seat tube angles

I see on here reference to seat tube angles often – this confuses me as you get to move your seat backwards and forwards on the seatpost to get the position of the seat adjusted relative to the BB – so why does seat tube angle matter? ( within limits obviously) as a slack seat tube with the seat forward will give the same position as a steep seat tube with the saddle back?

Can anyone explain to me what I am missing?

There’s a limit to how far backwards or forwards you can move your saddle and it also – side point – looks odd at extremes.

aye – but a few degrees in the seat tube surely only equals a inch or so on the saddle adjustment to get the same position?

I set the saddle to the same relative position on all my bikes – so does seat tube angle really matter? A slack seat tube I would have it forward, a steep one back? Indeed I have two bikes with very different seat tube angles but the seat is in the same position to the BB on both

For two bikes with the same reach a steep seat angle will mean the bars are also further forward, taking the whole body weight forward- no?

I designed a steeper seat angled CX bike for Nick Craig once.

As tj indicates, he just set his saddle x-mm behind the BB as that’s where he ran it.

He was wrong but he’s faster than me so I’m not going to argue.

Seat angle is relevant in relation to the golden triangle of seat, bars and pedals. And particularly comes into play on steep climbs. Even more so with longer travel suspension that sags into rear travel on steep sections. Steep seat angles maintain that correct bb position under those conditions where you really do want optimum pedalling power.

But that would mean a different seat to BB relationship?

I shall go and look at the setup of my various bikes a bit more closely

I think it’s more of an issue on slacker bikes with low standover and then 170mm+ droppers. If the SA is slack then that put you way too far over the back wheel which makes climbing a problem.

For two bikes with the same reach a steep seat angle will mean the bars are also further forward, taking the whole body weight forward- no?

Reach as a contemporary dimension is the horizontal distance from the bb to the top centre of the head tube. So no. It would be the same.

If you meant top tube length, then yes.

Ah – that makes sense Brant – so one is optimised for riding on the flat the other for riding up steep inclines?

But that would mean a different seat to BB relationship?

I shall go and look at the setup of my various bikes a bit more closely

The PlanetX Compo, sold in around 2001 I think, was the first bike I did with a steep seat angle as I decided I wanted the seat in the right place when going up steep hills, rather than having to tickle my arsehole with the nose of the saddle which was the case on many bikes at the time.

Ah – that makes sense Brant – so one is optimised for riding on the flat the other for riding up steep inclines?

Yeah that. With the option to adjust a bit to split the difference of course.

Brant – you are of course correct in that I meant TT. But have I got it right that a steeper SA will tend to mean the bars are further forward (I’m not a bike designer, better with electrical networks).

So is the next technological development a gadget to move the seat fore and aft so you can be optimised both for the flat AND for steep uphills?

😉

ta folks

🙂

Now you need to get that operated by some complex electronic or hydraulic system and you have the ext big thing! Sorry but not patentable now as I have described it!

aye – but a few degrees in the seat tube surely only equals a inch or so on the saddle adjustment to get the same position?

Of course you can adjust your seat +/- from whatever neutral position the frame geometry dictates. But that’s the same for any seat tube angle and the adjustment range is quite limited. So while there is inevitably some overlap from seat rail adjustment, a steeper STA will still generally allow you to sit more forward relative to the BB. Ideally you would want your seat to be clamped somewhere reasonably close to the centre of the rails rather than at either extreme end where you may simply run out of adjustment range. Plus it’s not ideal in either a mechanical or aesthetic sense.

Also a slack seat tube means there is more variation in fore-aft seat position as the dropper goes up and down (although not necessarily so on bikes with curvy seat tubes where the actual seat post angle is quite different from the effective angle relative to the BB) and there’s nothing you can do about that (except for developing an “active” seat rail system as described above)

As reach keeps on increasing, unless we are evolving longer arms, it’s inevitable that seat tubes will get steeper to compensate.

But with reach measurements getting longer alongside steeper sta’s it means your seated position remains similar to an “old” geometry bike, you get the benefit of the reach when out of the saddle.

I haven’t measured but I’ve gone from a bike with 420mm reach and circa 71° sa to a bike with 510mm reach and 77° sa. Seated feels quite similar but the new bike feels significantly longer when stood up on the pedals.

