Tarmac SL8 Sizing Help
 

I currently have a 2016 Tarmac Comp and I am wanting to get a new bike to get me through the next 10 years+. My bike is fantastic, but the new tech is calling my name. Florida rider so most rides are on the flats with some occasional hills in the Gainesville area. I have my eye on the SL8 Pro.

I currently ride a 54 frame and shops have told me that this is the correct size. However, I have had to move my seat pretty much as far as it will go forward to feel comfortable. I am thinking a 52 may be a better option. I use the stock stem with the 16 Tarmac Comp and a slightly narrower bar and it feels good with the seat all the way forward.

I am about 5'7 with a shin length of around 16-16.5 inches. My current seat height feels good at 28 inches. There is a 54 and a 52 to look at, but they are 3.5hr+ drive from me.

What are your thoughts on this? I am leaning toward 52. What are the main differences in frame sizes? Length? Height? Ever since I bought my tarmac in a 54, it just felt a tad large to me. I am not a serious racer. I just love to get out by myself and push it for 20 miles or an occasional group ride.

Here's a photo of my current setup where things feel comfortable.

https://cimg9.ibsrv.net/gimg/bikefor...83dd30702.jpeg

Why is your saddle so far forward? If it’s for KOPS that’s reasonable. If it’s to reach the bars you need a shorter stem and move saddle back for proper fit over pedals.

That’s just my bias regarding fit.

Can’t you test ride 52 vs 54 sizes to see which works best for you??

52

Reading OP's post would answer both your questions.

Having done the same bodges to get my fit 'right' I'd definitely go for 52, you'll probably need ~20mm of spacers for the lower stack but maybe you'll find you can actually go lower.

Yeah I expect you are right, and that he would probably do better with a 52.

My vote goes to 54cm. Did you compare your current bike's geometry with the new one? Bike Insights - Geometry comparisons and design analysis

At 6'1'', I ride a 58cm SL8 Pro, but with smaller components (narrower bars, shorter stem, shorter crank arms). The 56cm was a little too small unfortunately. Lots of seat post sticking out and too many headset spacers to my taste. My previous bike was a 56cm which was also borderline too small, but OK.

I will agree with the above. Go for the 54cm fram BUT get a shorter stem for sure. Whats on your bike now looks like a 110 or 120? This is the issue with buying an integrated cockpit. Unless this IS a fir for you, you will have to compensate. Maybe your shop can just swap this out for you since you are spending a pretty $$$$ with them.

OP could ditch the set-back sestpost.

My current stem is a 100mm.

With the current setup, if my seat is any farther back, I tend to move down toward the front of the seat, get numbness and have to keep repositioning myself. With the seat farther forward I feel the body staying put, locked in and comfortable.

I also spoke with Specialized and they said the stem length on the SL8 Pro at 52cm is 90 and the 54cm is 100. I just ran into my local shop and grabbed a cheap 90mm stem, dropped the bars lower on the stack and moved the seat back a bit. We will see how that goes.

I am in between sizes. 5’11’ 35 inch inseam. Every bike company says I ride a 56, but all of my bikes are a 58. I recently bought a 56 BMC but with the stack, I had way too many spacers under the stem to make it comfortable. Looked ridiculous, so I went back and got a 58 and it feels perfect with 15 mm of spacers under the stem.

As someone else pointed out, you have a set back seat post which would explain why the seat is forward. Swapping to a shorter stem would help.


https://www.bikegeocalc.com/#7Unname...3F172.5G30H30Z


If you really want to see a side by side comparison this site allows you input all the geo, plus components. This is what I used to determine that my 56 bmc woukdnt do, and that the 58 would actually be closer to my madone as far as set up.

You have to have patience and recommend using a computer or tablet to input the numbers.

https://cimg6.ibsrv.net/gimg/bikefor...82ef1ba6c.jpeg

56
https://cimg6.ibsrv.net/gimg/bikefor...7d2b18bde.jpeg
58

At the price point of the SL8 Pro, I suggest that if the bike shop does not have a Retul or fit expert to help you, then go to a fitter with your old Tarmac and have the fitter help with the sizing, integrated bar and stem selection and general fit parameters of the new bike and how you may want to alter your current position to take advantage of the new bike. The fitter can address the seat setback matter. It will be fun and will not add much on a percentage basis to the cost of the bike.

