Geometry of saddle to pedals and of front end

[Randommoose]

Hi,
I have a Gazelle Citygo, normally in the set up shown in the photo, with my 17kg child on the front seat most of the time. I have a couple of issues, am planning on changing bike but I don't know what I don't know and wondered if anyone can tell me what I need to be looking up?

Issue 1:
I have my saddle as far back as I can but have to keep shifting myself backwards to sit on the very rear of the saddle to make my legs feel 'right' pedalling. It isn't related to my arms or the slope of the saddle, I have experimented with different set ups. It is specifically the horizontal distance between my pelvic bones and the bottom bracket. What is this called? Or what types of bikes have a longer horizontal distance between these two points?

Issue 2:
The steering of this bike is really twitchy. Better with my child weighing it down but when unloaded the handle bars ping sideways with the slightest stone making trails quite scary. When on the stand with front wheel off the ground the wheel flops backwards if that is relevant. In the photo is has turned part way by itself even though the wheel is on the ground and the rear wheel off the ground. What is this called - is it trail? If so, I need a bike with a more stable front end - is this more hybrid/mountain style bikes?

I would be really grateful for any answers so I know what terms I need to look up and learn about.

Moose

 

[Chris S]

Issue 1:
I have my saddle as far back as I can but have to keep shifting myself backwards to sit on the very rear of the saddle to make my legs feel 'right' pedalling. It isn't related to my arms or the slope of the saddle, I have experimented with different set ups. It is specifically the distance between my pelvic bones and the bottom bracket. What is this called? Or what types of bikes have a longer distance between these two points?

The seat tube length determines the distance between your pelvic bones and the bottom bracket. Have you tried raising the seat?

 

[Randommoose]

The seat tube length determines the distance between your pelvic bones and the bottom bracket. Have you tried raising the seat?

Hi Chris, it isn't the height, it is the front to back horizontal distance. The height is correct.

 

[Twilkes]

The seat tube angle determines how far back you sit for any given saddle height, i.e. the distance from the centre of the bottom bracket/chainrings to the top of the saddle. A lower seat tube angle would move your saddle further back from the pedals for any given saddle height. 90 degrees would be a vertical seat tube, most bikes are 70-something.

Note that, in real terms, a lower seat tube angle would result in the saddle being further back AND lower to the ground, but the same distance from the pedals.

This may or may not be what you want, suggest you get someone who knows a bit about bikes to watch you ride, and/or for you to try riding someone else's bike(s).

 

[All uphill]

My first thought was similar to Chris; the saddle looks very low unless you have very short legs. How have you determined that the saddle is at the correct height?

As you are cycling with a child on board maybe you feel the need to be able to put both feet on the ground while sitting on the saddle?

The significance of seat height is that the seat will move backwards as it is raised, because the seat post is at an angle to the vertical.

 

[Randommoose]

My first thought was similar to Chris; the saddle looks very low unless you have very short legs. How have you determined that the saddle is at the correct height?

As you are cycling with a child on board maybe you feel the need to be able to put both feet on the ground while sitting on the saddle?

The significance of seat height is that the seat will move backwards as it is raised, because the seat post is at an angle to the vertical.

Hi, I have longer than average legs for my height (80cm/31" inseam for 168cm/5'6" height). My leg is very nearly straight when I sit on the saddle and put my heel on the pedal. I can only touch the ground with one tiptoe (which can make things... interesting... stopping and starting with my child on board).
That's a good point that the higher the saddle, the further backwards it goes. Maybe I need a smaller frame size to get that? I have never had this issue with previous bikes, only this one. But this is most upright bike I have owned as well so maybe it is related to that?

 

[Randommoose]

The seat tube angle determines how far back you sit for any given saddle height, i.e. the distance from the centre of the bottom bracket/chainrings to the top of the saddle. A lower seat tube angle would move your saddle further back from the pedals for any given saddle height. 90 degrees would be a vertical seat tube, most bikes are 70-something.

Note that, in real terms, a lower seat tube angle would result in the saddle being further back AND lower to the ground, but the same distance from the pedals.

