# Sizing help: Seat Tube Angle - Does it affect the bike fit?



## rian4224 (Jul 31, 2017)

I posted this in a different section but since this I'm a big rider I thought I'd post it here to see if any fellow Clyde's had any thoughts as well all deal with fit issues:

I'm a tall rider (6'7") and looking to add a full squish to my small fleet (1 XC race hardtail). I went to a professional bike fitter to get properly fit on my hardtail, so I have most of my measurements set up on where I need to be.

in looking at new full suspension bike geometry I see the obvious trend towards steeper seat tube angles and longer reach. My question is how does this effect the fit of a bike?

If I know the measurement of my "ideal" saddle tip position (setback from the BB) and saddle height from BB, wouldn't the seat tube angle be kind of irrelevant since I'll adjust the saddle fore/aft to meet this point?

with that said, reach would be the best indicator or room on a bike vs. the effective top tube measurement, correct? (Stack too but that doesn't apply here)

i ask because I see some of the "bigger" newer bikes with longer reaches (500mm+), but shorter ETT (650mm-665mm)...I'm used to seeing 685mm-690mm ETT so I'm just making sure I'm thinking correctly.

and I know riding on is the best way to find out about fit but being tall and where I live makes it tough to do that.

thanks for any info!


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## pctloper (Jan 3, 2016)

Sitting ----ala climbing ETT is the dimension that matters---and this is probably 90% of the time---reach is the dimension for descending since you are standing up--hopefully

I struggle with the new geo---being 6 foot and 345.5 inseam many new bikes are too short in ett----but long in reach----and I am getting on in years so need to climb well so need the proper ett---about 630 with reach I am good up to 460---beyond that I struggle

You need XXL for sure and I cannot comment at that height but suspect ett tends to be too short and stack too low


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## attaboy (Apr 4, 2008)

Yes, you’re thinking correctly. It effects fit because it places you more on top of BB and decreases distance from seat to handle bars. I demoed a L bike with 471mm reach and 76 degree STA and it was way too short even though reach is far greater than my current ride.


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## rian4224 (Jul 31, 2017)

attaboy said:


> Yes, you're thinking correctly. It effects fit because it places you more on top of BB and decreases distance from seat to handle bars. I demoed a L bike with 471mm reach and 76 degree STA and it was way too short even though reach is far greater than my current ride.


Thanks! But how much can you adjust the saddle fwd or back to make up for a steeper (or slacker) seat tube? I know if I drop a plum line from the saddle tip I want it 110mm behind the BB. That gets my kneecap right where I want it over the pedal.

So in theory a steeper seat tube should not matter, as long as you have enough wiggle room to slide your saddle back?

I know I'm totally overthinking it but just trying to understand if these newer geo setups are actually better for a taller rider. I'm looking to make the jump and want to make sure I get something I'm not cramped on.


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## LyNx (Oct 26, 2004)

Well, you're one of the few who seem to understand about saddle position not being something you use to help fit a bike, that's amazing. Yes, Reach AND Stack are the two measurements you're looking for, as two bikes could have the same Reach numbers, but one has a Stack that's 40mm lower, so you'd need to add those spacers to get to the same Stack as the other, in doing so, you then would be moving the Reach back because of the head tube angle, so in effect it would have a shorter Reach. 

Effective top tube is a waste of time and is dependent on STA, ignore it, unless you couldn't find a Reach number, then you can somewhat extrapolate Reach using it and STA.

If the STA is too steep and you can't get the saddle far enough back using rail movement, then you'll need to look for a setback post or one that can use a setback head. I am in this situation, not many posts out that have a setback head, the X-Fusion used to have, but don't anymore, so when I went for a dropper nearly 3 years ago I went for a 9point8 FallLine as you can change the head from straight to setback.


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## Fuse6F (Jul 5, 2017)

ETT does tell you alot. 

