[Closed] Getting my head round reach & stack after years of effective top tube.

In a side to side situation, think of it as a boat: weight above the waterline will make the boat less stable, weight below will add to stability

In front rear terms, think of it as a clock face...

The Front axle is the centre of the clock and the BB is the tip of the hour hand...

You are Cinderella and you want to party, but at 12 o clock, you're getting thrown out the front door. (we've all been there)

The lower the BB in relation to the front axle, the longer you can party.

If you get to the party before 9 o clock, you can party harder with less of a hangover

Of course, if you get to the party too early, your carriage is going to clatter on the rocks and mess stuff up, fairy godmother or no.

Basically, it's similar to having a lower centre of gravity; the further below the axle that a load is carried, the more stable~

A certain bike designer of repute would talk about balancing a stick on your palm to show that a higher c of g is actually easier to balance

Aye, but does he go around corners 😉

Sore wrists and the like are usually down to poor positioning... (or poor technique)

Forearm strength can help here as well as a relaxed, but firm, grip. And as with any exercise, endurance comes with practice.

But back on subject, the stack is also a pretty important measurement - it's not all about the reach. A low stack and a short reach will be low, cramped and uncomfortable, whereas a higher stack and short reach will push the body back, actually creating the illusion of more room.

Aye, but does he go around corners

this thread just gets better and better!

anyway

6'2" and alway ridden large & been happy as hell, never even sat on an XL. Current bike 600 ETT (which turns out to be different from the geo chart), a 20mm set back seat & a 70mm stem all add up to suggesting a large 620mmETT and 50mm stem 'shouldn't' be right.
Several demo's of bikes with 620ish ETT and 50mm stem feel fine, so tried a 50mm on my current bike and it's horrible, I'm pushing myself off the back of the seat.

It totally depends on the bike and the shape of your body, but I'm guessing at that height you'll be at the lower end of the scale for most XL frames but not totally off it. I'm buying me new bike blind too but I'm slap bang in the middle of the height range for the frame size. I've also just put a shorter 50mm stem on my too-short bike and I'm also having to try hard to keep my weight forward.

In a side to side situation, think of it as a boat: weight above the waterline will make the boat less stable, weight below will add to stability

The bike isn't a ship. There's nothing at the 'waterline'. The bike pivots side to side at the wheel/ground contact point so adjusting the BB in relation to the axles doesn't affect that.

It does affect the front to back part but then we were discussing reach. It's hardly rocket science to say that lowering the CoG in relation to the axles reduces the direct risk of OTB dismounts (and by the sounds of it, that's what you're talking about in relation to 'stability') but that wasn't the point being discussed - specifically, does increasing reach increase or decrease load on the front wheel in the real world (eg how does a rider actually react to this).

That increased load may well mean more chance of OTB which you can then reduce by lowering CoG in relation to the axles (bigger wheels or lower CoG position on the bike) but again, that wasn't the point (though clearly of significant importance in stopping you doing an impression of Superman).

It totally depends on the bike and the shape of your body, but I'm guessing at that height you'll be at the lower end of the scale for most XL frames but not totally off it. I'm buying me new bike blind too but I'm slap bang in the middle of the height range for the frame size. I've also just put a shorter 50mm stem on my too-short bike and I'm also having to try hard to keep my weight forward.

My experience riding a smaller bike is that the margin for error is smaller: i.e. you're likely to keep weight further back as a safety mechanism to lower the chance of catapulting over the bars.

I rode a borrowed XL Fisher at Cwm Carn several years back (after years of riding small bikes that were realistically too small for me) and was amazed at how well it both climbed and descended, and how much safer I felt.

This could be due to a number of things, such as wheelbase and a higher front end (or stack), but the big difference I felt was the sheer amount of extra room there was in the 'cockpit' area; I felt I could fine tune my body positioning a lot more over the bike.

"I don't think" doesn't count. Do some research and you'll find you're wrong.

Gosh, I see, every skills coach teaches 0% body weight on the handlebars it is then.

