Hi-I currently ride XL size TCR Adv Pro 0 Disc and entertaining the idea of getting a Tarmac SL7 at some point. I have gradually lowered my position over the past couple of years and still want to go a bit lower. However, I ran out of spacers and don't want to use a negative angle stem (let's just say for aesthetic reasons).

XL size TCR and SL7 61cm are quite similar geometry; TCR has a slightly lower stack and reach #s are almost the same. TCR has a more slack seat tube angle. Same with the head tube angles. I attache the geometry table below. I had to measure some numbers myself since can't find them on Giant website. I am 6-4 and 175lbs. My inseam is ~35''



When I go one size down to SL7 58cm I only lose 7mm of reach. 58cm SL7 has a 73.5° seat tube angle vs 72° for TCR which puts my body o bit more over the BB. This translates into a ~21mm forward position on the SL7 but since the SL7's seatpost has 20mm offset the end result is close to no change. (at least this is my uneducated assumption, I welcome comments if that's the case or not)

Stack numbers are 581mm vs 595mm for SL7 and TCR, respectively. This is perfect because it gives me plenty of stack height to adjust to the new fit. Also, seems like SL7's BB height is a bit lower than TCR's. I'm guessing this should also lower my center of gravity and improve my descending and cornering.

Again here I am looking at the bike picture. I appreciate comments on how to approach this purchase and what I said above makes sense. Is any fellow TCR owners here buying SL7? Thanks all.

I used to ride very large frames for my 5'11" size and not being able to get lower on the bike was one of the motivations for me getting a 56cm over a 58cm Tarmac. I was coming from a '91 Parmount with a 60cm frame.

I've not looked at the geometry numbers close enough, but with the 58cm Tarmac, I don't think I would have been able to get as low as I am now on the 56 cm bike with over half the spacers removed. This was a 2020 bike also, Specialized may have changed sizing methods with the 2021's.

Another thing to think about is crank lengths. Seems that on Tarmacs, those are where they go from 172.5 to 175 mm cranks. So that might be a consideration if you prefer one length crank over the other.

It was for me. I had the 172.5 crank removed and put in 165 mm cranks. With either frame, I would have done that anyway. But if you find not enough difference in stack for you between what ever you finally decide on, then look to be sure to note the size of the cranks and other stuff and figure those in with your frame bigger or smaller decision.

Keep in mind that the stack measurement listed for the new SL7 is WITHOUT the top bearing cover (which you have to use), so add 10mm to the listed number to get the actual stack.

Then seat tube and head tube angles should not be used to determine the sizing. With ST, you should always have your saddle set to the same position, in relation to the BB, so it should not be used to judge reach (i.e. you can't make up for excessive reach, by simply pushing your saddle forward). It should only be used to judge how much setback will be needed, and if you can achieve the desired setback.

HT angle will affect reach slightly, but only if you need to run a ton of spacers

Based on the stack numbers, I'd say go for the 58. But, you also have to consider the wheelbase, front center, and trail numbers as those will have a big impact on the handling/stability of the bike...and vary quite a bite between the two sizes.
So, a 61 with a -12 may actually be the best route for you.

Thank you. Frankly crank size is something I’ve never thought about changing but I’ll def put that into my list of things to consider.

Thank you too. This is super interesting. I remember seeing the spacer discussion in another thread but totally forgot about it when I was typing the post. I’ll update my spreadsheet.

I don’t think anyone can give you a very helpful answer without seeing your position on your current bike.

That being said, I’m having trouble wrapping my head around someone 6’4 riding a 57cm frame.

I”m 6’1”, and fit fine on a TCR XL setup with 4-5” of drop seat to bars and plenty of rom for more.

Unless you have gorilla arms, or are running an insane amount of drop from seat to bars, going down a frame size seems like it would be a difficult fit.

Again hard to sat without pics though

As others have noted, the STA is not a factor in figuring the reach difference. It only affects the amount of seat post offset you might require.

If you're using no spacers and a -17 stem, you need less stack height. If the smaller frame has 7mm less reach, you'll need a 10mm longer stem, assuming that the same bar reach is used.

One problem that can arise is the one piece bar and stem setups limit the stem angle and bar reach selection.

I agree that you seem quite large for a 58 frame.

That said, put the figures into BikeGeo.net and see what the difference would be in terms of handlebar stack and reach.

OP, I'm 6'-3" tall with a 35" inseam. I prefer to size down, and currently, I ride a Cdale Supersix HiMod 58 frame which I love. It fits me perfectly with a 130mm stem and setback seat post. I have been riding similar bike configurations for about 10 years and it works out well for me. However, I have felt that I am at the upper size limit of the bikes I have been riding. YMMV

It will come down to what length stem you currently ride. If you are already on a 130mm stem you won't want to be getting a frame with less reach.

This is a debate I have all the time when I'm buying a bike. I'm exactly 5'10", which for essentially every manufacturer and every type of bike means that I'm right between a medium and a large. Given that, I've bought both large sized bikes and medium sized bikes based on what I'm using the bike for. For a performance road bike, I went with a medium. By lowering the stem (a lot) and exposing more seatpost I got a better race-type position than would have been possible on a large while having more comfort (due to more seatpost) and sharper handling (due to the overall smaller frame). For my gravel/adventure bike, I went with a large to get a more upright/roomy position, which works great for more stability and relaxed positioning. For a trail MTB, I went with a medium because the trails around here are super twisty and tight and getting a larger bike around would be a pain. If I were riding wide-open trails fast, I would have gone with a large trail bike. Etc. So yes, going smaller sometimes makes sense to me, other times I go with the bigger option.