'm pretty sure its due to leg length but we can't change that - its all we know - it doesn't mean one of us isn't a better build for a runner than the other, or we don't improve our running based on the legs we have :P Sometimes I do wonder if MTBers go a little too far the other way regarding roadie bike fit obsession; I find it hard to believe that such different leg lengths are all suited to identical cranks, the engineer in me just screams that can't be the case. As to whether the difference in lengths is enough to notice/bother about or whether we just get used to what we know - I don't know...

diy

dhobiwallah wrote:

When I run with my 6'4" mate I bury him on the uphills and he trounces me on the downs - I'm pretty sure its due to leg length but we can't change that

Or gravity i.e. a difference in body mass?

nicklouse

diy wrote:

dhobiwallah wrote:

When I run with my 6'4" mate I bury him on the uphills and he trounces me on the downs - I'm pretty sure its due to leg length but we can't change that

Or gravity i.e. a difference in body mass?

or ability.

Cubist

Theres so much wisdom and experience in this thread so far. Heres hoping shimano are paying attention to the experts on bikeradar and, come 2013, stop offering a choice of crank arm lengths.
Theyre clearly a waste of time and the leg extension in relation to saddle height options they offer to riders must be ignored henceforth.
Imagine factors as ridiculous as mathematics and physics interfering with something as mystical and spiritual as rotating a crank in order to propell a machine forwards! And shame on the OP for having legs that dont conform to the industry standard!

cooldad

Obviously you missed the point.

Numerous times.

nicklouse

Cubist wrote:

Theres so much wisdom and experience in this thread so far. Heres hoping shimano are paying attention to the experts on bikeradar and, come 2013, stop offering a choice of crank arm lengths.
Theyre clearly a waste of time and the leg extension in relation to saddle height options they offer to riders must be ignored henceforth.
Imagine factors as ridiculous as mathematics and physics interfering with something as mystical and spiritual as rotating a crank in order to propell a machine forwards! And shame on the OP for having legs that dont conform to the industry standard!

Not at all I hope they continue to offer the limited choice they do. But it will make bugger all difference to the problems the OP is having.

If you read the topic you will see that.

craigw99

i checked last night on my bike and the center of the seat lies about half way between the bb and the rear axle - no amount of inline post and seat is going to position that over the BB. is that where the op is looking to put it? if so the only thing that i can think of that does that is a unicycle...

supersonic

And shame on the OP for having legs that dont conform to the industry standard!

How did you deduce that?