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Can saddle setback confirm if a frame is too small or large?

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Can saddle setback confirm if a frame is too small or large?

Old 06-04-21, 02:16 AM
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willhub
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Can saddle setback confirm if a frame is too small or large?

I've ignored the "KOPS" method and instead focused on the center of gravity, so I assume you want your center of gravity where it's right between the two wheels.

So I've attained this but I've had to slide my saddle all the way backwards until it's at the max limit on the rail, setback must be about 7cm from BB now.

This is a 52cm frame, I assume if I was on a 54cm frame, my center of gravity would be about 2cm further forward, so saddle 2cm closer to BB as the back wheel would essentially be 2cm further back?

So essentially a too small frame can mean that you might have to be setback too far so your knee is actually not in a good place in relation to the pedal thus frame size is incorrect?

It would be excellent (literally) if this is the case because I'd have to sell my Trek and get a different bike.
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Old 06-04-21, 05:57 AM
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Sorry, I'm not in agreement with your premise that balancing between the two wheels is how best to fit yourself on a bike. For bicycle fit the centre of the bottom bracket (BB) is the balance point and the zero, zero reference point for all measurements.
This is a fixed reference point. The centre of the two wheels is all over the place and subject to numerous variables that have nothing to do with how the rider fits on a bike.
I have heard of frame builders taking the weight distribution on each wheel into consideration in a frame design yet the is secondary rather than of primary importance to fit.
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Old 06-04-21, 06:02 AM
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The standard weight distribution figures I've usually seen quoted for racers on drop bar bikes are 55% rear/45% front.
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Old 06-04-21, 06:11 AM
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Interesting thread, I will be watching this. I'm no bike fitter but through my own trial and error I have found it to be a balance between center of gravity, how strong your pedal stroke is (ie riding style) and reach/flexibility. Very non scientific and definitely not the answer because how do you measure that. It suggests that it would be different for everyone because we have our own riding styles, flexibility, etc... So I'm here to learn as well.

Originally Posted by Trakhak View Post
The standard weight distribution figures I've usually seen quoted for racers on drop bar bikes are 55% rear/45% front.
Interesting, because that would suggest that the COG is even more to the rear than the actual balance point. I wonder if those figures are while riding hard? For example the harder your pedal stroke is or the harder you ride...it unweights your hands a certain level.
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Old 06-04-21, 06:37 AM
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Wouldn't a 54cm bike as opposed to a 52cm bike mean less setback for same center of gravity?

I'm trying to determine if a bike could be deemed too small due to the ability to get balanced or not. Also how does geometry effect this?

My Kinesis is more of a "traditional" bike, no curved, straight top tube, alu frame, whilst the Trek is curved top tube, wider, generally seems a real pig to get a good position compared to the Kinesis whilst I haven't found my "center" on that one, the Kinesis seems more "laid back" as in much easier to get some reliable position.

Originally Posted by AnthonyG View Post
Sorry, I'm not in agreement with your premise that balancing between the two wheels is how best to fit yourself on a bike. For bicycle fit the centre of the bottom bracket (BB) is the balance point and the zero, zero reference point for all measurements.
This is a fixed reference point. The centre of the two wheels is all over the place and subject to numerous variables that have nothing to do with how the rider fits on a bike.
I have heard of frame builders taking the weight distribution on each wheel into consideration in a frame design yet the is secondary rather than of primary importance to fit.
​​​​​​​
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Old 06-04-21, 07:22 AM
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Originally Posted by willhub View Post
Wouldn't a 54cm bike as opposed to a 52cm bike mean less setback for same center of gravity?

I'm trying to determine if a bike could be deemed too small due to the ability to get balanced or not. Also how does geometry effect this?

My Kinesis is more of a "traditional" bike, no curved, straight top tube, alu frame, whilst the Trek is curved top tube, wider, generally seems a real pig to get a good position compared to the Kinesis whilst I haven't found my "center" on that one, the Kinesis seems more "laid back" as in much easier to get some reliable position.
Sorry, my view is that you have taken an irrelevant detour. When talking about "fit", the reference point to everything is the centre of the BB. Balance between the wheels is an issue that can effect the handling of the bike, yet its not relevant to "fit". When your talking "fit", then its all about the balance over the BB.

EDIT: can a frame be too small? Yes, but its usually a quite simple factor of the smaller frame having a steeper seat tube angle which pushes the rider forwards. Its the seat tube angle from the BB which is key here.
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Old 06-04-21, 07:53 AM
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Why does a steeper seat tube angle push a rider forwards? Assume you've got 2 bikes with a totally different frame geometry, but you've got setback, reach, saddle height set, does the frame have any bearing on the comfort and position?

If I have a 52cm bike, a 54cm bike, I've got the saddle setback exactly the same as well as reach, saddle height etc.., would not balance not be off on one of the bikes with one been longer than the other?

