Why Is Everyone Being Told To Size Down On Steeper STA Bikes?

vikb · Aug 31, 2022

Mac_89 said:
I'm 6' with a +1 ape index. I’ve found on larger sizes, at least on the bikes I’ve tried/owned with LLS geo, that stack heights tend to be a bit low which can feel like too much reach.

I find it kind of crazy that people spend so much time talking about REACH and they rarely mention STACK. That's a problem when you calculate Reach based on the Stack. So you can have two bikes with the same Reach that are full size different in length due to higher and lower Stack.

stripes · Aug 31, 2022

vikb said:
I find it kind of crazy that people spend so much time talking about REACH and they rarely mention STACK. That's a problem when you calculate Reach based on the Stack. So you can have two bikes with the same Reach that are full size different in length due to higher and lower Stack.

You're not wrong. One of the problems I had with the Revved GG frames was the stack height (too low), and it made the size 2 too big for me.

dysfunction · Aug 31, 2022

Yea, I totally agree on the stack height. I really would like to have more stack. But I'm also riding L or XL frames. Same reason that I love having a steeper STA. Finally I'm not sitting over the rear hub.

plummet · Aug 31, 2022

The first question you need to ask is when do you get the most enjoyment riding a bike? What terrain, what style. Then size for that.

I like steep tight tech downs, ideally I have slack with a shorter reach and shorter chain stays to be able to get some maneuverability in the tight and also get weight back on the steep.

So I size for that. I'll take a more cramped ett for better control on the steep.

stripes · Aug 31, 2022

dysfunction said:
Yea, I totally agree on the stack height. I really would like to have more stack. But I'm also riding L or XL frames. Same reason that I love having a steeper STA. Finally I'm not sitting over the rear hub.

Short person here with neck and shoulder mobility. I love a bigger stack height for the same reason. I have found you can go too steep on the STA.. I'm not sure what the right value is, but I've done too steep and too slack before.

Bikeventures · Aug 31, 2022

Since we're on the subject of sizing, I will exploit for my own purposes. 😁

For DH bikes, their geometry is pretty different from modern trail bikes. A medium DH bike will have shorter reach, but much longer wheelbase than a medium trail bike. So I'm not sure which measurement is more important for sizing. Renting in Whistler and I'm right on the fence of small or medium.

kapusta · Aug 31, 2022

vikb said:
I find it kind of crazy that people spend so much time talking about REACH and they rarely mention STACK. That's a problem when you calculate Reach based on the Stack. So you can have two bikes with the same Reach that are full size different in length due to higher and lower Stack.

Very good point about stack effecting the effective reach. If you need to add spacers to raise the bar, you are shortening the reach. Not a problem as long as you are aware of it. I use various calculators to see what the reach will be at a given stack hieght. I've generally been able to predict within 10mm what length stem and how many spacers I would need for my MTB and Road/Gravel bikes.

That said, I had to throw everything I know about my fit out the window as I bought my first modern geo bike, and just go with what the manufacturer recommended, even though the frame reach numbers looked crazy long, and the STA was crazy steep. I did pay attention to the ETT (something I stopped looking at years ago), and that ended up being pretty consistent with my most recent (2012) old geo bikes. Anyway, it worked.

EatsDirt · Aug 31, 2022

Bikeventures said:
Renting in Whistler and I'm right on the fence of small or medium.

I've been in that same boat. Went larger and was very happy. Short adjustment loading the front wheel around corners but it wasn't really a problem. The fairly high speeds catered to a longer bike, and the tight stuff isn't really that tight.

Spring rate (assuming coil) might come in to play here also assuming the larger bike has firmer springs, and that seemed to work to my favor.

RS VR6 · Aug 31, 2022

jonshonda said:
But if my ACAD model isn't all jacked up, a bike my size with a 73* STA and 450mm reach should fit the same as a bike with a 77.5* sta and 510mm reach.

This is all it is. Your body is getting shifted forward. If your body doesn't shift forward...with all that extra front center...the bike will ride you instead of you riding it.

