29" vS 26" MTB

in message <[email hidden]>, Rob Morley

(') said:

On
uneven surfaces a larger wheel will float over bumps better than a
smaller one, but conversely a smaller wheel may have greater contact
area as it fits into dips in the ground better than a larger one.

The real difference in size between a '26"' wheel and a '29"' wheel is
quite small, however; 63mm on the diameter to be precise. I'd be
surprised if, for practical purposes, this makes much difference.

--
[email hidden] (Simon Brooke) http://www.jasmine.org.uk/~simon/

;; Diplomacy, American: see Intelligence, Military

"Peter Clinch" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Clive George said:

But all you're doing with the bigger wheels wrt agility, handling and
going over the back is effectively exactly the same as lengthening the
chainstays with the smaller wheels. Why is there a difference?

You're more likely to ground out the transmission with smaller wheels and
longer stays, though how often is that a deal-breaker?

Given the same BB height?

cheers,
clive

In article <[email hidden]>,

Roger () said:

So you would expect some people to love 29" and some to say they are
fad. A great holy war. Well there is a lot of debate, but it is clear
that 29" is much more favourable with larger people.

29s /must/ be good^w bad.

How do we know this?

Simple.

The UCI has banned^w rescinded the ban on them.

--
Dave Larrington - <http://www.legslarry.beerdrinkers.co.uk/>
External Transparent Wall Inspection Operative.

Roger came up with the following;:

Quoted message said:

Paul - [censored] wrote:

Quoted message said:

Quoted message said:

It appears to me that most of the advantages you cite are superficial and
are easily achieved using a correctly fitted frame with good geometry.

Many, not all. Given that the BB is at the same height, a 29" is not a
scaled up 26" as I always used to think.

I didn't say a 29" is a scaled up 26 at all, you appear to have brought that
up arbitrarily.

Quoted message said:

BTW, why does everybody think that changing the frame geometry is an
"easier" compromise?

I didn't say the frame geometry needs changing at all. Most decent bikes
already have good frame geometry that easily copes with the situations you
describe, provided it fits the rider properly. Your observations might make
a little sense at the very lowest end of the scale where mtb's are made to a
price point, with a generic frame size, and 'gimmicks' to attract punters.

? Making a frame for 29" instead of 26" is no

Quoted message said:

different from changing the geometry.

I didn't suggest changing geometry. Decent geometry is inherent in decent
26" mtb's

Quoted message said:

As for wheels/tyres, one thing I
found out while I was looking for "larger MTB" wheels was that 29"
rugged wheels and chunky tyres have been used for eons in cycle cross.

No they haven't. Similar sized wheels and tyres have, but with totally
different characteristics than mtb wheels and tyres.

--
Paul ...
(8(|) Homer Rules ..... Doh !!!

Clive George wrote:
[ground out the transmission]

Quoted message said:

Given the same BB height?

Think of the other end of the operation, with the derailleur hanger.
Irrespective of the chainstay length, a smaller wheel will have the
gubbins nearer the ground (though I suppose to some extent at least you
can get away with smaller sprockets for the same gearing).

Pete.
--
Peter Clinch Medical Physics IT Officer
Tel 44 1382 660111 ext. 33637 Univ. of Dundee, Ninewells Hospital
Fax 44 1382 640177 Dundee DD1 9SY Scotland UK
net [email hidden] http://www.dundee.ac.uk/~pjclinch/

Paul - censored said:

Roger came up with the following;:

Quoted message said:

Paul - [censored] wrote:

Quoted message said:

Quoted message said:

It appears to me that most of the advantages you cite are superficial and
are easily achieved using a correctly fitted frame with good geometry.

Many, not all. Given that the BB is at the same height, a 29" is not a
scaled up 26" as I always used to think.

I didn't say a 29" is a scaled up 26 at all, you appear to have brought that
up arbitrarily.

I concluded it from your arguments, given that the BB on a 29" is at
the same height as a 26" then it is impossible to create the same
conditions by otherwise altering the geometry, the rider has been
lowwered wrt the axle height whilst remaining in the same riding
position. Is this bit clear?