TBH I wasn't aware I was riding small frame for my size (except for the current one).. the sizing guides have always said I fit Large, even my newest choice, I'm in the middle of the sizing range for large.
It's when you start measuring things, that gotten me all confused.

PimpmasterJ.. Still riding an XL? (how tall are yuo?)

The bike isn't a ship. There's nothing at the 'waterline'. The bike pivots side to side at the wheel/ground contact point so adjusting the BB in relation to the axles doesn't affect that.

I see your point~ We're both kinda right on that one~ though the contact point of the wheels is the ground, the axles are where the force is being conveyed from the rider to the wheel.

The Fore/Aft situation is more clear cut.

I rode a borrowed XL Fisher at Cwm Carn several years back (after years of riding small bikes that were realistically too small for me) and was amazed at how well it both climbed and descended, and how much safer I felt.

This could be due to a number of things, such as wheelbase and a higher front end (or stack), but the big difference I felt was the sheer amount of extra room there was in the 'cockpit' area; I felt I could fine tune my body positioning a lot more over the bike.

That's what I'm trying to portray with the 2 gravity bikes above (as they exaggerate the body position)

A longer reach means your weight is more 'in' the bike, than 'on' it~ stretched out, your weight is lower down, both in absolute terms and in relation to the front axle.

PimpmasterJ.. Still riding an XL? (how tall are yuo?)

A whisker under 6ft.

I tend to ride a large / 19" seat tube. Currently on a L (19") Saracen Ariel and a L (19") Kona Unit, and recently sold a L Brodie Holeshot (18" ST). I used to ride 17-18" bikes with massive seatposts because they were more 'flickable'.

*sigh*

Saying that, I did appreciate the standover. And you could always stick a longer stem on...

I see your point~ We're both kinda right on that one~ though the contact point of the wheels is the ground, the axles are where the force is being conveyed from the rider to the wheel.

So is every structural point of the bike between the rider and the ground. The axle has no specific importance to that situation.

stretched out, your weight is lower down

Depends really doesn't it -and that's back to the question - if riders simply flatten their back/unbend their arms with a longer reach, that's true. If they effectively rotate their body forward a bit then not so much. And that's assuming that riders don't adjust their stack position at all to deal with longer reach.

The bike isn't a ship. There's nothing at the 'waterline'. The bike pivots side to side at the wheel/ground contact point so adjusting the BB in relation to the axles doesn't affect that.

But back to R+S, OP, try playing with this if you've not seen it before - http://www.bikecad.ca/quickapplet

Set up a bike and a rider, move the ETT or seat angle, see the reach change and the rider position etc.

Silly question time... Is reach (aprox range) a calculable measurement for a person, or is it purely an experience based decision?

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PMJ.. I've looked thru my old photo's and can see long stems, laidback posts, seat all the way back.. I've changed my order, going XL.

Nemesis is right, the idea that a lower BB relative to axles corners better is mistaken, it's only potentially better fore-aft stability that you get. It may feel a bit better as you start sliding and counter-steering but I can't even start explaining why I get that impression.

I can't explain it either but I swear there's something going on with it. On my 26 hardtail I'll hit slidey corners with my outside foot down whilst with my 27.5 full-sus I'll often ride them with pedals level - the 26 has a higher BB as well as less BB drop though so it's not an accurate test. Having my weight pushed through a low pedal allows me to balance a slide better (despite losing out on some leg suspension) but I don't need to do that to balance the lower, longer and bigger wheeled full sus.

You're only comparing two bikes there though and quite different ones at that - FS and Hardtail so that's not really a basis for comparison.

z1ppy, a rule of thumb could be if you have a long back and arms then more reach would be good as there will be more of you between bb and handlebars. I'm short legs and long back so a short seat tube and longer reach is good for me, plus I don't like slack seat angles and layback saddles to make the riding position longer as I get too much weight over the back. I imagine if you go for a proper bike fitting reach is one of the measurements they might give you though.

the idea that a lower BB relative to axles corners better is mistaken, it's only potentially better fore-aft stability that you get.