I have a 56 2025 Tarmac S-Works SL and I am 6 feet with longer arms. When I purchased it, I rode different size Tarmacs made videos on the trainer at the shop as well as in the parking lot and worked off of my older Cervelo. I altered my fit to a more compact position with more seat to bar drop. The only way to know what will work is to ride the fit. I spent a lot of time testing bikes.

One of the challenges with the new tech is that once an integrated handlebar and stem is selected and hydraulic lines run to the brakes, changes are expensive to make. Integrated bars are stupid expensive. Once the bar height is set, there is not much adjustment without redoing the hydraulic brake lines.

The other aspect of this tech is that one needs an expert mechanic who knows the tech and can make a good build. The current tech that you have is very forgiving and much easier and less expensive to alter.

My wife recently purchased a 2025 Cervelo P5. We took her old bike to the shop and they measured her bike and we looked at the Cervelo sizing charts and talked with Cevelo. The shops do have an inventory of bikes to ride. We purchased the bike and when it was built, the shop matched the new bike setup to the old one. We did not connect the brakes. We took the bike to our fitter and he checked her Retul numbers and the shop hit every metric correctly. We then had the shop connect the hydraulic brakes.

She is now in the aero testing phase of the final setup where she can modify the cockpit adjustments to minimize her CdA.

I love that bike.

I went through a similar situation deciding between the 56 and 58. I went with the 56 to have more seat to bar drop narrower handlebars and a compact position. Podacar and his gang are much more compact in their position with narrow bars such that when they are on the hoods with their arms parallel to the ground, they are in a TT position. I have about 3500 miles on my Tarmac and love the fit. I did have a period of strength and adaptation to the new fit. I have my Cervelo on the trainer and I want to redo the fit. I hate it. So it goes.

That is another good point. Fully integrated is very expensive to swap out. It’s a shame that bike manufacturers don’t allow this at time of purchase, especially when spending 7k plus on a bike. The bmc cockpit is 700 dollars, so I chose not to do it. It’s about 2 mm longer reach than my madone.

I also had to swap out crank arms, something else I wish you could do at time of purchase. That’s an argument for another thread.

I currently have a 70cm stem on my Tarmac. Though I have gone back and forth a couple times with the original 100cm stem that came on my 2020 Tarmac. When I put the bars higher, I seem to like the longer stem. With them low I prefer the short stem.

I agree that a zero offset seat post will be better. Not just for looks, but to get the clamp on a better part of the rails to endure the times you hit a big bump with your butt still in the saddle.

As far as what size, that's still a toss up. Shin length isn't useful for anything. Inseam and and total height are more useful. If anything the femur length is more useful than shin length.

If you can, you should ride both sizes for as long a distance as you can. 10 miles or better at least. The real issues you'll have with fit, won't show up till you are doing the long rides.

I’m able to hold that position on all of my bikes. Actually seeing a trend of pros moving away from slammed stems. Increasing the stack is a touch more aero in the sense you can get your head closer to your arms while on the hoods, decreasing the front profile. All depends on the person, body type flexibility etc. I recently swapped stems on my emonda. Went from 120 mm 17 degrees drop, to 110 12 degrees drop. Feels better and I am still able to hold the aero position while riding.

OP, some considerations...
as reference - on geometrygeeks.bike site - compare the two sizes of 2024 Tarmac SL8, and you'll see what I'm referring to.
Both size have the same seattube angle - 74 deg, About as steep/upright as one can find on noncustom road frames) both have 172.5 cranks... so IF you're setting up the saddle in reference to the BB & pedal position, your saddle will be at the same point on either bike (same rear set relative to crank) visually the saddle extension would be visually 'higher' at the same extension length... in other words, NO CHANGE to be at the same saddle position on both sizes !.
What would change is the cockpit position (stem length, maybe a bar difference ?) which would need adjusting.... other factors will also change the 'ride' Headtube angle, TT length, changes in reach & stack... will make a difference in feel and ride...
ALSO the Front center is 1 cm shorter - so depending on your foot/shoe size, you have a greater chance of considerable toe/wheel overlap - not an issue for some, but for some it's not desirable. Front-Center does affect the overall feel and ride of a bike, quite a bit... that judgement is up to the rider.
IF you have your saddle fore-aft adjusted to make your cockpit position more comfortable or 'better' in some fashion - that should be adjusted with STEM LENGTH, not changing saddle position.

