Making changes to a used bike

[makoti]

Looking at a used bike & wondering how much adjustment I can make before I toss off the handling of it. What I’d need doesn't sound like much, but it was built with these things in mind, apparently.
The TT is about 1cm longer than I'm used to. That isn’t changing. The setback is 14mm more, but it has a 20mm setback post, so just going with a 0 setback post should fix that. The reach (saddle-to-center of stem) is about 25mm too long (again, 0 seat post fixes most of that) & the drop is about 20mm too shallow. It has a good deal of spacers, so just reducing them gets me pretty close to fixing the drop, but I’d need a different stem to fix the reach.
None of these sound like huge changes, but the thing that worries me is the bike was built for THESE parts, this length stem, this amount of setback with this post. As long as I’ve been riding, I should know this stuff, but never had to deal with it.
Anything here that might be a concern? I'd be sitting on the bike more forward & lower than "intended". The bike will likely be gone before I figure this out, but for next time, guidance would be great.

[jmoore]

I don't think any of these changes would make the bike handle any worse.

[adrien]

You're moving the center of gravity towards the front wheel (by going to a straight seat post) and you're putting more pressure on the front wheel (by increasing the drop. That will make the front livelier, and may in extreme cases give you wheel slip in the back when pushing hard.

Unless the changes are radical, it shouldn't make much difference. It sounds like what you're doing is well within what the designers would have planned.

[oldguy00]

Do you have another bike that is setup correctly that you can copy the measurements from?

Just to be clear, you should be basing your saddle setback, and height, from the center of the bottom bracket.

Regardless of what seat post it requires, get your saddle setup in the same position as your previous bike based on those measurements from the bottom bracket. Then, if it is still a longer (or shorter) reach to the bars from the saddle, at that point you adjust with stem length.

Sorry if you knew all that, just wanted to be sure. :)

[makoti]

Do you have another bike that is setup correctly that you can copy the measurements from?

Just to be clear, you should be basing your saddle setback, and height, from the center of the bottom bracket.

Regardless of what seat post it requires, get your saddle setup in the same position as your previous bike based on those measurements from the bottom bracket. Then, if it is still a longer (or shorter) reach to the bars from the saddle, at that point you adjust with stem length.

Sorry if you knew all that, just wanted to be sure. :)

All the measurements I am comparing to are for the setup of my other 3 bikes. My saddle height, drop, setback, and reach are all very similar.

[Mark McM]

Looking at a used bike & wondering how much adjustment I can make before I toss off the handling of it. What I’d need doesn't sound like much, but it was built with these things in mind, apparently.
The TT is about 1cm longer than I'm used to. That isn’t changing. The setback is 14mm more, but it has a 20mm setback post, so just going with a 0 setback post should fix that. The reach (saddle-to-center of stem) is about 25mm too long (again, 0 seat post fixes most of that) & the drop is about 20mm too shallow. It has a good deal of spacers, so just reducing them gets me pretty close to fixing the drop, but I’d need a different stem to fix the reach.
None of these sound like huge changes, but the thing that worries me is the bike was built for THESE parts, this length stem, this amount of setback with this post. As long as I’ve been riding, I should know this stuff, but never had to deal with it.
Anything here that might be a concern? I'd be sitting on the bike more forward & lower than "intended". The bike will likely be gone before I figure this out, but for next time, guidance would be great.

Rather that saying, "the bike was built for THESE parts," it is more correct to say, "these parts were selected for A PARTICULAR rider". Since that rider isn't you, then there's no reason that you need to use the parts that came with the bike.

As far as dimensions: If the frame has 10mm longer top tube than your old frame, but it also has 1mm more setback (as measured at the seat cluster), then the new frame has a shorter reach than your old frame (by 4mm). That's pretty darn close. As you say, much of the saddle-to-handlebar reach. As for the saddle-to-handlebar reach, that will change once you set up your saddle with the your specific setback. If as you imply your saddle will be 15-20mm further forward than the original saddle, then the handlebar reach will only be 10-15mm more than before. That's basically just a single increment in stem size. What matters more is the the stem size that you are used to using. If the old and new stem sizes ending being close, then the effects on handling should be minimal, and you'll likely quickly adapt and this will become your new normal.

[Mark McM]

You're moving the center of gravity towards the front wheel (by going to a straight seat post) and you're putting more pressure on the front wheel (by increasing the drop. That will make the front livelier, and may in extreme cases give you wheel slip in the back when pushing hard.

In this case, the straight seatpost is being used to restore his normal saddle setback (due to the new frame's shallower seat tube angle), and will therefore not change the weight balance.