This has been ticking over in my head and I reckon I've sussed it...

Here we have a hastily scribbled diagram, (on a British Cycling envelope to make it seem more official 😀 )

[img] [/img]

(The arrows coming from the axle and BB are the cornering force and The moment of leverage arrowed is a dimension line... the actual moment will be the centre line of the wheel)

In simple terms, the lower the BB is below the axle, the more force that is going into making the tyre grip and the less force that is trying to stand the bike up.

No worries Bigjim, was just a thought.. going from other ppl's bike I've tried, I believe I'm roughly 'average' (neither long leg or long body) for my height.
I know what reach I can live with (current/old bikes), but don't have experience of anything any longer which is where I'm heading, in order to avoid a laidback seating position

jhj - how does the bike know where the axle is?

the lower the BB is below the axle, the more force that is going into making the tyre grip and the less force that is trying to stand the bike up.

This doesn't really mean anything. The tyre doesn't 'know' where the axle is and the force being applied to it is not dependent on the BB drop (axle to BB measurement).

Not to mention that the initial discussion was over weight distribution in a straight line rather than cornering...

The axle is where the force is being transferred from the bike to the wheel...

The BB is where the weight is concentrated

The bike doesn't know a whole lot about anything, wonderous creation though it is.

If you only ride your bike in a straight line:

a) I'd like to know where

and

b) BORING!!

Not to mention that the initial discussion was over weight distribution in a straight line rather than cornering...

well I don't think it was, which is why I was so highly amused by 0% weight through the handlebars statement, I can't imagine going into a corner with no weight on the bars at all, but I didn't want to say anything apart from maybe the person shouldn't go around telling people not to put any weight at all on the bars. personally I think weight transfer to the front end through the bars is one of the most useful skills to learn, but then I'm just a very average mountain biker.

The axle is where the force is being transferred from the bike to the wheel...

And so is every bit of the wheel and fork and frame between the ground and the rider. What's actually special about the axle?

jivehoneyjive - Member

The axle is where the force is being transferred from the bike to the wheel...
The BB is where the weight is concentrated

Yes. Well kind of, the wheel is part of the bike so force isn't transferred from the bike to the wheel.
No. The weight isn't concentrated anywhere, but the centre of gravity of the bike and rider is nowhere near the BB.

In simple terms, the lower the BB is below the axle, the more force that is going into making the tyre grip and the less force that is trying to stand the bike up.

Also in simplistic terms, could this be applied to wheels too? (Hub axles replacing BB axle, obviously)

I feel a bit bad because we're straying off topic, but it's all interesting stuff...

You could have exactly the same BB height and Centre of Gravity, but with 2 different wheel sizes, the larger wheelsize will give more stability~ taken to extremes the smaller wheelsize could have the BB+COG above the axle, which would mean cornering forces automatically try to stand the bike up.

Go back to the ship analogy... though the pivot point is different, the overall principle is the same

Back to weight distribution and bigjim does have a very valid point; you selectively weight the bars to improve traction.

When you start analyzing, there's a heck of a lot of variables...

taken to extremes the smaller wheelsize could have the BB+COG above the axle, which would mean cornering forces automatically try to stand the bike up.

The pivot is at the ground, not the axle. The axle is just part of the structure, same as the rest of the fork and frame transferring load/forces from the rider to the ground. The overall forces on the bike 'trying to stand it up' are the same regardless of BB drop.

The tyre isn't just the pivot point, it's also where the load is concentrated...

How does that load get there and what plane does it act in?

Here's some more scribbling:

[img] [/img]

What does 'load is concentrated' mean? The contact point in your diagram is the pivot (assuming the tyre isn't slipping).

The 'load' goes from the rider (to simplify things), through the bar, pedals (and saddle if seated) to the frame and fork, through the wheels where it contacts the ground.