A short reach may still accomodate a large rider if the ett provides a decent amount of negative reach. A larger rider can accomodate that offset alot better than a smaller one. 

i did some measurements using paint where you draw a horizontal line from top of head tube back, then vertical up from the bb and also along the seat tube. it was interesting to see how some bikes while the angles are laid back or steep, can actually be offset from the bb. not all measurements are the same. instinct, fuse, hightower, fuel ex

see how far back the nose of the seat is, regardless of the sta.
also where the seat tube angle passes through the bb. that makes a difference to your feet as much as where your hands go on the bars. A set inseam height will have you more forward or back. 

all these bikes have almost identical chain stays and they stay the same across all model sizes in each frame design.


bike fit matters alot.


FWIW and IMO regarding excessively short negative reach!
to climb better forward geo bikes shift the power point further into the crank rotation. When on a slope, that is offset by gravity. When on long rolling flat single track you end up forcing more weight into your hands on the bars. Eg an exaggeration but think of a seat ahead of the bb. Sta affects the style of riding you want to do.


i think steep sta make it harder on taller riders as your cg is forced up higher and they just dont adjust the stack all that much. you end up riding upside down. GG bikes have alot of stack for that reason i suspect.

interestingly, the pole seat tube angle passes through the bb like that of the fuse. the pole also has a huge 455mm chain stay. something everyone says is unrideable and a terrible idea. Not poppy and hard to manual etc. its also got. A 670 stack and a 535 reach. but ett is only 662. So negative reach is 127mm. 

if gwin (5'8") is riding a xl downhill bike frame, what should a larger rider be on???


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## jeremy3220 (Jul 5, 2017)

The idea is that the longer your inseam the further back your saddle is on the bike. A steeper STA definitely helps put you in a better position on the climbs. It can be the difference between having to lean forward to actively keep the front tire on the ground versus casually spinning up on the same grade. I have an XXL Hightower LT and the seat is slammed forward. I would like the seat tube about another 2° steeper but the reach would need to be appropriator longer to compensate (fine by me). 

Longer chainstays would also help out taller riders by make the fit more proportional across the height range. Also, longer travel bikes need a steeper STA than shorter travel or hardtail bikes since on the climbs they'll rotate more (sink into the travel more). A good shock also helps a noticeable amount by keeping the bike calm while climbing, an under damped shock will cause you to have to deal with the bike wallowing back on climbs.


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## LyNx (Oct 26, 2004)

ETT doesn't tell you sweet fvck all, not compared to reach and Stack combined and if you only go by ETT, then you will end up with bikes the wrong size, I know -_ early on that's what I did and damn was I wrong._ I had one bike with a 25.4" ETT and then when I was looking at another, I went soley by ETT and bought the other, bother size XL, the difference, the first had a Reach of only 448mm, the second's Reach was 475mm. Never could get that bike to handle the same with saddle in the same position relative to the BB until I started looking at Reach and figure it out.

FYI, ESTA - Effective Seat Tube Angle, is a line drawn directly up from the centre of the BB to the centre of the Saddle rail clamp. It is used to help give a more accurate idea of angle where frames have bends in them or are straight and attach in front of the BB etc, but it is also listed at a specific height up, which no one seems to list/tell you, so only sort of helpful.



Fuse6F said:


> ETT does tell you alot.


It tells me that first up, Gwinn is riding soley DH, doesn't pedal that bike up hill and that is what HE likes, there are many more riders who prefer smaller fitting bikes and also remember that one brands XL is anothers Medium, so don't be so silly as to look at the pros and then just try to emulate them, it doesn't work.


Fuse6F said:


> if gwin (5'8") is riding a xl downhill bike frame, what should a larger rider be on???


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## Fuse6F (Jul 5, 2017)

first, what ever works for you is all that matters.



LyNx said:


> ETT doesn't tell you sweet fvck all, not compared to reach and Stack combined and if you only go by ETT, then you will end up with bikes the wrong size, I know -_ early on that's what I did and damn was I wrong._ I had one bike with a 25.4" ETT and then when I was looking at another, I went soley by ETT and bought the other, bother size XL, the difference, the first had a Reach of only 448mm, the second's Reach was 475mm. Never could get that bike to handle the same with saddle in the same position relative to the BB until I started looking at Reach and figure it out.


