slack (73.5*) seat tube on a road bike...

razorjack Posts: 98
edited February 2021 in Road general
recently i've started riding more on a road bike (Spesh Tarmac), and i'm a bit annoyed by a slack seat tube (comparing to MTB).
Have any one of you tried seat posts with `set forward` ?

also i'm in a process of changing antediluvian 175mm cranks to 170 :)

If anyone is curious, i think the frame size is ok. 56cm, for 181cm guy with legs about 84cm.


  • rafletcher
    rafletcher Posts: 1,235
    Seat tube angles will differ between MTB and road frames, and so will the riding position. You'll naturally be a lot more stretched out on a road bike, more upright on an MTB. There's nothing "wrong" with the seat tube angle on your Spesh. The head tune angle on the Spesh will be a lot steeper than the MTB as well, and the stack height less. It's just the way it is. Maybe you should swap to a hybrid with flat bars for the road? Or get used to the position required for road riding.
  • but it's not about being stretched - i can regulate that by a stem and by rotating handlebar. in fact 'length' of the bike suits me.
    only ST angle for pedalling is strange, saddle is 4-5cm more behind BB than on my MTB, anyway, i'm changing cranks to 170mm, something will change.

    but also i see i can test in on my old seatpost - just the tilt is not perfect :/

  • me-109
    me-109 Posts: 1,915
    Road bike SA is in the range 72-74 typically, so I wouldn't say 73.5 is 'slack'. With a quick look at MTB geos they are 72-75, although I only found one that steep. They are more often a compromise of packaging for suspension design (if any) and travel, balance and front-centre lengths. Can't imagine 1.5 degrees translating to such a huge fore-aft variation as to need to reverse a layback seat post.
  • razorjack
    razorjack Posts: 98
    edited February 2021
    MTB (i'm not talking about XC, which looks and has angles closer to road/gravel bike than to proper mtb :) ) goes like 76-80* now...
    and i've checked my saddle (on Transition Sentinel), it's like 5-6cm more above BB than on my Tarmac .... and that's the feeling i don't like.
    every degree translates to about 2cm (at 76cm)
  • ragged_trousers
    ragged_trousers Posts: 8
    edited February 2021
    73.5 is bang on average for a road bike tbh.

    Traditionally, seat angles started steep (74-74.5) in a small frame, and gradually slackened out to c.73 as you went up the size range*. These days, a lot of manufacturers stick to a default seat angle (usually about 73.5) and simply change the seatpost layback (50&52, zero; 54, 10mm; 58, 20 etc) because it's cheaper to manufacture 3 varieties of post and limit the variables in frame manufacture.

    Road geometry in theory is about putting your pelvis and knee in the most powerful/efficient position, whereas MTB geometry has to account more for significant slopes and making it easier to shift your weight about on the bike.

    But in short you should be fine with the Tarmac geo (it's pretty vanilla, if quite racy) unless the bike doesn't fit.

    *probably TMI but the reason is because people's femurs tend to increase disproportionately with height, so the slacker seat angle is needed to facilitate knee over pedal spindle. If you ever look closely at a geo chart, look at how quickly seat tubes and head tubes increase as you go up the sizes, which reach tends to increase in much smaller increments.
  • razorjack
    razorjack Posts: 98
    yeah, i know that most road bikes have ~73 (in size large/56) and even they give setback seatposts !! well.. so they thought about adjustability only in wrong direction...

    now, after watching some videos and riding more with other ppl, I'm starting to understand why roadies love riding and more often climbing out of the saddle so much... because their saddle is in a wrong position .... (something that doesn't happen on my MTB).
  • pilot_pete
    pilot_pete Posts: 2,120
    I’m assuming you are a wind up merchant? Show us a picture of you on your bike, side on. Only then can we ascertain if you are actually serious about your position on your road bike....and possibly spot why you think it’s so wrong...