Or can one ignore the frame if their reach, saddle height, setback, bar height is all exactly the same?
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Old 06-04-21, 08:02 AM
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If you can get the same saddle height and setback on a 52cm frame and a 54cm frame then its the same, but remember that this is all being measured from the centre of the BB.
Smaller frames next to always have a steeper seat tube angle (they shouldn't yet it a common fudge) so the saddle starts out with less saddle setback and in order to get it in the same position as a larger frame(with a more relaxed seat tube angle) you will need a seatpost with more setback and/or set the seat further back on the rails.

How have you measured saddle setback?
This measurement is once again, measured from the centre of the BB and it takes a deliberate effort to get it the same on different bikes.

EDIT: If the seat and handlebar positions on two seperate bikes is indeed exactly the same in relation to the BB, then you really should balance exactly the same on both bikes. How you balance over the wheels however will differ and this is ONE factor that will effect how the bike handles.

Last edited by AnthonyG; 06-04-21 at 08:05 AM.
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Old 06-04-21, 09:05 AM
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I'm measuring setback with a plum line, so if the setback is 100% the same the center of gravity will be 100% the same despite one bike having longer top tube?
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Old 06-04-21, 09:30 AM
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If you need more setback than the manufacturer provided for that bike, then you might just have the wrong type of bike for the way you ride a bike. Not so much the wrong size bike.
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Old 06-04-21, 10:32 AM
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Puzzling, it just seems I need like 2cm more setback saddle tip to BB on my Trek compared to the Kineses to feel stable.

I can't accelerate as fast but it feels more comfy, not sure what it'll be like on hills yet.
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Old 06-04-21, 12:07 PM
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I’ve recently experimented with the same ideas! I have 3 road bikes to compare measurements on. My fit on each bike, relative to the bottom bracket, is within 0.5 cm.

Putting a bathroom scale under one wheel with a 2”x4” under the other, I found between 40/60 and 45/55 percent weight distribution front/rear. But that was very hard to measure! If I move 1/4” to the front or back while seated on the saddle, it changes ~3 percent.

The bike with a 1 cm longer top tube and wheelbase, my fit is roughly the same but I put more weight on the front wheel. My more compact bike has slightly more weight on the rear.
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Old 06-04-21, 12:31 PM
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I wouldn't use Saddle setback to compensate for frame size. The saddle set back should be used to correctly alien our power stroke with the crankset. To compensate for frame size swap out the handlebar stem, with a different length unit.

Once you attain the correct saddle position the weight distribution is what it is and varies with the frames geometry, handlebar and stem. On my comfort bike a lot of my weight is on the seat, enabling me to climb steep hills without standing on the pedals. I'm guessing a 40/60% split vs perhaps a 45/55% for most bikes.

I used KOPS as a baseline for my saddle position and it worked perfectly. No more knee pain. KOPS may not be perfect for everyone but it is as good a starting point as any other method IMO. Weight distribution doesn't enter into it.

Last edited by xroadcharlie; 06-04-21 at 12:40 PM.
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Old 06-04-21, 01:04 PM
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To me after some testing it seems the best balance is when my setback is at such a point I'm 1cm behind KOPS.

At this if I lift my hands off the bars I don't fall forward my cadence doesn't increase drastically and I don't seem to slip forward.

But I'm concerned that been 1cm behind this KOPS perhaps may effect my power output, it certainly does accelerating.

Unsure about hills
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Old 06-04-21, 01:17 PM
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Originally Posted by willhub View Post

But I'm concerned that been 1cm behind this KOPS perhaps may effect my power output, it certainly does accelerating.

Unsure about hills
You don't need to be constrained to one spot on the saddle. You can slide yourself forward when you need to. If I'm not riding hard, I'll sit in one place but if I'm getting after it I'll slide forward while I lower my position.
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Old 06-04-21, 05:03 PM
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I never understood why the position of the saddle's nose was a reference point. The relevant reference point on the saddle is where one rides - that is, where one's sitbones or pubic bone fall. A bone can be placed at varying distances from the nose, but the bone will fall approximately the same distance from a fixed reference point if the bike is set up to the rider's preference.

I also don't understand the focus on COG as determined by ability to ride no-hands without falling over. I've got greater core strength now than a year ago and in October of 2020, at the end of last year's riding season. With luck, good health, and a good bit of riding, my core strength will be higher in October of 2021. I can lean over farther now than a year ago. In 4 months, with good health and a good bit of riding, I'll be able lean over still farther without falling. My COG changes, and I imagine everyone's does.

Bike fit captures a moment in time, doesn't it? But life is change and adaptation.

If you're worried about power, my reco is to get a power meter. If you can't afford a power meter - it wouldn't fit into my budget at this point - I recommend riding more, monitoring what you can monitor, and aiming to improve what you can measure until your budget can include a power meter.
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Old 06-04-21, 06:03 PM
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I'm focusing on center of gravity because it seems to avoid my hands going numb it needs to be at that point. When I get that center of gravity right my hands seem to have less weight on them
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Old 06-04-21, 06:13 PM
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Originally Posted by willhub View Post
I'm measuring setback with a plum line, so if the setback is 100% the same the center of gravity will be 100% the same despite one bike having longer top tube?
If the setback measured with a plumb line is the same and you have positioned the handlebars the same with a shorter stem then yes, your balance over the BB is the same.
"Fit" is all about your balance over the BB but yes your balance over the wheels can be different, and you can prefer one over the other, yet I'm not calling this a "fit" issue.