This may or may not be what you want, suggest you get someone who knows a bit about bikes to watch you ride, and/or for you to try riding someone else's bike(s).

Thank you, that is useful to know. I will have a look at the seat tube angles on some websites. Lower to the ground would be useful for me, I can barely touch the ground on any bike once I have the seat at the right height for the pedals. Not an issue for me by myself but with my child it would be handy to be able to touch the ground better if possible. Not essential though.

 

[Randommoose]

I have just measured and the seat tube height from centre of BB is 21.5"/54.5cm, it is size Medium.

 

[cyberknight]

I think what you are actually talking about is saddle setback, this is normally measured from the tip of the saddle to the centre of the bottom bracket.
Saddle height and how far you have the saddle forward or backwards on the saddle rails can affect this .
Many basic fit guidelines use " knee over pedal spindle KOPS" to get you in the right ball park once you have set saddle height correctly

 

[Twilkes]

I think what you are actually talking about is saddle setback, this is normally measured from the tip of the saddle to the centre of the bottom bracket.
Saddle height and how far you have the saddle forward or backwards on the saddle rails can affect this .
Many basic fit guidelines use " knee over pedal spindle KOPS" to get you in the right ball park once you have set saddle height correctly

I did think about mentioning that, but that saddle looks as far back as it'll go, and there's barely any room for a setback seatpost as there's not much post showing as it is. That's why I suggested getting someone to watch them riding it, it looks an odd bike to me, probably very much a sit-upright bike, and possibly a touch too big for them if the seatpost is to go by.

 

[mickle]

Your saddle is almost certainly too low. You might want to try levelling it too - its a bit up at the nose.

 

[Randommoose]

I think what you are actually talking about is saddle setback, this is normally measured from the tip of the saddle to the centre of the bottom bracket.
Saddle height and how far you have the saddle forward or backwards on the saddle rails can affect this .
Many basic fit guidelines use " knee over pedal spindle KOPS" to get you in the right ball park once you have set saddle height correctly

Thank you, that is useful to know the term 'setback' so I can look up and try to learn more about it.
I have the saddle as far back on the rails as it will go. I will get my husband to check knee over pedal spindle at some point. I think he has before but I will check again.

 

[Randommoose]

I did think about mentioning that, but that saddle looks as far back as it'll go, and there's barely any room for a setback seatpost as there's not much post showing as it is. That's why I suggested getting someone to watch them riding it, it looks an odd bike to me, probably very much a sit-upright bike, and possibly a touch too big for them if the seatpost is to go by.

It is very upright, no weight at all through the handlebars. I think it is too upright for me hence why I am thinking of changing bike. In manufacturer charts my height is generally Small frame size, legs Medium and arms (for the few that show it) Medium or Large. This is a medium frame but maybe a future bike could be medium or small provided I can get the handlebars far enough in front. On a previous bike I changed to butterfly bars leant forwards to get more reach.

 

[wafter]

Fore-aft position of the saddle is often referred to as layback. From a frame geometry perspective if you want more layback for a given height above the pedals you want a slacker seat tube angle; however most modern stuff seems within a pretty narrow range.

I have similar proportions and manage as best I can with the saddle in the absolute rear-most position; you can push this to the limit by replacing the seatpost with one that gives max layback and a saddle with rails that extend as far forward as possible relative to the area you're actually sat on.

Hard to tell from the pics but the forks look very upright / lacking trail and the head angle unremarkable, so the former certainly in keeping with your suggestion its twitchy. Generally for stability over responsiveness you want higher trail numbers and slacker (less vertical) head tube angles. I'd generally expect utility bikes to push more towards stability than responsiveness so it's a bit odd in this regard.

 

[Randommoose]

Your saddle is almost certainly too low. You might want to try levelling it too - its a bit up at the nose.

How high should it be? I currently have it so my knee is very nearly straight if I put my heel on the pedal. I have actually just put the saddle up nose higher like this as I am so fed up of sliding off the front. It feels flat when I am on it. I keep experimenting with different saddle angles but any flatter than this and I slide forwards.