Some of the reviews of new bikes with long reach and steep seat tube angles (76/77) are describing a slightly cramped feeling when seated and pedalling along the flat.

Increased reach has pushed the bars further from the feet but steeper seat tubes have moved your arse closer to the bars.

I went to a presentation about geometry with Chris Porter a few years back. While others were saying slack head tubes made climbing difficult he said climbing was only about the seat tube.

If most of your riding is seated and flat(ish) effective top tube is a useful measure. If it’s winch and plummet, you’ll want a steep seat tube, slack head angle and a generous reach.

I haven’t measured but I’ve gone from a bike with 420mm reach and circa 71° sa to a bike with 510mm reach and 77° sa. Seated feels quite similar but the new bike feels significantly longer when stood up on the pedals.

Your bike’s wheelbase is almost certainly longer and you are just sitting further forward relative to the BB. I presume your arms are the same length as they were before you got your new bike. On really old bikes I remember having to hang way off behind the seat on steep downhills just to avoid going OTB. It wasn’t pretty at all!

If most of your riding is seated and flat(ish) effective top tube is a useful measure.

Yeah, I always look at the effective top tube length when comparing frame sizes. If you simply went off reach alone, you would think everyone had turned into knuckle dragging primates over the last 5 years.

Yes my wheelbase is significantly longer but as you say I’m more centred on the bike and seated still very comfortable

A 3 degree change in the seat angle is like moving the saddle about 40mm on the rails. So:

“I haven’t measured but I’ve gone from a bike with 420mm reach and circa 71° sa to a bike with 510mm reach and 77° sa”

That’s like moving your seat 80mm forwards, which is quite a long way! Further if you’re tall with long legs, would be over 100mm.

It’s possibly worth mentioning that seat tube angle is a dynamic thing, at least on full sussers. For example I have a 155 mm travel bike and a 130 mm bike with seat tube angles within half a degree (the 155 is 0.5 deg steeper). When I ride them both the 130 mm bike feels like it has a much steeper seat tube simply because it sags less.

Lengthening the reach and then compensating by steepening the seat angle so you end up with the same effective top tube does not result in the same pedalling position. The seat is more nearly above the bottom bracket and this will affect how far, relative to your pelvis, your femur is rotated as you pedal. Take this too far and you are not pedalling so efficiently as your gluteal (buttock) muscles are operating outside their optimum range. You will naturally compensate for this by hunching down over the handlebars more when you need pedal power. But with the high bars of a mountain bike, this might result in a hunched-up feel. That applies whether uphill or on the flat, but uphill your hands will be taking less of your weight so it might feel more comfy. On the other hand, the more forward body weight will help on really steep climbs, less having to perch on the nose of the saddle to get your weight over the pedal and avoid wheelies.

https://redshiftsports.com/dual-position-seatpost

I think it’s more of an issue on slacker bikes with low standover and then 170mm+ droppers. If the SA is slack then that put you way too far over the back wheel which makes climbing a problem.

Irrespective of a bikes slack or lowness or dropper this has been a tall person problem since the beginning of mtb. IF everyone wants a certain hip to pedal angle then the long of leg will be much closer to the rear axle.

Made worse with full sussers where the tube itself is not inline with the theroetical seat tube – ie a line through the middle of the bottom bracket. Manufacturers then quote an “effective” angle at an indeterminate height, with no explanation on how this changes on the different sizes.

Although regardless of what the stat sheet says your angles are, when you’ve got a big 170 full susser, if youve set it up 30% sag both ends when stood up, when you settle in to a long grinding climb sat on the saddle, you’ll be looking at 50% rear sag and 5%. with big travel theres a couple of degrees there.

I set the saddle to the same relative position on all my bikes

What do you use the different bikes for?
I have 2 bikes, rigid 29er which gets used for usually solo fitness type rides in quite flat terrain. Efficient pedalling on the near flat is important here, as it gives a good increase in speed and efficiency on the terrain I ride it on. On the few steep hills I’ll likely be stood up and sprinting in the bottom 1:1 gear. its seated position is very much like a road bike (roadies do have this pedalling thing down)

trail/AM full suss bike, has a steeper angle (as designed). As its used on trails, most of the riding is stood up, save for the climbing, where the steeper angle helps. on the rare occasions it needs to travel on the flat road, I can feel that it isn’t as comfy or efficient a position. Reduced power here doesn’t hugely bother me in this application as I understand where else I have gained it.

