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Most important fit measurements?

Old 01-28-23, 10:11 PM
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Most important fit measurements?

Looking to buy a new ride and am happy with the fit of my current ride so i'd like to keep the fit as similar as possible. in comparing the geometry of my current ride i find that naturally i am one of those in betweeners. i have this unfounded impression that the two most important measurements would be stack and reach. would that be a correct assessment? or are there other considerations i should also add emphasis to like tube angles or tube lengths?
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Old 01-28-23, 10:34 PM
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Also look at chainstay length and front center. (Front center - distance BB to front hub.) At a given fit, your weight is at some (small) distance in front of the bottom bracket. It is then distributed between the front and rear wheels depending on the distances of that weight center to the respective hubs. A bike that changes this distribution will change that weight balance.

You can find your weight center and weight distribution using just a typical bathroom scale and old fashioned phonebooks. (Boards, bricks ...) Place the scale beside a hallway wall with the same height's worth of phone books your bike's wheelbase away. Set bike on scale and book. Hop on. Using the wall, sit in your usual position and read (or better, have someone else read) the scale, then swap scale and book and repeat. Now you have the weight distribution with simple math and with little more, the distance forward of the BB.

If you are satisfied with your bike's handling, especially how "planted" your wheels feel on turns, these are numbers to remember.
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Old 01-28-23, 11:56 PM
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Check this site out, you can compare geometry to your present bike. If your bike or the bike you are looking for is not there, you can put the metrics in. I used it when I was looking for a new bike and found it very helpful ruling out models that would really be outside my known bike. I would not considered it a be all end all, but it was a very helpful tool. It also has to option to give you a list of bikes in your range.

https://geometrygeeks.bike/
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Old 01-29-23, 12:44 AM
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As an old guy I can handle very little change in my bicycle set up but of all the measurements I have found the "Seat to Bars" distance most critical for comfort.

Just Sayen...
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Old 01-29-23, 01:56 AM
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stack and reach
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Old 01-29-23, 02:21 AM
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Originally Posted by spelger
Looking to buy a new ride and am happy with the fit of my current ride so i'd like to keep the fit as similar as possible. in comparing the geometry of my current ride i find that naturally i am one of those in betweeners. i have this unfounded impression that the two most important measurements would be stack and reach. would that be a correct assessment? or are there other considerations i should also add emphasis to like tube angles or tube lengths?
Stack will tell you whether you can get the bars at the same height without a crapload of spacers. Reach will tell you whether you can put the bars the same distance away without resorting to an 80mm or a140mm stem. These days, it seems like getting your saddle the right height and setback is just a question of getting a seatpost that allows you to put the saddle in the right place. Head tube angle affects handling more than fit, and it seems like seat tube angle these days primarily determines whether you need a seatpost with setback or not.
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Old 01-29-23, 08:38 AM
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Stack is the place to start. When I buy a frame, I want a stack of about 540mm WITH the headset top cover and spacers, if I can use a -17 stem. If I'm using integrated bars with a -6 degree angle, I only need a total stack of about 525mm. Reach can only be compared at one stack height. If two sizes are being considered, subtract 3mm from the reach of the smaller frame for each 10mm of stack height difference, This assumes that spacers will be used on the smaller frame, to produce the same total stack. The stack values on geometry charts don't include the headset top cover or any spacers.

I also have a range of reach of 370-380 that I look for, to keep the stem length in the 100-110 range.

With a 725mm saddle height, I get a saddle to bar drop of about 115mm.
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Old 01-29-23, 08:49 AM
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As long as you can match your current position, the actual frame dimensions don't matter. Getting too big of a frame often results in not being able to get the bars as low as you want them. This seems to really slow me down. It makes sense to check the seat tube angle to make sure that you aren't going to need a seatpost with ridiculous setback. The other thing that can happen with a too-big frame is you might need a much shorter stem and your friends will make fun of you. But at least if the frame is too big, slamming the stem shouldn't be too difficult.

Something else that's important is to check to make sure that the frame sizes actually are what you think they are. For example, Kona uses actual seat tube length on bikes with sloped top tubes. Their sizes can be almost 10cm off from what every other brand would label the bike. I saw that with their largest size, but smaller sizes are going to be off too.
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Old 01-29-23, 08:50 AM
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Primarily Stack and Reach.
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Old 01-29-23, 12:21 PM
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Everything about the geometry will work to make one bike feel different depending on how picky you are or set in your ways.

Stack and reach along with head tube and seat tube angles is a good start. Wheel base and trail to a less extent, as those are somewhat dependent on the head tube angles and reach. BB drop and crank length will alter your perceptions too somewhat.

What type tubing bike is made of and it's characteristics whether carbon, steel, aluminum or titanium will alter your perceptions about the ride too that you might blame as not fitting you correctly.