There are riders that do size down...but seems like it's the ones that race EWS. You'll see riders that are 5'10/'11 ride mediums. Jack Moir at 6'3" rides a small...because the wheelbase on his Canyon got so long. People on Pinkbike were losing their minds when Josh Bryceland at also 6'3" was riding a medium with a 450mm reach.

Dirt or Die · Aug 31, 2022

vikb said:
I find it kind of crazy that people spend so much time talking about REACH and they rarely mention STACK. That's a problem when you calculate Reach based on the Stack. So you can have two bikes with the same Reach that are full size different in length due to higher and lower Stack.

Yes, but the problem with looking at stack height is that it is determined by fork travel and BB height, so you really need to look at fork travel and bb drop along with the stack number, reach and ett.

vikb · Aug 31, 2022

singletrackmack said:
Yes, but the problem with looking at stack height is that it is determined by fork travel and BB height, so you really need to look at fork travel and bb drop along with the stack number, reach and ett.

Stack in the bike's geo chart is determined by the fork the manufacturer is specing for that frame. So if you plan to use something different yes you would need to take that into account. That's not a problem. Last frame I bought the geo chart included numbers for 140mm, 150mm and 160mm forks since that was what was suggested by the manufacturer. If I was going to run something else I would have had to adjust the geo chart to take that into account. I'd call that pretty normal.

jonshonda · Sep 1, 2022

vikb said:
I find it kind of crazy that people spend so much time talking about REACH and they rarely mention STACK. That's a problem when you calculate Reach based on the Stack. So you can have two bikes with the same Reach that are full size different in length due to higher and lower Stack.

This is an interesting topic of fit for riders who require higher stack. If you require significantly more stack then the frame you are riding offers, and make up the difference using stem spacers instead of riser bars, that can be an issue. Using spacers puts the stem further back effectively decreasing reach, but using riser bars won't change reach. For example, a 10mm change in stack on a 66*hta bike decreases reach by .25" or 6mm.

vikb · Sep 1, 2022

jonshonda said:
This is an interesting topic of fit for riders who require higher stack. If you require significantly more stack then the frame you are riding offers, and make up the difference using stem spacers instead of riser bars, that can be an issue. Using spacers puts the stem further back effectively decreasing reach, but using riser bars won't change reach. For example, a 10mm change in stack on a 66*hta bike decreases reach by .25" or 6mm.

I'm one of those high grip placement chasing riders. Stack and Reach doesn't change because you added spacers/higher bar. Bars can change the distance from the saddle [held constant] depending on the sweep and the roll combined with rise. If you don't change the sweep and the bars are oriented vertically the distance between saddle and bars doesn't change much with rise going up and down. I don't run my bars perfectly vertically so typically when I add higher rise bars I am bringing the grips back towards the saddle. At high rise a small amount of bar roll moves the grips a decent amount in space. I am also using some spacers so that also moves the grips up and back.

OTOH with bikes where I use the highest bars [~80mm rise] I can't get as much backsweep as I would like ~16 deg. So in that way the grips are not moving as far backwards as compared to a lower bar available in higher sweep.

That said these days when choosing between the two frame sizes I can usually fit on they are both pretty long in the big picture. I can't recall the last time I sat on a frame and was concerned it was too small given I am not allergic to running a stem in the range 31-50mm and I prefer an effective STA that's not crazy steep so I can use a 1" setback dropper on most frames. I have run up against frames where they are on the edge of being too long to fit once I am down to a 31mm stem and maxed out on the bar adjustments. Hence I'm now shooting for the smaller of the two sizes as my default choice unless there is some other compelling reason to try and make the bigger frame fit.

Ultimately I just choose a frame that gets the saddle to bar distance into a zone where I feel reasonably confident I can adjust my way to happiness. Geo charts are not perfectly accurate in terms of what actually gets built and with a bent seat tube you are not going to know where the saddle will end up at your ride height [in most cases].

My medium 2018 FS bike is notably longer than my XL 2009 FS frame. So even downsizing I'm not ending up on a short bike.

Brad · Sep 1, 2022

vikb said:
I can almost always ride two sizes of frame comfortably. Choosing to go up or down a size would change how the bike rides. Agility vs. stability. I've tried longer and now my preference is to go to the shorter of the sizes I can ride comfortably. I don't think there is a "right" answer in that. People have to look at the various elements of the frame geo and decide what's going to be better for them.

best answer ever

kapusta · Sep 1, 2022

vikb said:
Stack and Reach doesn't change because you added spacers/higher bar.

