Mountain Bike Seat tube angles - a full explanation

A common trend over the past few years has been the move to steep seat tube angles on mountain bikes, especially in the Enduro / Trail Bike segment. This would mean bikes with 120-160 mm travel roughly. 

Historically, mountain bikes have featured on average a 73 seat tube angle and many are still really close to that. I think there are a few around with 72 degree STA but more common is the steeper ones around 74 degrees and plenty more examples around 75-78 degrees. For reference, a slack seat tube angle would be seen on a beach cruiser around 65 degrees or so and a very steep seat tube angle would be on a triathlon bike around 78-80 degrees. The higher number means your hips are further forward...think of a unicycle with the seat directly over the cranks, that would be 90 degrees. 

Before we go too deep here, be aware that there can be a virtual and actual seat tube angle on many bikes…the virtual one is what is most important. The diagram from Turner below is a great representation of this.

Anyway, I think the push towards steeper STAs have been in part driven by slacker head tube angles. The slack HTA makes for better stability and confidence on nasty downhill stuff but a wandering front wheel on climbs. By moving the rider further forward with the steep STA there is more control on climbs. 

Most bikes in this category also have dropper seat posts which means the saddle is out of the way on downhills so the angle of the seat tube doesn’t really matter all that much descending.  The primary time STA matters is during seated pedaling, which most riders do LOTS of. If you want to dive into this topic more, look over articles from Steve Hogg or Keith Bontrager on this.

Let’s go over a few practical details:

Respect the Design: If you already own a mountain bike, my best advice is just start with the saddle in the middle of the seatpost. Bikes are designed by very smart people and I like to stick to the intended fit and design without going too crazy. Big travel trail bikes will have the seat more forward than a cross country hardtail, there is no way around it. Don’t try to make the bike something it isn’t. Often riders with a short torso and long legs will like the seat moved forward some and riders with more normal proportions or a long torso and shorter legs will prefer the seat moved back for reasons mentioned in the Body Proportions section below. Bikes with a steep STA tend to feel too short in the reach from saddle to bar when seated and this is really just a compromise of modern geometry.

Terrain: Are you always climbing steep, then descending steep? A steeper STA is great here as you aren’t worried about flat ground pedaling all that much. A STA that is too steep will cause too much weight on the hands in hard singletrack riding and really make for some weird handling. Around here in the Southeast I’d lean towards more of a slacker STA due to how much seated pedaling we have. Also consider the wheelbase of the bike compared to your favorite trails. Bikes are getting longer and this isn’t always a good thing. 

Body Proportions: Everything else being even, a rider with a shorter torso will want a steeper STA! The shorter torso requires less distance for the hips to be behind the bottom bracket to counterbalance the position. Heavy feet, light hands ya know? Long torsos will usually demand a slacker STA to achieve this. Also, the rider with a more aggressive forward lean while pedaling will often like a touch slacker STA since the more horizontal torso position projects more weight forward onto the bars. As we talked about above, this is somewhat predetermined for you based on what bike you have.

Personal Preference: Go test ride some bikes! Decide for yourself versus letting Specialized or Trek dictate what YOU should be on. Riders that do lots of accelerating out of turns and out of the saddle climbing will be more sensitive to differences due to the frequent body position changes. Often, you will need to change your riding style a bit to suit the bike that you are on.

Bike Choice: In general, longer travel bikes have a steeper STA and a more upright position MOST of the time. There are exceptions to this, but I’ve noticed I like the steeper STA on my old Fuel EX but would HATE it on my new Procaliber hardtail. Full suspension bikes will often make better use of a steep STA than a comparable hardtail due to the suspension sag climbing which makes the bike even slacker.

Getting Used To It: Anatomy wise, the forward seat position means more quad usage and less hamstrings, glutes, and low back. This means the knee is further forward over the pedal spindle, which I’m not too worried about as KOPS is kind of an arbitrary relationship. This means the pedal stroke is a bit “knee dominant” which isn’t a bad thing in my opinion. 

How to adapt?

Firstly, make sure your seat height is correct. Too low and too far forward is BAD. Also doing some solid off bike training is a good idea. Goblet squats, step ups, and split squats are good options. The seated knee extension machine is solid also. Don’t be scared of it. Also regardless of what STA the bike comes with riders with long legs will need a more forward seat position vs the long torso crowd. Also riding a foot position a bit more forward on the pedal than usual can help a steep STA feel better. 

Hopefully this serves as a more objective guide to seat tube angles. It is way more than just accepting that really steep STAs are the future for everyone that rides trails. 

MOuntain bike bar sweep: all you need to know.

Let’s talk sweep for mountain bike bars. There are lots of options out now which is great as there is something for everyone!

First of all, let’s get it out of the way. People are different. I feel smart just saying it. There WILL be someone that disagrees with this and rides super sweep bars, flat bars, or drop bars on their mountain bike and love them. Keep loving them. 

Now, moving beyond “people are different” and “it depends”.

Let us assume here that bar width is close to what I consider natural, about the same width you would do pushups at and the bar drop and reach isn’t crazy. If you hold a bar in front of you about halfway to your chest you will notice your upper arms “bird wing” about 30 degrees out from your torso. Notice if you really force them you can bring them up to about parallel to the ground or if you force the elbows down with the bar in the same position you can maybe get them to 15 degrees out from your ribcage.