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Old 01-29-23, 05:21 PM
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ok, thanks. will very likely go with the small size since most dimensions for the two frames i've been looking at are spread equally among both small and medium. makes sense that it is easier to make a small larger than make a medium smaller. but really no dimension is drastically different so likely either would do. i always seem to fall borderline. i fall border line with power/weight ratio too so the few races i've done i either finish too well or have my @55 handed to me.
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Old 01-29-23, 08:37 PM
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I also keep in mind the stem angle. A 540 stack with a -17 stem will produce about the same bar height as a 520 stack with a -6 stem.
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Old 01-30-23, 02:40 PM
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Shorts, you don't want to look fat.
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Old 01-30-23, 04:38 PM
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if your fit works look at measuring

center of seat to center of handlebars
stem length
saddle height (saddle to BB center
saddle to floor
handle bars (at stem to floor

then if you can recreate this you should be very close
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Old 01-30-23, 05:14 PM
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Originally Posted by Pratt
Shorts, you don't want to look fat.
You mean like this?
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Old 01-31-23, 02:02 PM
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Originally Posted by spelger
ok, thanks. will very likely go with the small size since most dimensions for the two frames i've been looking at are spread equally among both small and medium. makes sense that it is easier to make a small larger than make a medium smaller. but really no dimension is drastically different so likely either would do. i always seem to fall borderline. i fall border line with power/weight ratio too so the few races i've done i either finish too well or have my @55 handed to me.
Keep in mind that the smaller size of the same model bike will have a lower stack height and therefore more drop from saddle to bars. And that is one of the reasons why I took the smaller size of my bike when I was between the two size suggestion of the manufacturer. But I did test ride both sizes.

I think most new bikes come with about 25 to 30 mm of spacers under the stem that will raise your bars. However you need to be sure that will be enough. Otherwise you'll be doing fugly things to your bike to make it fit.

You cannot just lower the saddle to match where the bars are. Saddle height is pretty much the all important thing and will be pretty much the same on most every road bike you get regardless of the size of the frame.

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Old 01-31-23, 02:14 PM
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I use the RAD system.

https://www.leelikesbikes.com/rad-bike-setup.html

My hard tail mountain bike and full suspension mountain bike have completely different geometries but the RAD dimension between the two bikes is within a few millimeters of each other. When I hop between the two bikes they feel nearly identical.
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Old 01-31-23, 09:11 PM
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You can get exactly the same fit on a bike with nine-foot long chainstays, so if you like the steering and handling of your current bike, matching it will require more than getting the same 'fit'.
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Old 01-31-23, 09:28 PM
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Originally Posted by Fredo76
You can get exactly the same fit on a bike with nine-foot long chainstays, so if you like the steering and handling of your current bike, matching it will require more than getting the same 'fit'.
gee, thanks. that was very helpful.
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Old 01-31-23, 11:38 PM
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Try to approximately match the head and seat tube angles, the wheelbase, the fork rake, and the BB height, for the best chance at similar handling.

Better?
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Old 02-01-23, 08:28 AM
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What you really want is a similar steering trail, that many geometry charts leave out. I've owned a Colnago with a larger trail and a LOOK with a smaller trail at the same time. It wasn't a problem. I like the stability of larger trail values over the "magical" 58mm across the board value on some models. I'd be surprised if any major brand sells models that handle poorly. I catch comments from experienced riders that leads me to believe that they don't know how to steer a bike.
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Old 02-01-23, 03:08 PM
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Of course the seat tube length is the most important measurement. Which is why bikes are sized using that number. Reach to handlebars and proper leg extension are adjusted using stem and seat post length. This mostly applies to classic road bikes, of which "stack height" has little meaning, and usually very little adjustment.
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Old 02-01-23, 04:31 PM
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Originally Posted by smd4
Of course the seat tube length is the most important measurement. Which is why bikes are sized using that number. Reach to handlebars and proper leg extension are adjusted using stem and seat post length. This mostly applies to classic road bikes, of which "stack height" has little meaning, and usually very little adjustment.
No. Road bikes are usually sized using the top tube length. A size 56 has a 56cm top tube, not a 56cm seat tube.

Though mountain bikes are usually sized using seat tube length.
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Old 02-01-23, 05:06 PM
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Originally Posted by Lombard
No. Road bikes are usually sized using the top tube length. A size 56 has a 56cm top tube, not a 56cm seat tube.

Though mountain bikes are usually sized using seat tube length.
That is not correct, road bike sizing is by seat tube length a 56 cm bike has a seat tube of 56 cm
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Old 02-01-23, 05:34 PM
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Originally Posted by Lombard
No. Road bikes are usually sized using the top tube length. A size 56 has a 56cm top tube, not a 56cm seat tube..
Wow. Just…wow.
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