Hello,
I'm 179 cm tall, with shorter legs (saddle height of 714mm on 170mm cranks), longer torso, short arms (negative ape index: 175 cm wingspan) and rather average flexibility for my 48 years (I'm fit, lean, and ride regularly, but my saddle to bar drop is only around 3 cm, due to my short arms and short legs). I've been riding a 2013 Specialized Roubaix SL3 in size 56 with 90mm negative 12 degree stem for years and (having been professionally fitted) this bike just feels great to me. I can easily do a century and still feel fresh at the end.
Now I've been tempted to pick up a new old stock Specialized Tarmac SL6 rim brake bike (the last of its kind) at a very good price, so I'm having difficulties trying to make a rational decision. The problem is the following: the Tarmac is also a size 56, like the Roubaix, but it has an 8mm longer reach and 25mm lower stack. Since I would use 18-20 mm more spacers on the Tarmac, I could decrease the effective reach difference to only 3 mm, but the stack would still stay lower by 1-2 mm too. This is probably still manageable, especially as I plan to use the same bars on both (and also the same saddle & saddle setback, as the seat tube angle difference is almost negligible: 0.25 degree steeper on the SL6).
BUT, and this is main concern, I would also need to use a 90mm stem on the Tarmac (actually 85mm would be ideal, but that's far too short and not readily available anyway), on top of 30mm spacers, to achieve this (i.e. the 3mm longer reach and 1-2 mm lower stack compared to my Roubaix). AND the head tube angle on the Roubaix is 1 degree slacker (72.5 degrees) than on the Tarmac (73.5 degrees). Both the Roubaix and the Tarmac came with 100mm stock stems in size 56, but on the Roubaix I went down to 90m after my Retul fit many years ago, and on the Tarmac the seller fitted a 90mm (negative 6 degrees) stem for me.
What really bugs me is whether the inherent twitchiness of the SL6 geometry (especially in larger sizes, such as the 56) much worse. As I said, 90mm is the max stem length I could use on this long bike, and while the 90mm stem on my slacker, endurance-geometry Roubaix feels perfect to me, I wonder if this could be a major issue on the Tarmac? I would use the Tarmac for the same purpose as the Roubaix (no racing, just all around rides for fun and fitness).
The fork trail figures between the two bikes are otherwise very similar (Tarmac 55m, Roubaix 56mm, according to Spesh charts, though I suspect the actual trail on the Tarmac might be a bit less than 55mm), but there's the problem of short stem + tall stack of spacers + steepish head tube angle. Do you think this combination is less than ideal? I've only been able to take the SL6 for a very short spin (15 minute test ride), set up according to my Retul fit numbers, with the 90mm stem, and it felt good, with balanced weight distribution, though I definitely noticed the somewhat more nervous steering compared to my Roubaix. It also felt just a little too long (tension in my lower back), even though the effective reach at the handlebar should be ~3mm longer and 2mm lower only according to my calculations).
I don't bomb downhills at high speed (50 km/h max, and then I pull the brakes), but I'm still wondering whether the SL6 in this size and with this setup is a good choice for me. I am aware that endurance bikes fit me better, but there's not much choice for a modern carbon bike with rim brakes and warranty (since the SL6 is new old stock) these days.
Here are some Bikegeo pictures too:
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