Originally Posted by willhub View Post
To me after some testing it seems the best balance is when my setback is at such a point I'm 1cm behind KOPS.
At this if I lift my hands off the bars I don't fall forward my cadence doesn't increase drastically and I don't seem to slip forward.
But I'm concerned that been 1cm behind this KOPS perhaps may effect my power output, it certainly does accelerating.
Unsure about hills
My experience is that being behind KOPS puts me in a more comfortable position but it DOES effect my power output. This is why KOPS is considered a compromise position between power and comfort.
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Old 06-05-21, 01:45 AM
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My Trek and Kineses at the same settings before I looked at balance, my Kineses was more stable than the trek slightly. The Trek always veered to one side when setting off and it was harder to keep it straight. Trek has a 11cm stem, Kineses has a 9cm stem to get the same fit.
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Old 06-05-21, 04:20 AM
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There are SOO many things going on with these two bikes and the only way to get on top of what's going on is to carefully seperate out different issues yet the problem I see is that you are conflating them altogether.
The only way you can hope to get on top of it is to know that the centre of the BB is the one and only zero, zero point that everything else is measured to.
I don't want to say that your bad at measuring yet at this point I would have to have the bikes in front of me and measure them myself to know.
Without having the bikes in front of me, I cant say this for certain, yet my expectation is that you would need to make quite an effort (probably getting yourself a special set back seatpost) in order to get the saddle on the smaller frame in the same position as the saddle on the larger frame and at this point I don't think you have.
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Old 06-05-21, 05:05 AM
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I measured from rear of saddle to wall and center of BB to wall, then I did the plum line from tip of saddle and cross checked both measurements.

I also placed tape on the frames where the distance from the BB is.

I can easily look at the BB been the balance point I can understand that considering we're rotating our feet around it, it's a pivot point almost.

I'm going to focus on the Kineses for the moment on the assumption that hopefully the Trek is too small for me.
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Old 06-05-21, 05:17 AM
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Originally Posted by willhub View Post
I measured from rear of saddle to wall and center of BB to wall, then I did the plum line from tip of saddle and cross checked both measurements.
I also placed tape on the frames where the distance from the BB is.
I can easily look at the BB been the balance point I can understand that considering we're rotating our feet around it, it's a pivot point almost.
I'm going to focus on the Kineses for the moment on the assumption that hopefully the Trek is too small for me.
Sorry, I'm not 100% sure of what your trying to describe to me.
Assuming the same saddle, the way you measure saddle setback is to place a rule/scale at the tip of the saddle, in the horizontal plane, measuring forwards towards the head tube, and then drop a plumb bob down to intersect with the centre of the BB. Then measure the distance from the centre of the BB back to the tip of the saddle. You have to do this carefully as the plumb bob will sway around.

It's perfectly normal and standard practice in the cycling World for smaller frames to have a steeper seat tube angle which means that at the same saddle height, the smaller frame will have less saddle setback than a larger frame. It's very normal to have to use a set back seat post on a smaller frame to get the saddle setback to be the same as on a larger frame.
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Old 06-06-21, 01:44 PM
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The angle of the seat tube might be significantly different placing the bb and saddle in a different relationship to each other from the other bike.

You mentioned you are trying to get pressure off your hands for numbness. That's not just balance doing that to you. Reach and bar height play a part too. I just recently changed to narrower bars than what conventional wisdom would say are appropriate to me and eliminated some wrist numbness I was having on long rides.

Might help others help you with your numb hands if you describe that actual issue that has you chasing just one possible solution that might be better addressed with another solution.
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Old 06-13-21, 10:34 PM
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Answer to the OP title is "NO." I have a frame perfectly sized for me which needs a 30mm setback post and really probably more setback to get the balance right. I still have too much weight on my hands. It all depends on one's personal body proportions. There are no rules, not really. KOPS is an interesting fitting standard but doesn't apply to everyone. There's no mechanical reason for KOPS being a good thing. It's just a thing which sometimes gives good balance on the bike. A person with a big chest and shoulders will need more setback. It's not simple. Just go by feel. Note that having the seatpost clamp far from the center of the saddle rails can lead to an embarrassing saddle rail failure, an end of ride kind of thing.
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Old 06-16-21, 12:16 PM
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I've now got my seat pushed backwards to its max as specified on rails, that combined with an apparent reduction to saddle height from 73cm to 72.5 and the movement backwards of my cleats I appear to have some extra stability. After rides my hamstrings and glutes appear to ache a bit more and my power readings are way down which partly is attributed to my current fitness. But I don't seem to be going hugely slower.

If anything my power is down on the flats but I can still keep a steady 320W on ascents.

Mech pain seems a problem, no lower back ache today however. Might have to flip my stem to increase the bar heights by 0.5 but I'm not that aero as it is at the moment.

Hand wise I'm getting numbness in the but below the thumb and it can radiate into the fingers.

Seems to be a little less weight on my hands but it's not perfect.
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