Measuring saddle Fore/Aft on Different Bikes

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Witterings
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Measuring saddle Fore/Aft on Different Bikes

Postby Witterings » 9 Sep 2019, 1:34pm

Has anybody found any reliable way of measuring saddle Fore/Aft so if you get a position on 1 bike that's really comfortable and is friendly to your knees etc. you can easily replicate it on another bike and also where do you measure from / to on the saddle ... the nose or the rear.

I've tried using plumb lines but where the middle of the bottom bracket is further out from the line of the top tube it makes it difficult and holding the line against the nose of the saddle it doesn't drop straight down without hitting / being deflected on various parts of the frame so you end up tilting the bike which I'm not convinced helps give an accurate reading.

If anybody's got any "bright ideas" I'd be really interested in hearing them.

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Tigerbiten
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Re: Measuring saddle Fore/Aft on Different Bikes

Postby Tigerbiten » 9 Sep 2019, 2:01pm

With the bike on a level surface.
A spirit level on the nose of the saddle forward above the top tube.
Drop the plumb bob line from this directly to the BB.
This will give you both the height above the BB plus the setback of the saddle.

Luck ......... :D

MikeDee
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Joined: 11 Dec 2014, 8:36pm

Re: Measuring saddle Fore/Aft on Different Bikes

Postby MikeDee » 9 Sep 2019, 2:17pm

This was written by frame builder Dave Kirk:

Most of us have more than one bike and I often hear from riders who say that they have a hard time getting two bikes to have the same exact contact points. The three contact points, and how they relate to one another, are what determine how one sits on the bike...they are the only things that matter. It can be easy to get confused trying to balance a different seat angle and a different top tube length but there is no need to go in that direction. The only things that matter are the three contact points and how they relate to each other.

It seems like getting good solid numbers is part science, part art, and part black magic but itʼs very simple. The trick is to reference the three points (bottom bracket, saddle, and handlebar at stem) to each other and the easiest way to do that is by referencing these points to the floor and a wall.

I will throw this caveat out there before outlining the process if you are using different saddles, handlebars or crank lengths then you can have a hard time and it will be an apples-to-oranges comparison at best. That said
We need to start with a ʻhome baseʼ if you will and that will be the bottom bracket everything springs from the bottom bracket. Hereʼs what you can do -


1) Saddle height most riders know how to get this number but just in case...you want to measure from the BB center along the center of the seat tube and up to the upper surface of the saddle. Itʼs important that the tape be held inline with the seat tube centerline to get a solid number. Write down this number.

2) Saddle setback or fore/aft position typically people try to do this with a plumb bob hung off the nose of the saddle but this can be really tricky to get just right. The following way is much easier and more accurate.
a. First you want to place the bike standing next to a wall and back it up into a corner so that the rear tire is up against the adjacent wall. This will allow you to measure the saddle and BB to the rear wall. So first hold your tape horizontally between the rear wall and the BB center. Itʼs key that the tape is more or less horizontal but you need to be WAY off level for this to have a large effect. Get it fairly level and you are good to go. Write this number down.
b. Next measure horizontally from the nose of the saddle rearward to the wall holding the tape horizontally. Write this number down.
c. Now simply subtract the saddle-to-wall number from the BB-to-wall number and you have an accurate saddle setback number.

3) Cockpit or saddle-to-bar reach This one couldnʼt be simpler. Hold the end of the tape against the nose of the saddle and measure to the bar center where it enters the stem. The bar centerline is typically the slot between the main part of the stem and the faceplate so itʼs easy to find. Write this number down.

4) Bar drop or saddle-to-bar drop this time we are going to reference to the floor to get an accurate number.
a. First measure from the saddle straight down to the floor. For the sake of consistency you should measure from the lowest part of the center of the saddle (often called the hammock) to the floor. Write this number down.
b. Now measure from the center of the handlebar where it enters the stem directly to the floor and write this number down.
c. Now to get the actual bar drop do the math and figure out the difference of the saddle-to-floor compared to the bar-to-floor numbers. This is your saddle to bar drop.

