Why is a slack seat angle so rip-able?

Hey Lee,

I know what I like but don’t know why. I have been riding a few demo bikes and find the slack 71.5-72 seat angle to feel great and very rip-able. When I jump on the 73/74 deg angle it feels … off even with similar head angles and bb heights. Any reason why? I also feel like I can ride with my seat lower with the slacker seat tube. Climbing and pedaling has the same results. Everyone keeps telling me “oh 73-74 will climb better.” I don’t see it. Lay it out for me bro.

My 2008 Stumpjumper Pro Carbon has a pretty steep seat angle. I pseudo-slacken it by sliding my seat back about an inch on its rails. Photo for MMBSii by Yosei Ikeda.

Great question. As a matter of fact, MMBSii will contain lot more on this topic.

A slack seat angle places your seat farther back in relation to your bottom bracket. That has these effects:

– It effectively shortens your top tube when you’re out of the saddle. The entire top tube is shifted back, and that brings the bars closer to you.

– Given an equal distance from the seat to the bottom bracket, a slacker seat tube places your seat lower vertically. This makes it easier to rip with your seat at full pedaling extension.

– Most of us have a tendency to keep our butts near our seats. This is almost always too far forward. A slacker seat angle encourages you to locate your hips a bit farther back. Maybe not as far back as I’d like, but farther back and closer to correct.

– Pushing your seat back changes the way you pedal. Compared with the triathlete-directly-over-the-pedals style, pedaling from the back of the bike recruits more glutes and hamstrings.

– Most of us can learn to pedal just fine that way. As my riding style and knees have (d)evolved over the years, my seats have been getting farther and farther back.

– Last winter I did a few indoor training sessions wired to a power meter, and I was more powerful in the saddle with my Stumpy’s slack seat tube than my Tricross’ steep one. Hey, whatever works. PowerMax indoor cycling workouts at the Boulder Center for Sports Medicine

Sweet bike + dialed cockpit = rip-ability. At the Lyons Bike Park. Photo for MMBSii by Yosei Ikeda.

OK, time to quantify
This wants to be a diagram, but I have to get back to MMBSii. Deadlines!

– Seat distance from bottom bracket: 28.5 inches (about my distance)

– Seat angle comparison: 71 degree vs. 74 degree (slack vs. steep)

– Vertical seat height: The 71-degree seat angle places the seat almost one-half inch lower. That’s significant. When it comes to smashing your stuff, a millimeter is as good as a mile.

– Seat setback: The 71-degree seat angle places the seat about 1.5 inch farther back. It also brings the bars about that much closer when you’re ripping in attack position. Also very significant.

Making adjustments
You can create this much range by sliding your seat back on its rails and/or rocking a layback seatpost. My Stumpjumper and Enduro have pretty steep seat tube angles; I run my saddles back on their rails.

OK, back to work.

Rip it!

— Lee

Know more. Have more fun!

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14 replies
  1. Tjaard says:

    Two other things to consider:
    1 If you are tall you will have the seat further back over the bike than if you are short, this changes weight distribution when seated (climbing) so tall people might want their seat more forward and short people more rearward in order to keep the weight distribution the same.

    2 People have different lengths of their femurs, so a certain set back will have a different biomechanical effect on their pedaling.

  2. leelikesbikes says:

    Oh yeah, that whole biometrics/seat positioning thing is hugely detailed/complicated.

    I think, if you’re a pedaling-style rider, first find a good pedaling position, then learn to rip that way.

  3. Farid says:

    Hey Lee, Specialized has Effective and Actual Seat Tube Angle on their Geometry. Could you please explain the difference between each? Thanks!

  4. Scott says:

    Farid, the Effective is the angle of an imaginary line drawn from the bb thru the seat clamp, whereas the Actual is the actual angle, from horizontal, of the seat tube.

    But Lee,
    “The 71-degree seat angle places the seat about 1.5 inch farther back. It also brings the bars about that much closer when you’re ripping in attack position.”
    -I don’t get this — wouldn’t the effective TT get longer as the ST gets slacker and/or saddle gets farther back??? Guess it’s still begging for a diagram.

  5. MW says:

    Scott, it brings the bars closer when you’re in attack position because the bottom bracket is closer in relation to the bars (aka, reach and stack)

    Tajjrd, Im 6’2″ with a 35″ inseam and having a slacker seat tube puts me lower on the bike in relation to the bars, where a steep seat tube bike puts me higher and requires me to run more spacers in my stem to fit correct. For me, the lower the better for handleing as long as the hands dont get heavy(this causes them to go numb on long rides) Climbing tech is no problem with just a slide of cheek 🙂

  6. Sean says:

    Scott is correct. Slacker seat tube angle = LONGER effective top tube. Slacker seat tube puts the seat FURTHER from the head tube, which makes the top tube effectively longer.

    Also, slacker seat tube angle = more quadriceps, less glute. It also affects how much hip flexor and adductor/abductor you use. The closer your pelvis gets to the head tube, the more your big muscles (glutes, lower back, hammies) are forced to work.

    Roadies worked this stuff out decades ago. I know a lot of MTB riders reflexively avoid and/or hate roadies, but the effects of the geometry of a bike are the same regardless of whether you ride it with big 2.5 inch tires or tiny 21mm tires.

  7. MW says:

    I totally agree that the road world had the perfect output angles dialed, but that also is limited to a single riding position (well two, sitting and standing.) Mountain biking is so dynamic and so much more is involved in making a MTB handle in varied terrain than just optimum output. Very different beasts for sure. That’s why I don’t think people should use road fit systems for mountain bike fit. Having that slacker seat angle may loose some theoretical output but its all worth it for that rip-able feeling ( aka Brrrrrrap!)

  8. leelikesbikes says:

    Great points all around.

    Seems to me, on a road bike you can build the ultimate engine, then fit the chassis around it. On a mountain bike — especially one meant to be Ridden (capital R) — you have to build the right chassis, then make sure the engine works within it.

    While a DH bike might be all chassis, and an XC race hardtail might be more engine, every mountain bike has to find the right marriage of handling and power output.

  9. leelikesbikes says:

    And btw:

    Last night I was rocking some road climbs on my very slack Enduro, and I noticed how far back my seat was. It’s pretty far back there, but I guess I’m used to that position.

    As a matter of fact, I stomped a serious-minded roadie up the steepest section. I just sat back and spinned, and he pooped off the back. I was in no hurry, and I was comfortable. That’s about all I expect from my climbing these days.

    But: This makes me want a computerized fit. It’d be interesting to see what the machines say.

    XC Worlds team member Judy Freeman got a fit from Pro Bike Center — http://www.probikecenter.com/home/index.php. She says Nat Ross moved her way forward, which felt weird, but she’s now more powerful and efficient.

    So there you go. No clear conclusion! 🙂

  10. Dan says:

    I’ve been experimenting with moving my seat forward because I felt a little too stretched out. It did seem to allow me to stay on the seat more on long climbs, and keep the front end from wandering, but I can see the downside of being taller as well. Maybe I’m better off with a shorter stem?

    I’m 5’9, short legs (30″ inseam), on a 19″frame.

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