Sliding off back of saddle.

General cycling advice ( NOT technical ! )
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AugustusWindsock
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Sliding off back of saddle.

Postby AugustusWindsock » 18 Feb 2018, 11:57pm

Please be gentle with me as this may be a stupid and/or badly-phrased question...

I often feel like I'm sliding off the back of the saddle on my bike. I suspect it's a sizing issue as I have some ...ahem... embarrassing proportions. I'm a very short bloke (5 foot 6 inches) with comically short, stumpy legs (29-and-a-half inches). This means that I struggle to get a bike with any kind of standover clearance. I currently own a Marin Muirwoods steel-framed hybrid that I bought new about 12 years ago. It's a small, 15 inch frame and I can just about stand over the sloping top tube but when riding it the overall effect looks like a particularly out-of-shape gorilla perched atop a child's bike.

When riding, I find that my behind wants to slide off the back of the saddle. I've got it set at what I believe to be the right height (leg almost fully straight at the bottom of the pedal's travel) so it seems illogical to me that I am sliding backwards so much, given that I should be running out of leg length to actually get off the back of it.

I've tried adjusting the saddle angle with no real difference noted. I've only just realised that you can get different sized bar stems (actually I didn't know what a stem was until I joined here) so I was wondering if getting a longer stem would effectively "lengthen" the bike and make it fit my distorted physique a little better. The current stem says it's 90mm and seven degrees, so I was thinking of trying a 100mm one. Any ideas if this will help?
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eileithyia
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Re: Sliding off back of saddle.

Postby eileithyia » 19 Feb 2018, 8:56am

It might well be that the length is too short for you and you are pushing backwards? I presume you have ensured the saddle is flat? What about tipping the nose down slightly, does that help? Pushing the saddle back on the rails and possibly a longer stem may help.
Do you have photos, side on shot of you on the bike?
Have you considered a bike fit?
Bit puzzled by the fact you need such a small frame for stand over height, as a 5'2" woman I can comfortably stand over my traditionally built 20.5" old steel frame without a sloping top tube that is more common on newer bikes? Some of my male cycling friends have been of smaller stature and don't need such a small frame to accommodate their frame?
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ANTONISH
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Re: Sliding off back of saddle.

Postby ANTONISH » 19 Feb 2018, 9:57am

I'm 5'5" and 29" inside seam for trousers. That isn't the figure you need for cycling which is crotch to floor without shoes.
I haven't experienced the standover problem you describe. I have a number of bikes around 50 to 52 cm. One has a sloping top tube.
I'm a bit mystified as to how you feel you are sliding off the back of the saddle. At a guess I'd say your 'bars are too high and the reach too short.
As has been said in previous post "531 Colin " gives some excellent advice on positioning.

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mjr
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Re: Sliding off back of saddle.

Postby mjr » 19 Feb 2018, 10:54am

ANTONISH wrote:I'm 5'5" and 29" inside seam for trousers. That isn't the figure you need for cycling which is crotch to floor without shoes.

It varies by fitting method, but inseam less 9" for road bikes and 11" for MTB used to be the rules of thumb. Now you have to check manufacturers' charts for sizing.

Colin's tips are on http://www.wheel-easy.org.uk see "Bike set-up" on the left.
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LollyKat
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Re: Sliding off back of saddle.

Postby LollyKat » 19 Feb 2018, 11:15am

A lay-back seat post, which allows you to put the saddle back, might help more than a longer stem. Try Colin's tips and see how you get on.

Freddie
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Re: Sliding off back of saddle.

Postby Freddie » 19 Feb 2018, 12:29pm

The tiny frame you are riding probably has a very steep seat tube and that is why you cannot get far enough back on the saddle. The relation of your behind to the pedals has little to do with forward reach and can't be fixed by adding more forward reach with a longer stem. Lollykat's advice is good if you want to modify your current frame.