Now you have four numbers (saddle height, setback, cockpit length and bar drop) that will allow you to set up bike B to fit just like bike A...or at a minimum have a real understanding how the two differ. Note that there isnʼt a single reference to seat tube or top tube length, head tube length or seat angle as these really do confuse the matter when trying to define the location of the three contact points.

With these four numbers in hand whatʼs next? Well if you are trying to set up bike B exactly like bike A youʼll no doubt need to start moving stuff and I suggest you do it in the same order that you took the above measurements. Start with saddle height and then move to setback and then to cockpit and finally bar drop. Note that you will no doubt need to go back and tweak one setting after making the second adjustment and it could take a few rounds of back and forth to get them both to be right. Setting the cockpit length can mess with the drop and visa-versa so you will need to no double check, tweak, and recheck a number of times to get everything in the right place. One place many will have issues it with the cockpit length and this is because stems come in fixed lengths typically in 10 mm increments so judgment calls often need to be made and you need to decide if you want to be a few millimeters longer or shorter.

I hope you find this helpful in getting an accurate set up of your contact points. Once you get used to it you can set up bikes very quickly this way even rental bikes can be tweaked quickly in a hotel room easily using this method. There are no doubt other ways to do this I find this works well for me. I hope it does for you.
Most of us have more than one bike and I often hear from riders who say that they have a hard time getting two bikes to have the same exact contact points. The three contact points, and how they relate to one another, are what determine how one sits on the bike...they are the only things that matter. It can be easy to get confused trying to balance a different seat angle and a different top tube length but there is no need to go in that direction. The only things that matter are the three contact points and how they relate to each other.

It seems like getting good solid numbers is part science, part art, and part black magic but itʼs very simple. The trick is to reference the three points (bottom bracket, saddle, and handlebar at stem) to each other and the easiest way to do that is by referencing these points to the floor and a wall.

I will throw this caveat out there before outlining the process if you are using different saddles, handlebars or crank lengths then you can have a hard time and it will be an apples-to-oranges comparison at best. That said
We need to start with a ʻhome baseʼ if you will and that will be the bottom bracket everything springs from the bottom bracket. Hereʼs what you can do -


1) Saddle height most riders know how to get this number but just in case...you want to measure from the BB center along the center of the seat tube and up to the upper surface of the saddle. Itʼs important that the tape be held inline with the seat tube centerline to get a solid number. Write down this number.

2) Saddle setback or fore/aft position typically people try to do this with a plumb bob hung off the nose of the saddle but this can be really tricky to get just right. The following way is much easier and more accurate.
a. First you want to place the bike standing next to a wall and back it up into a corner so that the rear tire is up against the adjacent wall. This will allow you to measure the saddle and BB to the rear wall. So first hold your tape horizontally between the rear wall and the BB center. Itʼs key that the tape is more or less horizontal but you need to be WAY off level for this to have a large effect. Get it fairly level and you are good to go. Write this number down.
b. Next measure horizontally from the nose of the saddle rearward to the wall holding the tape horizontally. Write this number down.
c. Now simply subtract the saddle-to-wall number from the BB-to-wall number and you have an accurate saddle setback number.

3) Cockpit or saddle-to-bar reach This one couldnʼt be simpler. Hold the end of the tape against the nose of the saddle and measure to the bar center where it enters the stem. The bar centerline is typically the slot between the main part of the stem and the faceplate so itʼs easy to find. Write this number down.

4) Bar drop or saddle-to-bar drop this time we are going to reference to the floor to get an accurate number.
a. First measure from the saddle straight down to the floor. For the sake of consistency you should measure from the lowest part of the center of the saddle (often called the hammock) to the floor. Write this number down.
b. Now measure from the center of the handlebar where it enters the stem directly to the floor and write this number down.
c. Now to get the actual bar drop do the math and figure out the difference of the saddle-to-floor compared to the bar-to-floor numbers. This is your saddle to bar drop.

