What dimension do I need?

[Bob Ross]

LOL, well, I tried asking this question over on the BikeForums.net fitting subforum and of course, like nearly every thread on BikeForums.net it quickly devolved into a bunch of second-guessing, misguided "advice" on an unrelated issue, and a bunch of folks whose reading comprehension scores in the lowest percentile.

So lemme give it a shot here: I've had several well-respected framebuilders
-- most of whom are regular contributors here --
tell me that the most critical dimensions for getting a new bike to match the fit of one you already have and like are

- saddle height (top of saddle to center of bottom bracket, measured along the seat tube)
- saddle setback (measured with a plumb bob from the front of the saddle nose, and then measured horizontally from the plumb line to the center of the bottom bracket)
- saddle-to-bar drop (measured by subtracting the bar tops-to-floor measurement from the saddle top-to-floor measurement)
- saddle nose to center of stem face (i.e., the seam between the two halves that bolt together)
- saddle nose to tips (tops?) of the lever hoods (measured on an angle)

But all of ^^^that is based on the presumption that you're using the same make/model saddle on both bikes.

What if you aren't?

What's the missing information that would help set up two bikes so the contact points are in the same location if they don't have identically-sized saddles?

My gut tells me that knowing the saddle nose to the center of the seatpost clamp dimension would allow one to compensate for the different sized saddles...but I suck at math so I can't quite figure out how, or why, or even why that might not be correct.

Anyone know what I'm looking for?

Thanks.

[sheepdog84]

Originally Posted by Bob Ross

LOL, well, I tried asking this question over on the BikeForums.net fitting subforum and of course, like nearly every thread on BikeForums.net it quickly devolved into a bunch of second-guessing, misguided "advice" on an unrelated issue, and a bunch of folks whose reading comprehension scores in the lowest percentile.

ahhh, old BF.net... we hardly knew ye

Jokes aside, I've been working through the same exact thing. An indoor trainer makes for a great fit experimentation lab, but still ends up being tricky, so I hear ya there!

Originally Posted by Bob Ross

What's the missing information that would help set up two bikes so the contact points are in the same location if they don't have identically-sized saddles?

My initial thought here are X/Y coordinates of the saddle, and secondarily, X/Y coordinates of the handlebars in relation to the bottom bracket. I think most fits miss out on this, and assume that you're using the same components between bikes so they go off that. There's a tool that can help you measure this, but it's expensive and a laser w/ a metric measuring tape + simple maths will do the job just as well.

There's a few caveats here. I assume you're working with saddles of different lengths, and possible different thicknesses as that has potential to throw off any number of your measurements. That's also to say that the handlebar width influences your reach as well, and if that's different at all between bikes. Even different groupsets have different lengths of hoods, which can impact your fit as well. The seat tube angle between the bikes will also in part dictate how fore/aft you can get your saddle with a given seatpost.

That list of complexities can go on and on, so I think I'll try to keep this simple for discussion's sake:

My current conclusion is that the goal is to have the seating area of the saddle result in the same "fit" between your different components.

So for example, A saddle with a shorter nose will give you a greater "effective setback" measurement to get your sitbones in the same spot as you would with a longer saddle.

You definitely *don't* want to move the saddle forward in an attempt to reduce the reach (saddle tip to any handlebar measurement) - as it has the potential to mess up the relationship between your pelvis and the bottom bracket/cranks.

A few side notes, I have is that saddle construction and shape will change the way you interact with it as well, such as tilt, fore/aft, etc.. For example, some saddles allow you to roll your pelvis further forward without too much frontal pressure, and others are a nightmare for trying to accomplish the same thing.

I have my dimensions pretty much nailed down- but I've been experimenting between a Selle Italia Boost SLR, measuring 248mm long, and my original fit was done with a S-Works Toupe, measuring 275mm or so. Not a huge difference, but I pretend that "if" the Boost had a 275mm length, I would get the same reach measurements as the Toupe. Comfort wise, I think I prefer the Boost over the Toupe - the large cutout just works for me. The Boost is narrower at 145mm, and the S-Works is 155mm (varying a bit OT here)

I would recommend picking up a cheap-o self-leveling laser, and a metric tape measurer and start documenting your fit between bikes.

Keep in mind that getting them to be precise to exact fit numbers-wise might not yield the same results (as hinted at earlier), as most people can tolerate a particular range in any given direction.
The real fun begins when you can start to feel if something is off, and then correlating that feeling to what it is in actuality

Overall, you'll learn a lot in the process and will definitely help you understand your fit and what works for you.

Hopefully some of that makes sense. Let me know if this helps.
Aubrey

[Peter P.]

If the saddles aren't the same, then you're going to measure your KOPS on the existing bike, and duplicate that measurement on the bike being set up.

It doesn't matter whether you believe in KOPS for setting your position; all you're interested in is the measurement. Every saddle shape is different and you will first find where you're comfortable on saddle "B" then duplicate the measurement you would have taken on the bike with saddle "A".

[e-RICHIE]

Originally Posted by Bob Ross

But all of ^^^that is based on the presumption that you're using the same make/model saddle on both bikes.

What if you aren't?

What's the missing information that would help set up two bikes so the contact points are in the same location if they don't have identically-sized saddles?

Well, if you aren't - just make a note of that and send both dimensions. I often deal with clients whose dimension J on my form (the distance from the saddle nose to the stem clamp, measured on the diagonal, along with dimension H, the nose setback) doesn't seem to align with the rest of it. So I simply ascertain that they're using a conventional 28.5mm saddle and, if they aren't the actual length and maybe brand and model helps to clarify the details.

[fourflys]

so, one thing I was told a while back when trying to measure your saddle fore/aft is to measure 120mm from the rear of the saddle (measure 120mm from the rear and then then measure from that point to your bars/etc), as that's where most saddles are designed to sat upon.. and that would negate any variance with the nose of the saddle..

hope what I said makes sense