Clive George said:

And serious cyclocrossers
don't use terribly rugged wheels - everything's a lot more light weight and
hence disposable, since you spend more time carrying the thing.

The terrain tends to be a bit smoother, so you can get away with a less
rugged wheel - provided it's of decent quality (I use 32h Open Pro rims
front and rear with d/b stainless spokes, Hope front hub, Campag Record
rear hub, Michelin Mud 2 tyres) and so still copes well with the
conditions. Would take issue with things being "disposable", though;
because of the need to keep the weight of bikes down in 'cross, parts
tend to be well-made, higher-priced lightweight stuff, and so not the
kind of kit you'd regard as being throwaway or of little consequence if
it gets trashed. E.g. not so long ago, pricey 3 or 4 spoke carbon
wheels such as Spinergy products were all the rage for top-level
'cross, but I don't know if UCI regs still permit them.

David Belcher

in message <[email hidden]>, Roger

(') said:

There has been a lot of discussion flying around about this issue,
often neglecting to mention perhaps one of the most significant
effects, the geometric differences on steep slopes.

This article attempts to redress the balance:

http://users.libero.it/irwin/C1/c1.html

H'mmm... this is nonsense, and not very useful nonsense either.

Let us assume for the sake of argument that we have two XC bikes, both of
which have minimal chainstay length, one of which has '26"' wheels
(559mm), and one of which has '29"' (622mm). When the bike is
horizontal, the hub of the '29" bike is 31.5mm - an inch and a quarter -
behind that of the '26"' bike. But when the bike is tilted up at 30
degrees it is 31.5mm * cos( 30) = 27.27mm or just over one inch. More
significant movements to the centre of gravity can be made by adjusting
your saddle clamp, or, as other people have pointed out, by lengthening
the chain stays (few if any mountain bikes have minimum-length
chainstays, anyway).

Going downhill it's even wronger. Firstly, at cos(60) the difference in
the distance between the two lines, if due to the difference in wheel
size alone, is 15.75mm - a bit over half an inch. Secondly, the critical
issue isn't where the axle is, it's where the contact patch is, and
while the contact patch is vertically below the axle when the bike is
horizontal, tilting the whole thing up at 60 degrees actually brings the
horizontal distance back by sin( 60) so it's actually only 13.63mm
different.

That's a very small difference. Very subtle changes to the steering head
angle can make more difference than that, without changing the wheel
size.

Finally, nobody but nobody goes downhill with their front end locked out.
It's an obviously daft thing to do.

29" wheels are, as far as I can see, pure marketing [censored]. Yes, larger
wheels must roll better over bumps, and there is benefit in larger
riders having larger wheels, but the benefit isn't enough to outweigh
the cost in lack of access to standard parts.

--
[email hidden] (Simon Brooke) http://www.jasmine.org.uk/~simon/

Morning had broken, and we had run out of gas for the welding torch.

"Peter Clinch" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Clive George wrote:
[ground out the transmission]

Quoted message said:

Given the same BB height?

Think of the other end of the operation, with the derailleur hanger.
Irrespective of the chainstay length, a smaller wheel will have the
gubbins nearer the ground (though I suppose to some extent at least you
can get away with smaller sprockets for the same gearing).

ah yes - fair enough. I was thinking of the grounding problems we get, which
are often chainwheel related - but then that's on a rather longer bike than
normal :-)

cheers,
clive

in message <[email hidden]>, David

E. Belcher (') said:

Roger said:

As for wheels/tyres, one thing I
found out while I was looking for "larger MTB" wheels was that 29"
rugged wheels and chunky tyres have been used for eons in cycle cross.

As I pointed out in your other thread re. 29"-wheeled MTBs, the sizing
isn't *quite* the same as 700c

Are you sure? They're quoted as Etrto 54-622. 622 sounds like 700c to me.