Now you have four numbers (saddle height, setback, cockpit length and bar drop) that will allow you to set up bike B to fit just like bike A...or at a minimum have a real understanding how the two differ. Note that there isnʼt a single reference to seat tube or top tube length, head tube length or seat angle as these really do confuse the matter when trying to define the location of the three contact points.

With these four numbers in hand whatʼs next? Well if you are trying to set up bike B exactly like bike A youʼll no doubt need to start moving stuff and I suggest you do it in the same order that you took the above measurements. Start with saddle height and then move to setback and then to cockpit and finally bar drop. Note that you will no doubt need to go back and tweak one setting after making the second adjustment and it could take a few rounds of back and forth to get them both to be right. Setting the cockpit length can mess with the drop and visa-versa so you will need to no double check, tweak, and recheck a number of times to get everything in the right place. One place many will have issues it with the cockpit length and this is because stems come in fixed lengths typically in 10 mm increments so judgment calls often need to be made and you need to decide if you want to be a few millimeters longer or shorter.

I hope you find this helpful in getting an accurate set up of your contact points. Once you get used to it you can set up bikes very quickly this way even rental bikes can be tweaked quickly in a hotel room easily using this method. There are no doubt other ways to do this I find this works well for me. I hope it does for you.

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Mick F
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Re: Measuring saddle Fore/Aft on Different Bikes

Postby Mick F » 9 Sep 2019, 3:18pm

I spent a while with Mercian measuring accurately, then recreated the exact riding position on Moulton.

I recorded them, as no doubt I'll need the figures again some day.

Critical Dimensions
Top of Handlebars to Floor 95cm
Bottom Bracket to Top of Saddle 75cm
Saddle Nose to Handlebars 49cm
Saddle behind Bottom Bracket 58mm

The handlebars are nigh on identical, and the saddles are both Brooks Team Pro.
Mick F. Cornwall

NetworkMan
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Joined: 25 Aug 2014, 11:13am
Location: South Devon

Re: Measuring saddle Fore/Aft on Different Bikes

Postby NetworkMan » 9 Sep 2019, 3:33pm

Mick F wrote:I spent a while with Mercian measuring accurately, then recreated the exact riding position on Moulton.

I recorded them, as no doubt I'll need the figures again some day.

Critical Dimensions
Top of Handlebars to Floor 95cm
Bottom Bracket to Top of Saddle 75cm
Saddle Nose to Handlebars 49cm
Saddle behind Bottom Bracket 58mm

The handlebars are nigh on identical, and the saddles are both Brooks Team Pro.

Only valid if BBs are the same height above floor of course.

Trouble I have is that all three saddles are different! Still don't know which I like best; one day one,one day another!

gazza1286
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Joined: 6 Sep 2018, 11:11am

Re: Measuring saddle Fore/Aft on Different Bikes

Postby gazza1286 » 9 Sep 2019, 6:22pm


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RickH
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Location: Horwich, Lancs.

Re: Measuring saddle Fore/Aft on Different Bikes

Postby RickH » 9 Sep 2019, 7:07pm

I may be an exception but there are slight problems with the first 2 of these.

MikeDee wrote:1) Saddle height most riders know how to get this number but just in case...you want to measure from the BB center along the center of the seat tube and up to the upper surface of the saddle. Itʼs important that the tape be held inline with the seat tube centerline to get a solid number. Write down this number.

On the rear of my Circe Helios tandem the rear bottom bracket is not at the bottom of the rear seat tube, it is located behind it, with a very shallow seatpost angle. The "virtual" seatpost angle is therefore different depending on the height of the saddle.

MikeDee wrote:2) Saddle setback or fore/aft position typically people try to do this with a plumb bob hung off the nose of the saddle but this can be really tricky to get just right. The following way is much easier and more accurate.
a. First you want to place the bike standing next to a wall and back it up into a corner so that the rear tire is up against the adjacent wall. This will allow you to measure the saddle and BB to the rear wall. So first hold your tape horizontally between the rear wall and the BB center. Itʼs key that the tape is more or less horizontal but you need to be WAY off level for this to have a large effect. Get it fairly level and you are good to go. Write this number down.
b. Next measure horizontally from the nose of the saddle rearward to the wall holding the tape horizontally. Write this number down.
c. Now simply subtract the saddle-to-wall number from the BB-to-wall number and you have an accurate saddle setback number.