As for standover, it is not necessary to stand over a bicycle with two flat feet on the ground and x room between yourself and the top tube. For most of cycling history people rode frames where they had to tilt them slightly to one side upon mount/dismount. Standover is a new concept, invented perhaps by American companies fearful of litigation. It is OK if you can get it, but 'standover' is no way to size up whether a frame fits or not. A bicycle is for riding, not standing over.

Even with your 'comically short legs', I imagine you could probably fit a 48cm frame on the seat tube, you'd just have to learn how to tilt the bicycle slightly on dismount. As long as the top tube is not far too high, then your...um...undercarriage has a bit of malleability and shouldn't disagree too much with the odd bit of gentle contact with the top tube, providing you take time to learn to mount and dismount properly.

andrew_s
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Re: Sliding off back of saddle.

Postby andrew_s » 19 Feb 2018, 2:47pm

Bear in mind that seat tube lengths are verging on useless when looking at new or relatively recent bikes, because the actual seat tube length is used, but there's very rarely any mention of what the top tube slope is, or whether the end of it is bu the top tube or some way above.

Back when seat tubes were a valid indication of sizing, all bikes had horizontal top tubes, and the seat tube ended level with the top of the top tube.

Short of a new bike, you can get replacement handlebar stems up to maybe 130 mm fairly readily, but it's also likely that you will need a seatpost that moves the saddle clamp behind the seatpost. Unfortunately seatposts with much more than 20-25 mm of setback are uncommon, and fairly expensive when you find them (eg Nitto S84)

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531colin
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Re: Sliding off back of saddle.

Postby 531colin » 19 Feb 2018, 6:09pm

Have a read of my bike-fitting tips linked below my signature. I try to concentrate on how it feels to me to ride with my bike set up the way I like it, and to avoid formulae involving multiplying a body dimension by a magic number of some sort.
Bike fitting is an iterative process, and one adjustment is likely to influence another adjustment, however I recommend that you try making your adjustments in this order..... saddle height....... saddle setback........ reach.
Saddle height is set for comfortable and efficient pedalling. Saddle setback adjusts how much weight you have on your hands when freewheeling....but moving the saddle back also moves it further away from the pedals, and may require the saddle to be lowered slightly.
Reach should be adjusted only by moving the handlebars. I like enough reach to brace my back and pelvis to support the effort of pedalling. If my bike is too short reach, I find myself sliding right to the back of the saddle, even though the saddle setback is what I always use.
10mm adjustment of handlebar stem length isn't very much at all (its often the smallest increment in stem length you can buy). I can happily tolerate reach being plus or minus 5mm from my "ideal" so I think if you need to adjust reach after you have adjusted saddle height and setback then you should probably go for more than 10mm change....if you can ride comfortably with the heel of your hand on the bars, you might need 30mm increase in reach.
The point about steep seat tube angles and seatposts with layback is valid.

JohnW
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Re: Sliding off back of saddle.

Postby JohnW » 19 Feb 2018, 8:11pm

Consider the possibility of your upper leg being disproportionally long (compared with your lower leg). There's more to it than saddle height. The relationship between your knee and pedal spindle is equally important. I'm a couple of inches shorter than you, but I have the same inside leg dimension and I have no trouble with top-tube/crutch relationship on horizontal top tubes. I ride classic diamond pattern frames and the one that sits beside me here in the dining room is a 21¼" frame (540mm).

I have a disproportionally long upper leg and I can't get a frame with a steep seat angle comfortable for me using Brooks saddles on 'normal' seat pins. You're probably not conscious of upper leg/lower leg proportions unless you have this issue - you can't see it and you don't notice it when walking/running etc. I wasn't aware myself until a couple of years ago (I was over 60 by that time) when, after a lifetime of 73º and 72º seat angles, I tried a 74º frame.

This became an issue when I couldn't get my Brooks saddle far enough back on the 74º frame using my previously perfectly acceptable seat pins. A long lay-back seat pin solved the problem.

It could be that your frame is too steep.