--
[email hidden] (Simon Brooke) http://www.jasmine.org.uk/~simon/
Copyright (c) Simon Brooke; All rights reserved. Permission is
granted to transfer this message via UUCP or NNTP and to store it
for the purpose of archiving or further transfer. Permission is
explicitly denied to use this message as part of a 'Web Forum', or
to transfer it by HTTP.

Roger said:

tim said:

Part of me is wanting to jump up and down shouting "No! No! No!"
but my A-level physics is now too rusty to be sure I'm right🙂 My
understanding of physics suggests that for the rider to topple backwards
climbing or to go OTB descending is dependant (at a simple level) only
on the CofG of bike & rider and it's position relative to the downhill
contact patch. If the CofG is behind the downhill contact patch the bike
and rider are going to start to topple. The position of the axle is
irrelevant.

Could you explain to me where my understanding is wrong?

The contact patch area you refer to is directly below the axle, but
when the bike is at an angle (on a slope) the larger wheel means that
the center of the axle moves laterally, hence the CoG goes behind the
axle with the bike at a steeper angle.

This is what had me wanting to shout "No! No! No!". If the axles
are in the same vertical plane when the bike is on the flat (and so the
wheelbase is the same) the contact patches will be in the same place for
the 26" and 29" bikes. The CofG is in the same place (in my mind and in
your diagrams). When the bike is on a slope, the CofG and contact
patches are in the same place for the 26" and 29" wheeled bikes.

I don't understand why you think the axle position is
significant: my rusty physics suggests that the point where the bicycle
contacts the ground is important and not the axle position. Why do you
think that the axle position is relevant and not the contact patch? Am I
incorrect in thinking the bicycle would tip about the point at which it
is in contact with the ground?

Quoted message said:

Quoted message said:

I'm not sure how having the axle further from the ground with
CofG (largely determined by rider position) and contact points with
ground (largely determined by chainstay and toptube length) in the same
place is going to have the influence you suggest.

What is causing a lot of confusion is that people imagine a 20" to be a
scaled up 26". It isn't, the BB is at the same height so assuming the
rider is in an ideal riding position the riders weight is nearer the
axle height. This becomes relevant when the bike is at a steep angle.
The diagrams I made were supposed to make this concept clear, but
apparantly it has not been grasped :-(

Your diagrams indicate to me that in both cases the contact
patches are the same and CofG is in the same place. What isn't clear and
what I can't work out is why you think that axle height is important to
stability.

I do understand that a larger wheel can roll over larger
obstacles or the same obstacle with greater ease than a smaller wheel
but that is seperate from the claimed increase in stability.
--
Tim.

[email hidden]

in message <[email hidden]>, tim

(') said:

Part of me is wanting to jump up and down shouting "No! No!
No!"
but my A-level physics is now too rusty to be sure I'm right🙂 My
understanding of physics suggests that for the rider to topple
backwards climbing or to go OTB descending is dependant (at a simple
level) only on the CofG of bike & rider and it's position relative to
the downhill contact patch. If the CofG is behind the downhill contact
patch the bike and rider are going to start to topple. The position of
the axle is irrelevant.

Could you explain to me where my understanding is wrong?

You don't start to topple providing there's no braking. You and the bike
just effectively freefall together. That's how it's possible for people
much braver than me to descend slopes close to vertical. The problem
comes when you hit the brakes - and suspension forks (apart from those
with anti-dive geometry, which are pretty rare) /do/ make this worse
because when you hit the brakes the forks compress, shortening the
forward wheelbase.

Under braking, you've got a quite complex little equation of resultant
forces with gravity and deceleration to allow for, so the direction of
the force isn't directly down, it's a bit forward of directly down and
this again makes things worse. You are right in assuming that in this
condition what matters is the CoG and and the position of the contact
point, put relative to the resultant vector not to vertical.

What this all adds up to is on a very steep hill, once you're committed,
don't even try to slow down. On any slope the amount of braking force
you can safely apply is related to gradient and speed.