In your picture, you could replace the whole bike with a solid object with infinitely small wheels and it would still contact the ground at the same point and the BB would still be in the same relative place and the rider's centre of mass would be in the same place. The net force 'trying to keep the bike upright' would also be the same. The model is exactly the same. The axle position is of no more relevance than where the headset it or where the disc caliper mounting point is - they're just points in the structure. You drawing the dashed line from the axle to the ground is of no relevance, it's just a random point you've chosen.

If you don't understand that, this discussion can't really progress.

Wasn't it so much easier when we just measured the seat tube?!

What we need now is some sort of online calculator where you can put all the frame data in and get the relevant measurements out....

And then we can argue over the formulae used to get those measurements 🙂

Though if there was an argument, some people would be right and others would just think they're right...

😉

Or post conspiracy theories, innuendo and misdirection about it...

So, have you come up with a justification for why you've drawn a line from the axle to the ground in your pics? 😉

Darn tootin...

Is there any force being passed from the frame and fork through the top of the wheel?

nemesis - Member

The pivot is at the ground, not the axle.

Is it so simple? Bikes, when they turn, don't rotate on the contact point as a fulcrum- if you turn left, the contact point doesn't stay still while the bike leans left, as if toppling, the contact point moves right. So the bike's rotating around a point somewhere higher up, but also that point moves downwards as the bike rotates and leans...

Yeah, I know it's not that simple but I'm trying to make the point that in this instance at least, the BB drop doesn't affect what he says it does. Trail and so on are of course affected by wheel size and they will also vary on steering angle, etc so it's a complex situation once you start cornering.

So the bike's rotating around a point somewhere higher up, but also that point moves downwards as the bike rotates and leans...

Northwind is on the money, the bike is rotating about the radius described between where the centre-lines of the axles meet the ground and the contact patch of the tyres

Is it so simple? Bikes, when they turn, don't rotate on the contact point as a fulcrum- if you turn left, the contact point doesn't stay still while the bike leans left, as if toppling, the contact point moves right. So the bike's rotating around a point somewhere higher up, but also that point moves downwards as the bike rotates and leans...

It's really complex. When the bike is leant it doesn't do so around the centreline of the tyre, the pivot point moves outwards towards the side knobs. But when a bike is moving and you lean it the front wheel countersteers slightly to pull the contact patches outwards so that the mass of the bike+rider falls inwards.

Something I've noticed when swapping between bikes (16" Brompton, 20" BMX, 26" & 27.5" MTBs) is that the turn is much quicker to initiate on the smaller wheels. Does that mean the moment of countersteer is briefer? Does the bike just need to countersteer laterally by an amount which is proportional to the wheel radius?

Other things to think about - forks have offset so when you turn the bars the centre of the wheels are no longer in a straight line which passes through the BB. Does the BB drop related stability interact with the gyroscopic wheel force? That's something where the axle height definitely matters.

No it's not or at least not as you've described it. You could lean your bike as you've drawn and still ride in a straight line (well you'd actually have to countersteer slightly I reckon) or you could be riding various different arcs depending on other variables (speed, weight, geometry).

And what does that have to do with reach?

Anyhoo, back to reach and stack...

In a similar manner to BB drop, whereby a lower COG between the axles improves stability, the same will apply in terms of reach and stack, though ergonomics will come into the equation for comfort and manoeuvrability.

It's effectively the whole 'on the bike' vs 'in the bike' thing

You still haven't defined "stability"...

And you didn't show that a bigger BB drop improves anything.

So it's down to 'on' vs 'in'. Great 😉

I'm sure others can comprehend what I've already conveyed...

I get what you're trying to say but they're really points about the ability to hit obstacles with different size wheels which I think are well established fact.

You haven't shown anything about stability or even explained what it is yet...

As in the fore/aft example, cornering stability basically means the bike will hold a line better.

In many respects it won't just be down to Reach... for a given geometry (i.e. head angle/seat angle/chainstay length) a change in reach is automatically going to change the wheelbase.

Stack is another matter...