We live in an old stonebuilt property where most, if not all, of the walls are not vertical & there could be a cm or more wall error between bb & saddle heights. The floors aren't completely level either! :?

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NATURAL ANKLING
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Re: Measuring saddle Fore/Aft on Different Bikes

Postby NATURAL ANKLING » 9 Sep 2019, 7:43pm

Hi,
viewtopic.php?f=5&t=129884&p=1353376&hilit=saddle+fore+aft#p1353376

"Re: New frame. Steep seat angle. Unrideable.

Postby Brucey » 12 May 2019, 9:11am

FWIW it is incredibly difficult to measure seat angles exactly, and anyway if there is another error, e.g. fork length isn't quite right (eg if the lower part of the headset is a different height) then the top tube won't be horizontal (or at the intended angle) regardless. This means that you can (with difficulty) measure the seat tube angle to the nth degree and it is still 'wrong' from a riding position POV anyway.

The good news is that you don't need to measure the seat tube angle (knowing it just gets you in the right ballpark); you just need to get the saddle and the handlebars in the right place, which is not quite the same thing (it depends on the saddle and seat pin being used). If you look at most of my bikes you will find a small mark on the top tube; this is a 'top tube sighting mark' (TTSM) and allows you to get the saddle and handlebars in the right place without measuring any tricky angles, even if the whole bike isn't sitting at the correct angle for some reason.

The method is to choose a flat surface and to drop a plumb line to the exact centre of the BB. Mark the top tube where the line intersects it. Now turn the bike 180 degrees so that the tyres sit in the exact same positions on the floor/ground and repeat. If the ground/floor was perfectly flat the two marks will coincide; if not, take an average. This is your TTSM. (Marker pen will last long enough to get set up but if you think you might tinker in the future, some kind of more permanent mark isn't a bad idea.)

Now you can offer up a ruler to the saddle nose (or cantle plate with a leather saddle) and sight three points (like a gunsight) being 1) the BB centre, 2) the TTSM, and 3) the scale on the ruler. You can set the saddle setback (for any one brand of saddle) to within ~2mm quite easily. Then set the reach from the saddle to the handlebars. Double-check that the saddle height hasn't been changed by a large fore-aft adjsutment, and if it has, adjust the saddle setback again to suit.

This method allows you to transfer a riding position from one bike to another with a high degree of accuracy, even if you don't know the angle of the seat tube on either frame.

FWIW the seat angle and the type of seat pin/saddle you use all make a substantial difference; IME if you intend to use a brooks leather saddle this has (vs some other saddles) a similar effect on the range of saddle adjustment as about two degrees increase in seat tube angle, or a (real) 20mm change in seat pin layback; once you realise this, the typically slack seat angles of yesteryear make a lot more sense....

cheers

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Witterings
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Re: Measuring saddle Fore/Aft on Different Bikes

Postby Witterings » 10 Sep 2019, 8:47am

Cheers all .... some useful responses / methods .... I may give them all a try just to compare and see which works best and is the easiest to do :)

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Mick F
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Re: Measuring saddle Fore/Aft on Different Bikes

Postby Mick F » 10 Sep 2019, 10:00am

NetworkMan wrote:
Mick F wrote:I spent a while with Mercian measuring accurately, then recreated the exact riding position on Moulton.

I recorded them, as no doubt I'll need the figures again some day.

Critical Dimensions
Top of Handlebars to Floor 95cm
Bottom Bracket to Top of Saddle 75cm
Saddle Nose to Handlebars 49cm
Saddle behind Bottom Bracket 58mm

The handlebars are nigh on identical, and the saddles are both Brooks Team Pro.

Only valid if BBs are the same height above floor of course.
As it happens, they aren't, but the handlebar height (for me at least) doesn't have to be correct spot on.

