Can I use stack/reach to determine if a different frame size will fit?
 

My 2013 Domane Six Series 52 cm is damaged, and I need a replacement.
I can get a smoking deal on a 50 cm frame, 52's are very hard to find.
By researching stack/reach numbers from both a 50 cm & 52 cm frame, I believe that can do this.
Stack on the 50 is 15mm lower, that can be adjusted with spacers, I currently have a 5mm below the stem on the 52, it'll be two-10 mm spacers on the 50.
Reach on the 50 3mm shorter, I'll barely notice, or I can change stems.
The seat tube C-T is 25 mm shorter, but the long seat mast cap will be well within spec. I currently use the short version.

So, will this 50 cm frame fit?

Thoughts?

You can fit a range of frame sizes. In my case, with a 32 1/2 " inseam it's about 54-56, although I can make up to a 58 work but it's not optimal. And, don't trust manufacturer's stated frame sizes. My CAAD 12 is 52 w/sloping TT. But, when actually measured with a virtual TT it's actually a 54. I'd start with inseam and go from there.

Sounds as if you will be fine. 20mm of spacers below the stem is not too much. If you can keep your stem between 90mm and 110mm that will be good as well. Probably more toe overlap than you are accustomed to but most folks can live with that.:thumb:

That's what I thought, I'm just not sure if I'll feel the difference in frame size, not sure if the bike will feel smaller.
Even if I do feel a difference, it may be because I'll be going from a 2013 Six-Series to a 2021 SLR.
Because of the endurance geometry & relatively long wheelbase, I don't think that I'll get toe-overlap.
I should add that I use a 100mm stem on the 2013.

Thanks for the input

The entire reason for stack and reach measurements is to do just this...compare different frames and determine if/how one will fit.

I think this is why stack and reach exist.
Sure it'll fit, but will it feel the same? Compare bottom bracket drop, fork trail, head tube angle, and seat tube angle.
Typically, the smaller the bike the seat tube becomes more upright and the head tube slacks out. But with you comparing one size different, hopefully all these numbers are the same/similar.

cxwrench,

Thanks for the reply.
Yes, I can easily make the effective geometry the same on both bikes, as I've done with the 2013 Domane & 2011 Madone H2 fit, both 52cm frames.
Will I notice the difference?
I don't get toe overlap on the 2013 Domane, I do on the H2 Madone.
I've read that you're a Trek mechanic.
Do you think that I'll feel a difference in the two bikes, and will that difference be more related to geometry or 1st generation vs current frame?
I should add that I'll get disc brakes on the new Domane.

BB drop is the same, 8.0 cm
Trail: 6.0 on the 50, 5.9 on the 52
Head tube angle:71.1* on the 50, 71.3* on the 52
Seat tube angle: 74.6* on the 50, 74.2 on the 52

Wheelbase is 99.6 cm vs 100.3 cm

I'll do some research but I don't know how these affect the way that the bike will feel.

Thanks

Between the 50 and 52? You most likely won't notice any difference.

Exactly. Even if we ignore feel, which also depends on frame material and construction in addition to the measurements, you need to consider at least seat tube angle in addition to just reach and stack to get an idea on fit.

Even a small difference in seat tube angle means the saddle may be so much further forward or backward that the change in distance cannot fully be compensated by the offset, depending on the existing offset of your seat post and saddle.

A common mistake made with this type of comparison is failing to correct the reach to the SAME stack height on both bikes. I bought two new frames with an 18mm shorter stack and 10mm shorter reach than my previous frames. The reach must be corrected for the 18mm of spacer needed. Use 3mm for each 10mm of stack height difference. That makes the new frame reach another 5-6mm shorter. In the end, I had a comfortable fit with 15mm of spacer and 110mm, minus 17 degree stems that I had kept around. 120mm stems would have been too long.

The difference in the STA does not impact the reach, but will slightly change the saddle setback needed to get the same saddle nose to handlebar dimension.

Good points.
If I get this frame, I have enough stems on hand so that I can get the same effective reach. I won't change the saddle setback, since it's the same on all of my bikes and I don't get knee pain.

Man. I'm bad. I run at least a 120mm and usually a 130mm stem :)

Remember those stack and reach measurements are probably frame stack and reach to the top of the headtube/headset. So the other components on the bike that many times aren't included in the geometry specs can make enough difference to annoy you after the purchase.

While many times most mfr's use the same component sizes as other for a specified size bike, you still will be wise to pay attention as some don't. Take a close look at things like crank length, stem length, handlebar reach/drop and even try to find out how many mm of spacers under the stem that new bike will come with.

I'm only getting the frame.
I've got all of the components from my current frame, which is damaged.
I've got plenty of stems & spacers to be able to make the contact points the same in relationship to each other as they are on my current bike.

Thanks

Me, too. I ran a 110 for years, until it was suggested that I change to a 130 (by someone who knows about such things). I made the switch, and very quickly it was "Oh! THIS is how it's supposed to feel." 15 years later, I'm not as flexible as I used to be, but a low 120 still feels very right to my body.

It will probably feel different (depending on your sensitivity to such things), but not dramatically. A longer stem on a frame with shorter reach dimension will change your center of gravity over the bike a bit, but you're not doing anything extreme, so it should be all good.

stack and reach are same, head and seat angles are within half a degree. It will be fine.
the longer wheelbase will feel more stable, wanting to stay straight but may feel a bit less agile, depending on fork rake.

Wheelbase is shorter on the 50 than my 52, so the 50 will be a little less stable & a little more nimble.
Probably very little.

yup, sounds good