IF you've adjusted your saddle forward to achieve the best power delivery thru your cranks (not consideration reach/stretch to bars) THEN
CONSIDER getting a seatpost with LESS or ZERO offset - which will allow a better range of adjustment, than just slamming the saddle forward on the 25mm setback of current seatpost.
Get an aftermarket 10-15 or zero setback carbon seatpost and give a try on your current bike...
You could ride either size...
Before you decide... a seatpost switch may help you decide which frame size suits you best....
DOn't go by pictures of other bikes set up for other riders... Your current bike and setup seems to put you in a good performance position... except the saddle placement on the post - if that works
a simple seatpost swap my tell the tale.
'Visual' static bike fashion/setup is one thing, and only that... fashion... - your riding performance will be the determing factor and tell the real tale.
Ride On
Jurij

Great info, thank you. I did just grab a cheap 90mm stem from my local bike shop to try, moved the stem down to the bottom of the stack and moved my seat back a bit. Seems to have a similar feel while getting the seat back to a more normal position.

I think I need to start with getting a fresh bike fit and go from there.

Ok, now you know the possible issue was prolly cockpit setup, not saddle setup... (possibly...)
changing the saddle setup and not considering the cockpit is a common issue...
it's hard to separate the saddle placement from the effect the cockpit placement, so the saddle often becomes the site of 'change', because it's easier (that changing stem or bars), but often isn;t the best option...
Bike 'Fit' ... often creates more questions... not answers...
My personal preference as relates to bike model changes or sizing changes - get some riding time on the bike model and size you are considering... with realization that the cockpit might not feel ideal and needs adjustment...
The Tarmac design is very stable - meaning very small changes over a period of years... My 2014 Tarmac is very similar to my 2023, they do ride a bit different because of the actual frame material and tube dimensions, but my position on both is almost identical...
If you can get onto a 52 Tarmac from the recent model years 2020, 21 or 22, similar level to SL8, you'll get a good feel for how a 2024 52 might feel/ride....
Ride On
Yuri

some other considerations...
you haven't given info on your body dimensions, and asking to get recommendations, online, without that kind of info is always 'guessing', and most will go by what THEY like - NOT what might suit you ....
There's enough variation in rider body dimensions, geometry and position to warrant the current increments in frame sizing. I.E., at 5'9" (barely...) most would say a 54 would be my size. But because of my body dimensions a 56 is really the best/optimum for me...
There are other considerations for sizing, which are 'qualitative' not some defined parameter... The feeling of being 'In' the bike as opposed to being perched high 'On' the bike is part of the 'feel'.
Being High up over a bike/frame can be disconcerting, especially felt at speed and on descents where the front end is lower, a 'high' position causes some to push further back and lock the elbows, which on speedy descents is dangerous.
Of course you live in Fla. and descents are not an issue... but there's also a good side to having an angled component to your arm reach to the bars... very vertical reach has it's issues...
too much to go thru here... also why the process of setting seat position first, then adjusting the cockpit next is a good approach.

But all of this is 'depending', on your body structure and dimensions.
If the new bike or new size have different dimensions than what your current Tarmac has, then DO try to get some riding time before spending big money... And note all the little things which are different... Over time and distance, some things may not be as 'wonderful' as in a 1st impression...
Ride On
Yuri

At 5"7" you are a 52cm not including stem adjustments

I am 5'5" have a 29 SOH and take a 49cm just usually have to switch to a 90mm stem on their bikes since a 49cm Specialized comes with a 80mm one

The 52 is only 10mm shorter in top tube length, but 17mm shorter in head tube height.

I don't see the point in going to a 52. Use a shorter stem.

5'7" is either a 52 or 54 on most fit charts as it is right between the sizes.

As a short king and OP being 5'7", SOH probably will start to make a difference in which size they take. A 52cm 746mm SOH a 54cm 768 SOH. That .86" of a difference for shorter riders like us might be one of the determining factors.
At 5'5" Specialized puts me on a 52cm but the 746mm/29.37" SOH is above my 29" SOH clearance. Also while a 52cm would work reach wise for me at 377mm that's really at the upper range for me, thus even if SOH was not an issue it would come down to running a smaller frame with a larger stem which IMO looks more pro and contributes to more stable steering, or running a slightly larger frame with a smaller stem which IMO doesn't look as pro and can be more twitchy