I could be wrong... but... as i understand...
ETT gives you an idea of what size upper body your trying to fit into a bike. a reach of 500 with an ETT of 700 is not the same fit as reach 500 and ETT of 450. (exaggerating here)

in your example above, if both bikes had the same ETT but the reach was 27mm different, then your negative reach will be different also, shifting your body forward toward the bb. assuming the same seat tube and head tube angles. you then need to go higher on the bike to fit your legs in and the stack would feel alot lower. like riding on top of the bike and not inside of it. you would also feel a different front rear balance when seated. if you tried moving the seat back to compensate achieve the same position relative to the bb, it would make the bike super long/big.



LyNx said:


> FYI, ESTA - Effective Seat Tube Angle, is a line drawn directly up from the centre of the BB to the centre of the Saddle rail clamp. It is used to help give a more accurate idea of angle where frames have bends in them or are straight and attach in front of the BB etc, but it is also listed at a specific height up, which no one seems to list/tell you, so only sort of helpful.


thats why i was showing the lines through the seat tubes.

the seat always moves rearward with increasing height, but how much is variable between designs. what is consistent is that the rear triangles of all the bikes in a model are the same (almost all the time). so looking at ETT on a size to get the negative reach and then doing this little exerecise in paint gives an idea of what amount of space you might find behind the bb between different bike brand/models. and how that might change with seat height.

when i go to pick a bike, I choose a head tube angle for the riding style im after. next i want to find my standing positions fit. Stack and Reach. since standing and attacking is done with horizontal pedals, my seated position has to be lower to accommodate turning the cranks. this should push your seat back a bit to accommodate the length of your arms as you drop down into pedaling position. how far back you move IMO depends on whether you spend most of your time putting down power on the flats or going straight up. eg. and to exaggerate, a seat tube that leans forward puts a ton of weight on my hands for all day comfort in the flats. but feels great going straight up. a seat too far back has me pulling on the bars when powering on the same flats (think recumbent bike here) and is even worse on a climb. Everyones body is different and i think most bikes accommodate the average rider just fine. however, us tall people, no so much.


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## Joules (Oct 12, 2005)

If you look at old school, slack seat angle fit: When you go from seated to standing, you're shifting your CG up and forward. As droppers have proliferated, and suspension travel has gotten longer, people have realized it's advantageous to... not move forward as much when you go from seated to attack position; a steeper seat angle combined with a longer reach keeps your CG farther forward when seated, so you don't move as much when moving to attack position (i.e., you aren't shifting your fore/aft weight bias when standing)

My last bike had a huge problem with this. It had a respectable "effective" seat angle, but like most bikes these days, the seat tube was bent, and it was only effective at the proscribed saddle height (the actual angle of the post was like 65*). But tall people... especially those with long legs, run the seat way higher than the geometry chart, and the seat angle becomes effectively much less. Result is when I'm on the saddle, my weight is almost 100% on the rear wheel and the suspension rides much lower seated vs standing (standing I'm at a 55/45 or 60/40 bias, seated it's over 80% on the rear). So it's a compromise; suspension set for attack position or climbing position. Climbing is something I do so I can attack, so I focused on the former. That made an already slack bike even slacker when climbing - the one time you don't necessarily want to to be slack, and when I wasn't pedal striking because the sagged BB was only 9" high at that point, I was inadvertently doing wheelies up hill.

The short version of all this rambling is it depends on the bike and what your goals are. If it's a hardtail or short travel XC bike, and you're going to be earning your money while seated and climbing efficiency is all you're worried about, I think treating ETT the old school way is at least somewhat reasonable. For that matter, every bike fitter I've ever talked to only worked on this aspect - maximizing power. I've never met a pro bike fitter that even considered suspension performance when fitting bikes. 

