Seat tube angle

First post here. I’m 4 months into my first Trainer Road experience. Love it! Anyway…

I’m in the market for a new mountain bike. I’m on the fence between an XC bike and a longer travel “trail” bike. I’ve noticed that trail bikes are getting steeper seat tube angle, all the way up to 77 degrees, and XC bikes are staying in the 70-73 degree range. Marketing blurbs for trail bikes say the steeper seat tube angles improve climbing speed and pedaling efficiency–If this is true, why don’t XC bikes feature these angle? Help me read between the lines here.

Background: I have one season of local short track racing (2020) under my belt on a Giant Fathom hardtail, plus fat bike racing before that.

Jesse
Sioux Falls, SD

Jesse, greetings from the Black Hills. My only thought could be XC racing is a lot of out if the saddle time and trail riding is more in the saddle climbing. That’s based on my experience when we get to again.

I think it’s partly the relationship between reach, head angle and seat tube angle.

A trail bike will have a long reach, short stem and slack head tube angle. This means your weight is further back in the (longer) wheelbase for the same seat tube angle. This helps with steep descending, with fewer OTB type moments. Steepening up the seat tube angle moves weight further forward when seated, compensating for the tendency of long bikes to unweight the front wheel on steep climbs.

The shorter reach (but longer stem) and steeper seat tube angle of an XC bike put your weight further forward in the wheelbase without the need for a steep seat tube angle.

FWIW I think that really steep seat tube angles/saddle all the way forward are not optimal for power output and control, but might be optimal for steep uphill traction on a long, slack bike.

1 Like

Sean-- that makes sense, thank you.

Doc-- If you are craving some racing, check out Sioux Falls Bike Racing Series on Facebook. Fat, gravel, and MTB are all on the calendar for 2021!

1 Like

There are 3 main reasons for the steep angle, and it is going to be primarily about full suspension.

  1. Sag. The sagged angle is different than the static. With longer travel, this gets more extreme.
  2. Climbing puts more weight over the rear with standard angles, and forces the suspension to compress more, and eventually you tip over. Since mountain bikes are going up or down, the bias is for the climbing angle.

In the old days, people would get on the nose of the saddle, going up hill. The climbing position is more forward. Partly the combat the 2 points, and to orient the body relative to the angle to maintain the center of gravity.

2a) IF you’ve ever had the old single pivots and you hit a technical climb, you can easily loop out when the suspension squats as you try to get over a bigger than normal sized bump.
3) Reach, that’s when you are descending and out of the saddle. By having a longer reach (for descending), you also will tend to have a more forward geometry to maintain the geometry for a normal proportioned body.

Things like the amount of anti-squat desired also plays a part as well. To “ride high” in the travel, and firmer, can mean a less extreme angle.

I’m no expert, by any means, but these are the things I mostly understand. The suspension kinematics, less so.

On a trail bike you should only be pedaling on steep up-hills, if you are pedaling on the flat you’re doing it wrong. The seat position is therefore further forward to compensate and improve traction and reduce front wheel wandering. Downhill all riding will be out of the saddle, with the dropper post down, so the seat position is irrelevant.

On an XC bike you’ll be pedaling everywhere, so the seat position is more neutral.

This all makes sense, especially the sag part… When you’re sitting on the bike, the ST angle slackens, especially when you get up to mid travel or longer bikes.

Thanks to everyone for the replies.

Another way to think about it is that you compare the bike to similar bikes and not to XC bikes.

Even though XC bikes are rowdier than ever and trail bikes pedal better than ever, there’s still the concept of the right tool for the job.

As someone mentioned, trail bikes are optimized for optimized for descending in a way that even modern progressive XC bikes are not, with the long reach, short stem and very slack HTA. But then there’s the problem, you have a choice here between a very long wheelbase (steep STA) which is less whippy going down but let’s you climb the thing better, or shorter wheelbase with the slacker STA that’s more nimble but where you might get wandering front wheel while climbing. Riders value different things and every trail bike maker solves this differently, and that’s the beauty of bikes, they all have different character.

So short answer is when they say “steep STA for better climbing,” they are not making a claim that this characteristic is always better, they are rather telling you how their trail bike solves a trail bike problem, a problem that XC bikes don’t have because they use different tools to place you in the (usually shorter) wheelbase.

1 Like