Effectively, stack and reach DO change with spacers under the stem. Fit/handeling/geo-wise it is the same outcome in all respects as if the frame itself had that additional stack and reduced reach.

vikb · Sep 1, 2022

kapusta said:
Effectively, stack and reach DO change with spacers under the stem. Fit/handeling/geo-wise it is the same outcome in all respects as if the frame itself had that additional stack and reduced reach.

If you start using the same terms to mean different things discussions get very confusing. If you are using spacers to get the grips up higher for fit purposes talk about that. Then it's easy to follow.

yzedf · Sep 1, 2022

RS VR6 said:
This is all it is. Your body is getting shifted forward. If your body doesn't shift forward...with all that extra front center...the bike will ride you instead of you riding it.

There are riders that do size down...but seems like it's the ones that race EWS. You'll see riders that are 5'10/'11 ride mediums. Jack Moir at 6'3" rides a small...because the wheelbase on his Canyon got so long. People on Pinkbike were losing their minds when Josh Bryceland at also 6'3" was riding a medium with a 450mm reach.

Jack isn’t 6’3” tall LOL (he says just over 6’) His race bike is smaller than his regular trail bikes according to him also. It’s a special use case scenario. Same as his bar height..

jeremy3220 · Sep 1, 2022

jonshonda said:
Using spacers puts the stem further back effectively decreasing reach, but using riser bars won't change reach.

The important thing to remember is that the frame size is fixed. You can move your bars around relative to the frame but you can't 'cheat' frame sizing. I've always ran riser bars with the rise inline with the angle of the fork so for me it's mostly the same as using spacers. However, if you roll the bar forward so the rise is 90° you're moving the bars further away from the steering axis...just like a longer stem does. Yeah a riser bar set with no back roll maintains effective reach but it does so by increasing what you could call 'effective stem length' aka horizontal distance between steering axis and grips.

Dirt or Die · Sep 1, 2022

vikb said:
Stack in the bike's geo chart is determined by the fork the manufacturer is specing for that frame. So if you plan to use something different yes you would need to take that into account. That's not a problem. Last frame I bought the geo chart included numbers for 140mm, 150mm and 160mm forks since that was what was suggested by the manufacturer. If I was going to run something else I would have had to adjust the geo chart to take that into account. I'd call that pretty normal.

Right, but what I was trying to get at is that if you are looking at similar types of bikes, like xc bikes, with the same fork travel, then the stack on a all the bikes are going to be very similar. So really no reason to look at stack too closely. Because of this, I look at heat tube length and bb drop. That gives way more info on how a bike will feel and handle than stack alone.
One bike can have a high bb and tall head tube and have the same stack as a bike with a low bb and shorter head tube, but they will ride quite differently.

nobody special · Sep 1, 2022

I think people are advised to size down just because bikes have become so long in general. I have a medium full suspension trail bike with 448 reach, 590 top tube, 438 chainstay, 1216 wheelbase, it fits me fine seated or standing. I have a medium park bike, 432 reach, 600 top tube, 438 chainstay, 1200 wheelbase, it fits me fine seated or standing. I have a medium hardtail with 463 reach, 622 top tube, 435 chainstay, 1219 wheelbase, it is too long for me, it hurts my hands and back when I ride while seated for long periods of time, and standing I have to put in a lot of mental effort to keep the front end from sliding, so I tend to only use it as a road bike. I should have bought the small, the small is the same size as the other bikes in medium, but there were few bikes available mid-pandemic. I can find many mediums similar in size to my hardtail but few mediums similar in size to the FS bikes, but often the same model bikes in small are close.

Why Is Everyone Being Told To Size Down On Steeper STA Bikes?

vikb

stripes

dysfunction

plummet

stripes

Bikeventures

kapusta

EatsDirt

RS VR6

Dirt or Die

vikb

jonshonda

vikb

Brad

kapusta

vikb

yzedf

jeremy3220

Dirt or Die

nobody special

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