If you're going full enduro with a longer-travel bike and max power isn't your main goal, the question of how the 2 positions are going to affect your suspension, and if it's even possible to make a reasonable compromise between the 2... that's something you should think about. For me, that means a steep seat angle and long chainstays (at least longer than the "as short as possible" fad from a couple years ago).


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## Tjaard (Aug 17, 2007)

rian4224 said:


> I know if I drop a plum line from the saddle tip I want it 110mm behind the BB. That gets my kneecap right where I want it over the pedal.
> 
> *So in theory a steeper seat tube should not matter*, as long as you have enough wiggle room to slide your saddle back?


You have it exactly right. You set your saddle where you want it. As long as you have enough adjustment in your seat post to get it where you want, the listed ETT and effective seat angle is meaningless.



rian4224 said:


> Thanks! But how much can you adjust the saddle fwd or back to make up for a steeper (or slacker) seat tube?


About 1.5cm either way usually. With the same seat post. 
Trick you can do: on a Reverb or similar post, reverse the plates that clamp the saddle rails. If that's not enough, use an offset seat post mounted backwards.

As tall guys, we don't have to worry about it being to steep, only about it being to slack! This is because virtually every full suspension out there has a super slack true seat angle*, so when you raise the seat height way up (like we need to) you end up with a effective angle WAY slacker than the listed effective seat angle (which is a fairly useless number, since they don't even tell you at which height they measured it).

The other thing to remember, your bike fit was on a hardtail, and perhaps looking at general riding (I know, I'm a bike fitter myself), but if you are climbing on a fullsuspension bike, weight shift will cause the rear to sag more than the front, leading to a slacker effective angle.

So, I would suggest setting you saddle at least a cm or so further forward than on your hardtail (you're using the *same saddle* right?) and then you will need to raise it about 3mm to keep leg extension the same. Then it is your choice whether or not you wish to adjust your grips forward too.

*except for Pole.


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## Tjaard (Aug 17, 2007)

You must spread some Reputation around before giving it to Joules again.
You must spread some Reputation around before giving it to LyNx again.
You must spread some Reputation around before giving it to jeremy3220 again.


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## Tjaard (Aug 17, 2007)

pctloper said:


> Sitting ----ala climbing, ETT is the dimension that matters---


This is incorrect. ETT is a _virtual number,* not an actual measurement!*_
This is a common misunderstanding. If, like the OP mentions, *you set your saddle where YOU prefer it for pedaling*, not at some random spot where the seat tube happens to puts you, you will have a different "saddle to headset" measurement than the spec. 
Note that stack and reach started with Triathletes, who spend their entire time seated.

What you need to know for seated bike fit is Stack + Reach, and whether you can set the saddle where you want it (which has to do with actual seat angle and offset, unfortunately 'effective seat angle' is not very useful).



pctloper said:


> ---_reach_ is the dimension for descending since you are standing up


Only in combination with stack. Modern mtb's are so slack that the ratio is about 0.4x Stack=reach. So if you have a bike with a 20mm lower stack, it will fit identical to another bike with 20mm higher stack and 8mm shorter reach.

In the table of Bikes for the very tall, @Asollie has listed the distance from BB to top of Head tube, that's a great way to get a quick, at a glance, idea of true size. For example, the new Megatower XXL is the 8th largest bike listed there.



pctloper said:


> I struggle with the new geo---being 6 foot and 345.5 inseam many new bikes are too short in ett----but long in reach----


Again, as long as you set your saddle and grips the same position from the bottom bracket, the distance to the handlebars should be the same.


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## Tjaard (Aug 17, 2007)

moved


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## Tjaard (Aug 17, 2007)

Fuse6F said:


> ETT does tell you alot.
> 
> A short reach may still accomodate a large rider if the ett provides a decent amount of negative reach. A larger rider can accommodate that offset a lot better than a smaller one.


What do you mean with "negative reach"?

Where you (and very many others) go wrong, is in assuming a rider should set their saddle in the middle of the seat tube.

I am a bike fitter, and mtb'er, and while their are many reason to have your saddle further forward or back, the random location of the seat tube(which is mostly caused by design constraints of the suspension linkage) is not one of them.