This means about 30 degrees upper arm away from torso is pretty normal. Now that the hand is in position we can talk about the angle of it. We want the wrist here in a position where it is balanced in ulnar and radial deviation or if you think about throwing a punch, the wrist would be straight….basically it would take an equal amount of load on either side of the thumb or the pinky to deviate the wrist. THIS is neutral.

Also you can turn your forearm vertical with palm toward you and hold a bar with equal weights on each end and see what angle the bar lands at to balance itself. For me this was about 8 degrees relative to the forearm.

Guess what? I ride an 8 degree backsweep bar. This gives a comfy and efficient position where force can be transmitted through the bars and through the upper extremity without undue stress on the wrist. 

I would encourage everyone to try this as you might like a bar close to 15 or 20 degrees sweep. I have heard folks mention holding pencils out in front of you and seeing how they point. I think this is a poor test since there is NO load put through the joint and the other relevant upper extremity angles aren’t the same as on a mountain bike. Yet again, if you think that test rules and mine is awful, that is cool with me.

There ya have it. I think the stock 9 degree sweep on most bars is really nice and a good starting point. If you want to try down to 6 degrees or up to the higher sweeps feel free to but beware as the sweep increases it typically forces a riders elbows down to their ribs giving less leverage in technical riding. This could be fine for a commuter or touring bike maybe. Shoulder injury status can also impact bar choice here so if things hurt...try some different setup tactics. Same for hand and wrist pain or numbness. If a normal setup isn’t working, try a less normal setup!

For most bars the logo near the stem should face forward and you can fine tune that rotation to your comfort but be careful not to roll them too far forwards as this tends to make people lock the elbows. I also check and document upsweep measured at the grip. This improves setup consistentcy. Hope that helps and I’m happy to answer any questions!

Crank length. The long and short of it.

Are you on the correct length cranks?

Maybe a better question is “does one crank length work perfectly for everybody?”

I know that is overly simplified but I think it makes the point early on here. Of course it matters! Repetitive activities are more comfortable and efficient with a certain dimension for the feet to travel. Think about watching a toddler navigate stairs...a bit scary due to their balance but more relevant to this discussion is how awkward it is biomechanically due to the mismatch in step height and leg length. The stair climbing session would be much easier for them if the stairs matched their legs!

From a variety of studies over the years, there is a general consensus among bike fitters and manufacturers that riders can use a variety of crank length across a fairly broad spectrum with no detriment to power or gross efficiency while riding. There is also a tendency among researchers and fitters to recommend a shorter crank length when in doubt. I am not going to jump into the ratios between femur and crank length and all that as I think it is a bit of a distraction from more relevant issues here.

A 2017 study led by Ferrer Roca concluded -  “At the same time, the maximum flexion and range of motion of the hip and knee joints were significantly increased (1.8–3.4° and P < 0.05), whereas the ankle joint was not affected. In conclusion, the biomechanical changes due to a longer crank did not alter the metabolic cost of pedalling, although they could have long-term adverse effects. Therefore, in case of doubt between two lengths, the shorter one might be recommended.” There is a less often cited study from 2010 involving female mountain bikers that showed shorter cranks allowed them to return to peak power quickly from a coasting state. Think about how often that scenario plays out in cyclocross or mountain bike racing!

Now, from here, it easy to just toss the discussion into the bucket labeled “Oh Just Do Whatever You Want”.This is not totally wrong but we can travel a bit further in the quest to optimize the bike to the rider. There are a few formulas out there and honestly, the only one I really trust comes from the guys at BikeDynamics. Check their site out below:

http://bikedynamics.co.uk/FitGuidecranks.htm

I usually consider biomechanics of the rider as my primary guide for selecting crank lengths. This is most commonly an issue for riders under 5 ft 5 inches in my experience. What this really comes down to is this. Does the rider have the joint mobility to easily get through the pedal stroke? The easiest way to check this is the following:

Put the bike in the trainer

Get  in the drops or aerobars (whatever the lowest torso position is)

Align the cranks parallel to the seat tube

Unclip the top foot and rest it on the pedal

Raise the top foot off the pedal without contorting yourself. If this is really tough I get a bit concerned the cranks may be too long. Ideally there should be some surplus movement leftover. This is a combination of actual mobility of the hip joint in the socket AND the stretngth of the hip flexor muscles to pull the leg upwards. This exercise assumes the rest of the position is close to correct of course…

Another red flag for me is riders with complaints of difficulty finding a good saddle height as they are close to the border of too low AND too high at the same time. Other complaints I have seen associated with too long cranks are the feeling of always being in the wrong gear climbing and the hips rocking excessively side to side during standing climbing.

Between these tips and the Bike Dynamics link above, you should have a reasonable starting point for getting the crank length that matches you and your goals. As I mentioned before, if you are not sure, I generally steer riders into a shorter crank length. Another thing I will mention is that I do not think it is imperative the cranks are the exact same between all the bikes a rider has. For example I happily ride 175mm cranks on my mountain bike and 172.5 on my road bike...BUT if I rode a TT bike I would 100% be on 165 cranks. 

Side note for mountain bikers: Many folks are going to 165 or 170 cranks on mountain bikes to reduce pedal strikes on rocks. Definitely worth considering if you have this issue.

I hope this provided some clarity on what is a hotly debated topic in the cycling world that too often ends with the caveat of “Well, it depends.”