I did a bike fit with the LBS last year. Zero setback post on that bike. Change was made to swap out to a 70mm stem.

My new bike (exact same frame and geometry) has a 20 mm setback. The saddle was set up to be in the same relative position.

I can't quite tell if I should also change out the OEM 90 mm stem for another 70. It's fractions of an inch. Is a 20mm difference considered to be significant?

yes

Yes, 20mm is significant.

My question is why would you put a 20mm setback post on a bike with the same geometry as one that fits you with a 0mm setback post? Don't change the stem, change the post. The post is how you set your knee angle. Trying to accomodate for the extra 20mm of saddle setback with a shorter stem is likely just going to put more weight on your hands, and your knees still won't like it.

Yes, 20 mm is huge.

Is the new seat post better than the old one? In terms of how it positions you in relation to the BB?

70 mm is unusually short for a road stem. As a rule of thumb, shortening your stem will make the bike a little twitchier.

I think the saddle is 'in the same relative position?" Yes? Which, in itself, would be odd because it would mean the saddle is slid forward to mimic the same position as a zero setback post. As a matter of aesthetics you can do what you choose, but I'd be looking to swap out the seatposts. Get your saddle position dialed in relative to the bottom bracket in a way that works for you, THEN start looking at your stem for reach/height based on your final saddle position. IOW, you fix reach and bar height issues at the stem, not at the saddle or seatpost.

As for the 20mm difference in stem length, depending on your fit issues it could be about as bad as a mile off. Millimeters matter.

So if I'm understanding correctly, you have the same frame with the same geo. The new frame came with a 20mm setback post, but you have the saddle slid way forward to mimic a zero setback post so effectively the saddle is in the same position relative to the BB. The new frame came with a 90mm stem and you're wondering if you should trade it for a 70mm stem like your old frame (all other things being equal)? Well, if you were comfortable on the old frame then yes, you're gonna want that 70mm stem. 20mm is over 3/4 of an inch. I doubt you'd notice a 5mm difference, but 20 is a whole 'nuther thing.


-Kedosto

I am just wondering why you are on such short stems?
Unless you are rather short or something it would suggest you may be on the wrong frame.

I wouldn't ride with a 70mm stem. Even 90 is a bit short for my tastes, but sometimes that's the only option I guess.

What I'm gathering from the responses is that it matters. And that 70mm is more unusual rather than common.

The old bike was a 4.3 Domane, new one an SLR7. I'm not short at all, being 5'9. Now I'm wondering if the LBS really wasn't all that swift on this task last year. The fit was more informal, rather than hours long and I assumed the guy knew his stuff. Maybe not!

I'm mostly okay on the bike as it is. Actually one of my concerns was if we shortened the stem, that now may put more pressure on the hands.

The point of the question was that I thought I would be duplicating whatever had worked last year. Now I'm not so sure and will think the solution.

Thanks...

Yes, 20mm is as others have stated a significant amount. My stem is 120mm which IIRC is 10mm longer than the stock stem that came with my 62cm bike. (I am 6'4")

That 20mm on the stem is a lot.
Just think what 20mm setback on seat post means.

I used to think cyclists were crazy; but fit is literally a game of millimeters.

70mm stem is short, like mountain bike short. I consider 90mm to be short. What size frame did your LBS put you on?

Wow really? That's almost an $8k bike and the shop seriously slammed the seat forward and now you're considering a short stem? Yikes

Did you buy the 56cm version when probably the 54 would've been better?

FWIW I've been working on my fit in my old LeMond, that has a top tube length a couple cm longer then it should. I swapped a 110mm stem that was level (-17°) for a flipped up 17° 90mm stem and it is noticably more "fast turning". I wanted a more upright position for longer days in the saddle. Also I replaced my setback post for a zero setback. The LeMond is a 55 ST x 56 TT so in the future I'll be looking at TTs that are 54CM.

70mm stem for a guy 5'9" tells me something isn't right.

For reference, I'm 5'5" and ride an 80mm stem (on a 52cm bike that came with a 90cm stem), and it's just right.

Yup, as everyone already mentioned. I didn't think there would be that much of a difference, but 10mm was a huge difference for me.

A guy my height posted he rides a bike 45mm bigger than mine with the same length stem.

A guy 280mm shorter rides a bike 55mm smaller than mine.

Just based on that alone, I'd have to say 20mm isn't that significant in the grand scheme of bike fit, however a 70mm stem is rather short and may feel sketchy.

It is also true that if you needed a zero setback post on bike A, you need it on bike B, and C, etc... Most bikes will have the same seat tube angle for a given size. (about 72-74 depending on bike size) meaning at your "fitted" seat height, nearly all road bikes your size, will require a zero setback post.

Where your butt is, is what matters in a bike fit. The rest is about your own comfort. I could swap my 120mm stem for a 100 or a 140 and id be used to either after a few miles.

All of that ^^^ right there. And the older you get, the more the millimeters matter

Moving your seat changes the geometry of your hip, knee and ankles, which are all under load and constantly moving while cycling.

Moving where your hands rest isn't anywhere near as significant. If it were, the drops wouldn't be there and we couldn't drop our stack and shorten our reach mid ride (by switching to the drops) or shorten our reach (by holding the tops) without consequence.

Where your butt is affects your performance and wear and tear on your joints.

Where your hands are... not so much.

To pile on.... yes, 20mm is significant.

Whether a 70mm stem is appropriate really depends on your fit needs. Also depends of the reach of the bar you are using. Needing a stem that short puts one on the tail of the bell curve, but does not mean it is necessarily wrong. But it does warrant making sure you are on the right size frame.

I have atypical fit needs. I run a 70mm stem on my road/gravel bike. Handles fine.

What many here are saying is it's important you have the same length stem as everyone else, or you're doing it wrong. What a load.

If your setup is working for you, that's what matters. If it isn't, then it's time to re-evaluate.

One aspect not mentioned yet is that you can have some wiggle room in stem length if you look at bars with differing amounts of reach. You can move the top of the bars up to 30mm or so, while the hoods and drops stay where they are. Something to consider...

I tend to go for a longer stem (I'm 5'10" and rock a 120) with shorter reach bars, cause I don't mind leaning over a fair bit (stem's slammed) as a minimum, and I like how the 120 makes the steering feel on the tops.

I'm 5'10" and ride a 56cm with a 110"mm stem.

70mm will be really twitchy on a steep decent.