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## Tjaard (Aug 17, 2007)

Fuse6F said:


> in your example above, if both bikes had the *same ETT but the reach was 27mm different*, then your negative reach will be different also, shifting your body forward toward the bb. assuming the* same seat tube* and head tube *angles*.


*This is impossible!*
The way you get different reach but same ETT is by adjusting the seat tube angle. If the seat tube angle is identical and the ETT is identical, reach will be identical also.

So from now on, let's assume you meant: same ETT but longer reach (and thus, steeper seat angle one one):



Fuse6F said:


> On the bike with a steeper seat angle, and longer reach, you'd be shifting your body forward toward the bb. You then need to go higher on the bike to fit your legs in and the stack would feel a lot lower,


Again, I would hope that if you raise your saddle, you raise your handlebars(or rather grips). You are arm-chair reasoning this. The unfortunate thing is, many riders do just that in real life too, rather than taking the time to properly set up their bikes (on their own or with a professional fit)



Fuse6F said:


> if you tried moving the seat back to compensate achieve the same position relative to the bb, it would make the bike super long/big.


Yes, it would indeed. A bike with _longer reach will feel longer_, both seated and standing. That's exactly the point us "_stack and reach proponents"_ are making!


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## Tjaard (Aug 17, 2007)

Joules said:


> For that matter, every bike fitter I've ever talked to only worked on this aspect - maximizing power. I've never met a pro bike fitter that even considered suspension performance when fitting bikes. .


I am a pro bike fitter, I've never met you , but as I wrote above (before I read your post) I absolutely do consider suspension performance in bike fit.

The first thing any fitter should do is ask the client what they are trying to achieve, what they are bothered by and what they are willing to compromise on. That holds true for a Tri bike fit just as much as for an enduro mtb fit.

The very first thing I was taught for suspension bike fit(over 10 years ago) is to set sag. After all, if you do that later, everything else changes. Then I set rebound and compression at baseline settings for their weight if they have not dailed it in themselves yet.

Typically my clients with XC bikes want max climbing and comfort on level sections, but for anyone who brings in a trail or enduro bike, I absolutely ask them what their priorities are, and discuss pro's and cons of different set ups.

Finally, I lend them a Sussmybike suspension data logger to get real time suspension set-up info, so we can fine tune their suspension set up at their follow up.


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## Tjaard (Aug 17, 2007)

Joules said:


> When you go from seated to standing, you're shifting your CG up and forward. As droppers have proliferated, and suspension travel has gotten longer, people have realized it's advantageous to... not move forward as much when you go from seated to attack position; a steeper seat angle combined with a longer reach keeps your CG farther forward when seated, so you don't move as much when moving to attack position (i.e., you aren't shifting your fore/aft weight bias when standing)


This I find to be a great benefit of the steep seat angle, that I wasn't expecting. It really helps on steep, technical climbs, where you simply stand straight up, vs having to "pull" your hips forward as you move to standing. Much easier to keep switching from seated to standing on the climb this way.

If this sounds appealing, but you find the reach to the bar to short in this set up, try "bar-ins"*


Joules said:


> If you're going full enduro with a longer-travel bike and max power isn't your main goal, the question of how the 2 positions are going to affect your suspension, and if it's even possible to make a reasonable compromise between the 2... that's something you should think about. For me, that means a steep seat angle and long chainstays (at least longer than the "as short as possible" fad from a couple years ago).


Everyone needs good weight distribution, no matter what the bike. Even DH bikes need enough weight on the front to corner well**, so keeping weight from being to far back is advantageous in both climbing and descending for us tall folks.

That means, long enough chainstays,steep enough seat positions, and not too long front centers. Since we do want a comfortably long reach, this could mean steeper headangles, and certainly short offset forks.

*https://www.leelikesbikes.com/bar-ins-the-new-bar-ends.html
*https://sqlab-usa.com/collections/innerbarends/products/inner-position-comfort-innerbarends
**https://www.pinkbike.com/news/behind-the-bike-developing-the-xxl-santa-cruz-v10-2016.html


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## Fuse6F (Jul 5, 2017)

Ipad poster. So too hard to cut and paste.