Be aware that Brooks saddles are notoriously short on front/back adjustment. If it's leather you want, Spa saddles may do the job.

You could find a decent framebuilder, and have a frame made to measure - they'll cost more but I'm still riding one built in 1981. Beware being talked in to a steep seat tube.

JohnW
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Re: Sliding off back of saddle.

Postby JohnW » 19 Feb 2018, 8:29pm

andrew_s wrote:Bear in mind that seat tube lengths are verging on useless when looking at new or relatively recent bikes...................[/url])

+1 to that.

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AugustusWindsock
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Re: Sliding off back of saddle.

Postby AugustusWindsock » 19 Feb 2018, 8:36pm

Thank you so much for all the answers. These will take me some time to digest. Like I said, apologies for my ignorance and lack of knowledge. I'm genuinely trying to read up about all this stuff but it's a lot to take in at once for someone who doesn't yet know his bottom bracket from his ..err.. bottom. I'll spend a lot of time investigating @531colin's bike fitting advice and maybe look at different seat posts. I'm also beginning to appreciate that this is a lot more complicated than it first appeared!

I also apologise for my freakish dimensions. I am genuinely 5'6" tall with a 29.5" crotch-to-floor measurement and this is what makes things difficult. I also know that my bike isn't exactly a top-spec model but although I'm thinking of upgrading this year, the whole embarrassment of being fitted would probably stop me entering a bike shop!

Here's an old picture of my comical push-iron. I've not got one of me on it but imagine some kind of monkey perched on top and you'll not be far wrong.

Image
Last edited by AugustusWindsock on 19 Feb 2018, 10:13pm, edited 1 time in total.
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JohnW
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Re: Sliding off back of saddle.

Postby JohnW » 19 Feb 2018, 8:45pm

Your bike looks OK, top spec or not...........except that the seat tube looks steep to me. Don't be deterred from having a bike-fit. I've never had one - when I was starting, the old guys helped a youngster through the trial and error process.............it took quite a while.

I was always a bit sceptical about bike-fits myself, until a cycling colleague had one because he couldn't keep up with the bunch and his legs hurt. The bike fit transformed him and cured his back ache. He'd not been cycling long - about 10 years - and he just thought that it was lack of fitness with him. Now he half-wheels most of us, including me. (He's 10 years younger - so that's my excuse :lol: :lol: :lol: :lol: :lol: ).

jimlews
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Re: Sliding off back of saddle.

Postby jimlews » 19 Feb 2018, 9:22pm

Just a thought...

Have you considered crank length? Most bikes seem to come supplied with 170mm cranks as a default size or worse still 175mm if it has offroad pretentions. Given your inside leg measurement, I would expect you to be comfortable with something in the region of 162.5 to 165mm

Seriously cool bike, by the way!

MikeF
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Re: Sliding off back of saddle.

Postby MikeF » 23 Feb 2018, 9:19pm

AugustusWindsock wrote:
I also apologise for my freakish dimensions. I am genuinely 5'6" tall with a 29.5" crotch-to-floor measurement and this is what makes things difficult.
I'm just about 5'6" and probably similar crotch to floor measurements - at least anything longer than a 29" trouser leg is too long. I'm quite a light build though. When I've ridden a daughter's bike I too find that I'm sliding off the back of the seat. As others have said it's because the seat is too far forward. Digest Colin's good advice, but I think you need to look at finding a seatpost and/or adaptor that allows the saddle to be moved farther back.
"It takes a genius to spot the obvious" - my old physics master

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CyberKnight
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Re: Sliding off back of saddle.

Postby CyberKnight » 25 Feb 2018, 9:29am

You can get these ...
https://www.sjscycles.co.uk/saddles/vk-saddleadjuster/
Image
I think the bike is probably a bit small tbh too .
Im 5 foot 7 " with an inside leg half an inch longer and i have an 18 " subway and 54 cm road bikes which fit me fine
drop bar mtb.jpg
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