--
[email hidden] (Simon Brooke) http://www.jasmine.org.uk/~simon/

Error 1109: There is no message for this error

Simon Brooke said:

in message <[email hidden]>, Roger

(') said:

There has been a lot of discussion flying around about this issue,
often neglecting to mention perhaps one of the most significant
effects, the geometric differences on steep slopes.

This article attempts to redress the balance:

http://users.libero.it/irwin/C1/c1.html

H'mmm... this is nonsense, and not very useful nonsense either.

Let us assume for the sake of argument that we have two XC bikes, both of
which have minimal chainstay length, one of which has '26"' wheels
(559mm), and one of which has '29"' (622mm). When the bike is
horizontal, the hub of the '29" bike is 31.5mm - an inch and a quarter -
behind that of the '26"' bike. But when the bike is tilted up at 30
degrees it is 31.5mm * cos( 30) = 27.27mm or just over one inch. More
significant movements to the centre of gravity can be made by adjusting
your saddle clamp, or, as other people have pointed out, by lengthening
the chain stays (few if any mountain bikes have minimum-length
chainstays, anyway).

Like everybody else, you are looking from a horizontal viewpoint. The
difference is in the vertical. I took it for granted that everybody
knows that the BB height on a 29" is the same as a 26".

Given that the rider is to be put in his ideal riding position then his
weight will be lowwer wrt the axle height. This fact is often cited on
pro 29" sites as the reason theat the CoG is the same as a 26". Again,
that is true of the horizontal case, but looking in the vertical plane,
on a slope, the picture is different.

Reffering back to my diagrams, the point were the dotted line
intersects the red line is where the 26" axle will be. Where the dotted
line intersects the green line is where the 29" axle will be.

You cannot achieve all the same objectives by making the
chainstay/mainstay longer.

Simon Brooke said:

in message <[email hidden]>, David

E. Belcher (') said:

Roger said:

As for wheels/tyres, one thing I
found out while I was looking for "larger MTB" wheels was that 29"
rugged wheels and chunky tyres have been used for eons in cycle cross.

As I pointed out in your other thread re. 29"-wheeled MTBs, the sizing
isn't *quite* the same as 700c

Are you sure? They're quoted as Etrto 54-622. 622 sounds like 700c to me.

A muddle on my part, I think - it's probably the rim & tyre widths that
aren't compatible. I definitely remember reading on cyclingnews.com 4
or so years ago that 29" MTB and 700c road/cross rims & tyres weren't
interchangeable in some respects.

David Belcher

In article <[email hidden]>
Roger <[email hidden]> wrote:
<snip>

Quoted message said:

The contact patch area you refer to is directly below the axle, but
when the bike is at an angle (on a slope) the larger wheel means that
the center of the axle moves laterally, hence the CoG goes behind the
axle with the bike at a steeper angle.

Reduce the situation to its simplest form:
The wheels remain the same distance apart.
The centre of mass is fixed relative to the wheels.
The ground, regardless of gradient, is a plane tangential to both
wheels.

How can the centre of mass move differently, relative to the tyre
contact points, depending on the wheel diameter?

To convince yourself draw two bikes superimposed, one with big wheels
and one with small wheels, such that the contact points coincide (i.e.
the wheelbase is the same for both bikes, the seat/bars/pedals are in
the same place). Chose an arbitrary point (I suggest the seat) to be
your centre of mass. Now tip the picture at 45 degrees - does the
centre of mass for each bike still coincide?

Simon Brooke said:

but the benefit isn't enough to outweigh
the cost in lack of access to standard parts.

Similar to the situation with 650c wheels for road-bike use, I guess;
limited product variety and a lack of lower-priced tyres and rims seems
to have resulted in a fall-off in interest compared to a few years ago
(e.g. many lo-pro TT bikes seem to be 700c back & front these days,
instead of having a smaller front wheel as was once the fashion).

David Belcher

Tim Izod said:

Quoted message said:

Your diagrams indicate to me that in both cases the contact
patches are the same and CofG is in the same place.