If the head tube and seat tube angle is identical between bikes, but one st passes through the bb and one st is located ahead of the bb could they then have the same ett but different reaches. Would your knee to pedal relationship be different bike to bike. 

negative reach is the distance backwards from where positive reach is measured from. Or ett minus reach. A small negative reach means the seat is located closer to directly over the bb.

If you try to apply power when the seat is directly over the bb (assume level ground). Does it not force your body forward causing you to resist that motion by pressing back on the bars to keep you in the seat? Yet if your seated to far back of the bb then you end up pulling on the bars as your power is forcing you backwards. 

If you actually intend topedal your bike. Then keep your seated position in mind or you will not like pedalling very much. Sure a forward seat position is great for an extreme climb, but how often are you doing that.


Are you saying the best bike geometry is one where the standing position is identical to the seated position. Cranks adjusted so they match. You just lower the seat via dropper to gain clearance for aggressive riding!? Because of different body sizes, Wouldnt that require different seat tube angles and rear center changes to match the changes in front center across the size range? Also to keep the bike weight balance the same f/r? Why arent the manufacturers doing that? Handle bar position seated vs standing would be the same as well? Sounds like a good idea. 

Im a tall clyde, so i cant find a bike that fits period. To start it would need 34” wheels. Reach and rear center is always a compromise. That locked in rear triangle design across the size ranges just ruins a bike fit for me. Dh bikes in the xl sizes are being ridden by 5’8” riders. The smaller bike sizes are built for the kids as they grow into the sport. Dh is a fairly small segment. But the only reason the bikes are optimized for smaller riders is that they make up a larger target audience. Imo. We clydes are the outliers. 

As a professional bike fitter you should know that fitting a mtb for a tall clyde isnt really a bike fit. Its more of a bike accomodation.


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## Fuse6F (Jul 5, 2017)

Tjaard said:


> Yes, it would indeed. A bike with _longer reach will feel longer_, both seated and standing. That's exactly the point us "_stack and reach proponents"_ are making!


a longer reach will feel longer when standing. But the seat position (or negative reach) dictates how the bike feels when seated. 500 reach on a 500 ett is the same standing as a 500 reach 700 ett. But they are way different seated positions.

my fuse xxl w 680 ett 480 reach fits my body rather poorly. But its the best on the market for me. What i really want is 30mm longer chain stays, 710 ett 510 reach, Stack could go up another 50mm also. 710 would be nice. Lastly fit 29x3.0".


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## Fuse6F (Jul 5, 2017)

rian4224 said:


> Thanks! But how much can you adjust the saddle fwd or back to make up for a steeper (or slacker) seat tube? I know if I drop a plum line from the saddle tip I want it 110mm behind the BB. That gets my kneecap right where I want it over the pedal.
> 
> So in theory a steeper seat tube should not matter, as long as you have enough wiggle room to slide your saddle back?
> 
> I know I'm totally overthinking it but just trying to understand if these newer geo setups are actually better for a taller rider. I'm looking to make the jump and want to make sure I get something I'm not cramped on.


One problem with that assumption, while valid imo, is that the seat tube angles affect dropper operation. So the more they line up vertically under you the smother they work and last longer.

Modern fit full squish seems to ignore knee crank relationship because your trading forward seat position to rotate peak power in the crank rotation. Then when your on an angle it effectively becomes normal and you have a good climbing bike.

Since you race xc on a hardtail. I assume you put the miles on.

Id go for a bike that suits trail riding more than enduro. Santa cruz has a more traditional sta and i know/expect they have a new hightower coming. So hopefully they increase stack like they did on the megatower But trying one of their bikes may prove interesting.

I found the stack too low and the price too high so settled for an 18 stumpjumper as the stack was huge. They dropped the xxl size on the new ones and while i tried one. It didnt come close to fitting.

Cheers


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