Also, the frame geometry of the two is fairly similar - so I could transfer with confidence, rather than trial and error.
Mick F. Cornwall

pwa
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Re: Measuring saddle Fore/Aft on Different Bikes

Postby pwa » 10 Sep 2019, 10:06am

All positional measurements on a bike relate to the bottom bracket centre. It is in a fixed position and your pedalling action rotates around it, so that is your starting point. You have to be sat right in relation to the bottom bracket. The saddle will be a specific distance back from it, and a specific height above it.

Witterings
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Re: Measuring saddle Fore/Aft on Different Bikes

Postby Witterings » 10 Sep 2019, 10:57am

pwa wrote:All positional measurements on a bike relate to the bottom bracket centre. It is in a fixed position and your pedalling action rotates around it, so that is your starting point. You have to be sat right in relation to the bottom bracket. The saddle will be a specific distance back from it, and a specific height above it.


This is "a given" ... the question is in knowing that - how do you most accurately replicate it from one bike to another.

pwa
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Re: Measuring saddle Fore/Aft on Different Bikes

Postby pwa » 10 Sep 2019, 12:13pm

Witterings wrote:
pwa wrote:All positional measurements on a bike relate to the bottom bracket centre. It is in a fixed position and your pedalling action rotates around it, so that is your starting point. You have to be sat right in relation to the bottom bracket. The saddle will be a specific distance back from it, and a specific height above it.


This is "a given" ... the question is in knowing that - how do you most accurately replicate it from one bike to another.

Well, the fore-aft thing can be tricky. First, are we dealing with two identical saddles? If we are not, the position on the saddle to measure from is not the tip of the nose, but the sit bone position. And that can mean guestimation. If you have two identical saddles you can use the tip of the nose, which is a clearer point to measure from. Whichever you use, you have to line a spirit level up with the bottom bracket centre and measure from that horizontally to the point on the saddle you have chosen, and replicate that measurement from one bike to the other. You also need to get the height above the bottom bracket the same, which might for instance mean resting a cane with a mark on it on the top of the bb shell, then using the spirit level again to check saddle height. An extra pair of hands might be welcome for all this.

You can of course also measure saddle height above bb centre by measuring saddle height above a truly flat floor then deducting the height of the bb centre above the floor.

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NATURAL ANKLING
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Re: Measuring saddle Fore/Aft on Different Bikes

Postby NATURAL ANKLING » 10 Sep 2019, 2:14pm

Hi,
Witterings wrote:
pwa wrote:All positional measurements on a bike relate to the bottom bracket centre. It is in a fixed position and your pedalling action rotates around it, so that is your starting point. You have to be sat right in relation to the bottom bracket. The saddle will be a specific distance back from it, and a specific height above it.


This is "a given" ... the question is in knowing that - how do you most accurately replicate it from one bike to another.


Brucey's method I posted up post is the simplest and most accurate by far.
Because you don't need a spirit level (which has also to be rotated thru 180 to be accurate) which is always a bit touch and go (you need three hands) and the bike needs to be level too!
It will also work on a sloping top tube (who hasn't one of these today) but there will be a small error, just make sure that the two lines on TT are as close together as possible (you can place a book etc under a wheel at low end of floor), the error will be in the region of a third of the two lines gap, so very small.

Bob's you're uncle :)
If You Don't Try You Don't Do.....Don't Do You Don't Get...I'm Still Trying....Well Very..
You'll Find Me At The Top Of A Hill...............Somewhere...After Dark..

Witterings
Posts: 333
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Re: Measuring saddle Fore/Aft on Different Bikes

Postby Witterings » 10 Sep 2019, 2:45pm

OK ... so a bit of ingenuity

I have one of those laser spirit levels for putting pictures / shelves up straight and have attached it to the top of a camera tripod so the line is vertical and will now lean the bike against a wall and tape / blue tack a piece of paper in line with the seat post and mark where the line falls on the paper for both the bottom bracket and wherever I deem to be the best place on the saddle and measure the distance between .... and then do the same with the 2nd bike.

I'd always thought the line wasn't quite level on the laser when the bubble was centered and put a plumb line down the wall to double check it and I was right it's not but at least can line it up perfectly with the plumb line.
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