Contact patch is directly below the axle. That is where the red line
intersects the ground for 26" and where the green line intersects the
ground for 29". Does that look the same to you?

What isn't clear and

Quoted message said:

what I can't work out is why you think that axle height is important to
stability.

It isn't and I don't. The critical bit is how close the rider is to the
axle height. On a 29er the BB is at the same height as a 26er, so the
lateral CoG is the same and axle height has no relevance. However on a
slope we are in effect an inverted pendulum where the fulcrum is the
axle, but the weight on the pendulum is very lobsided such that
equilibrium is reached while the pendulum is at an angle (wheelie
position). Getting the rider closer to the axle line makes the pendulum
weight less lob sided so equilibrium is achieved at a sharper angle.

Put bluntly we would be able to balance the bike on a steeper slope,
and of course it is easier to keep things under control at lesser
angles.

Quoted message said:

I do understand that a larger wheel can roll over larger
obstacles or the same obstacle with greater ease than a smaller wheel
but that is seperate from the claimed increase in stability.

It's not a stability issue, it is just the maxiumum angle that can be
tackled in a given position, or how much the rider needs to move out of
a given position in order to tackle a given slope (the more a rider
moves out of position the more compromised he is in other respects).

Quoted message said:

--
Tim.

[email hidden]

Rob Morley said:

Quoted message said:

Reduce the situation to its simplest form:
The wheels remain the same distance apart.
The centre of mass is fixed relative to the wheels.

NOOOOO!!!!. This is where everybody get's it wrong. The BB on a 29er is
at the same height as a 26"er, so the centre of mass is fixed relative
to the ground, not the wheels.

Quoted message said:

The ground, regardless of gradient, is a plane tangential to both
wheels.

How can the centre of mass move differently, relative to the tyre
contact points, depending on the wheel diameter?

Because the axle height has increased whilst the centre of mass has
remained the same. The diffence is in the fore/aft balance on a slope.

Quoted message said:

To convince yourself draw two bikes superimposed, one with big wheels
and one with small wheels, such that the contact points coincide (i.e.
the wheelbase is the same for both bikes, the seat/bars/pedals are in
the same place). Chose an arbitrary point (I suggest the seat) to be
your centre of mass. Now tip the picture at 45 degrees - does the
centre of mass for each bike still coincide?

Yes, but this is not the scenario ;-)

I have allready been here, and done that (drawings). Now, go and do
your drawings with the BB (and hence ideal rider position) at the same
height from the ground but with the different sized wheels. Tip up your
drawings and you will see that you can tilt the 29er further before the
Center of mass is over the axle, which is what you are pivoting around
when you pull a wheelie or go over the bars.

Roger said:

Contact patch is directly below the axle.

Eh? Are you sure about that?

Quoted message said:

That is where the red line intersects the ground for 26" and where
the green line intersects the ground for 29". Does that look the same
to you?

No, the contact point on your diagram is where the dotted black line
intersects with the ground. Perhaps you should draw the ground on your
diagrams to make this clearer.

Quoted message said:

It isn't and I don't. The critical bit is how close the rider is to
the axle height. On a 29er the BB is at the same height as a 26er, so
the lateral CoG is the same and axle height has no relevance. However
on a slope we are in effect an inverted pendulum where the fulcrum is
the axle, but the weight on the pendulum is very lobsided such that
equilibrium is reached while the pendulum is at an angle (wheelie
position). Getting the rider closer to the axle line makes the
pendulum weight less lob sided so equilibrium is achieved at a
sharper angle.

I suspect you're confusing yourself by making this far more complicated
than it is.

Anthony

Roger said:

I have allready been here, and done that (drawings). Now, go and do
your drawings with the BB (and hence ideal rider position) at the same
height from the ground but with the different sized wheels. Tip up your
drawings and you will see that you can tilt the 29er further before the
Center of mass is over the axle, which is what you are pivoting around
when you pull a wheelie or go over the bars.

No it isn't, you're pivoting about the contact point.

Anthony