Rule of thumb… one degree in seat angle is 15mm in fore/aft saddle position

For 15mm/degree STA to be the ROT you’d need to be really tall. Nitpicking I know.. but it’s ~12mm per degree for my saddle height between 770-780mm and I’m a bit over 6′. For averages / ROT 10mm maybe 12mm per degree is handy.

Steeper seat angles are good on bikes that spend most time on steep ups then downs, you might want 75+ static STA on those so that the range of adjustment available works. I find the same 75+ STA hard to make work for longer XC rides on more varied terrain though, for long day comfort you generally want the saddle position a bit further back to balance you on the bike without pitching your weight too far forward. It can be fine for a few hours though.

Of course you can move the seat back and fore, but the centre point changes with the seat angle of course.

But, given that we usually want our arms a similar sort of distance away, the change in seat angle is actually moving the pedals backwards, which is the key development IMO. On my old bike with its 72 degree SA that’s what I notice. It means that when I move to stand up I have to move my body quite a way forwards. And when I put myself in a position to jump the bars feel quite close.

I could use a longer stem though. It’s currently 50mm, but if I go to 60mm the steering feel gets worse.

If EBB solutions weren’t so expensive I’d have one to grab 5mm more reach, and I’m tempted by one of those Superstar reach extenders for another 5mm 🙂

Just to add to this, I had my first proper ride on my new hardtail with the 77° SA yesterday. In comparison to my old bike climbing is considerably easier up steep and wet inclines. Obviously there are other factors such as wheel size and tyres/pressure but it was night and day. I used to have to perch on the end of my saddle hunched right over and would still get the front wheel lifting and then rear tyre spinning under power, new bike had none of these issues. I was taking a bit of a gamble speccing such a steep SA on a custom frame but I’m sold already.

I used to have to perch on the end of my saddle hunched right over and would still get the front wheel lifting and then rear tyre spinning under power, new bike had none of these issues.

Yes! That!

So what about rigid bikes – is a steep SA as important for climbing, or does it make less difference? I’m thinking about 72 on a Stooge and 75 on the new Kona Unit?

So what about rigid bikes – is a steep SA as important for climbing, or does it make less difference? I’m thinking about 72 on a Stooge and 75 on the new Kona Unit?

Seat angle is less important on singlespeed bikes of course. Can you think why 🙂

Seat angle is less important on singlespeed bikes of course. Can you think why

Ah, but I didn’t say ss 😉

The National Anthem in 1960’s Ilkley?

“I find the same 75+ STA hard to make work for longer XC rides on more varied terrain though, for long day comfort you generally want the saddle position a bit further back to balance you on the bike without pitching your weight too far forward. It can be fine for a few hours though.”

I’ve found tall stack height plus big riser bars helps with steep seat angles.

I rather like long chainstays nowadays because you can go fairly steep with the seat angle (about 75 deg) and then stop any wheelies by having 450mm+ of chainstay length. And I prefer the handling balance for cornering with that (my Levo is 455mm reach and chainstay length, which works great).

Most people just wop the saddle down in the middle of the rails and ride from there. Few get out the plumb line to set fore and aft on the rails. Seat tube angle will then have an effect. And it can be plus or minus 2 cm.

Seat angle is less important on singlespeed bikes of course.

Yes, big part of why I prefer my Jones as a SS. It’s got a comfy slack STA but I don’t climb anything close to steep sat down, I can’t.

CGG, agreed on what a longer CS does, though I meant (not that clear in my post) more about how I fit the bike, how I balance up between saddle, bar and pedals. ie the drawback of a steep STA is it can tip my c of g too far fwd and creates wrist / arm / shoulder pressure when a good efficient bike fit for long XC rides is needed, vs the positives it has for the winch+plummet use of an enduro bike. But who rides XC now anyway : )

And it can be plus or minus 2 cm.

plus where I sit comfortably on one saddle vs another can be +/- that much vs the same rails or post position also.