29" vS 26" MTB

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

Quoted message 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

Er - it's nonsense. All the differences you mention can be changed with
frame geometry. Nothing to do with the wheels, unless you're trying for the
shortest chainstays possible, in which case 29" loses.

cheers,
clive

Clive George said:

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

Quoted message 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

Er - it's nonsense. All the differences you mention can be changed with
frame geometry. Nothing to do with the wheels, unless you're trying for the
shortest chainstays possible, in which case 29" loses.

cheers,
clive

Thats what I thought for a long time. But it's not true, the position
of body weight with respect to the vertical line over the axle **is**
critical, and that axle position can only change with different size
wheels. Changing bike geometry to get body mass nearer the floorline
means either a poor pedalling position or a chainset that hits the
ground.

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

Quoted message said:

Clive George said:

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

Quoted message 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

Er - it's nonsense. All the differences you mention can be changed with
frame geometry. Nothing to do with the wheels, unless you're trying for
the
shortest chainstays possible, in which case 29" loses.

Thats what I thought for a long time. But it's not true, the position
of body weight with respect to the vertical line over the axle **is**
critical, and that axle position can only change with different size
wheels. Changing bike geometry to get body mass nearer the floorline
means either a poor pedalling position or a chainset that hits the
ground.

That's where you're going wrong. You can put longer chainstays on to get the
same effect - no need to change the pedalling position or chainset height.
Frame geometry is about more than just angles.

People choose not to put the longer chainstays on because I think they tend
to feel that the increased problems climbing steep stuff are offset by the
more agile handling afforded by the short wheelbase.

Interesting that you say "many amaturs go down steep slopes with the forks
blocked". I've never even thought of doing that - going down a steep slope
is where I'd definitely be wanting the most suspension action.

cheers,
clive

In article <[email hidden]>

Roger said:

Clive George said:

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

Quoted message 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

Er - it's nonsense. All the differences you mention can be changed with
frame geometry. Nothing to do with the wheels, unless you're trying for the
shortest chainstays possible, in which case 29" loses.

Thats what I thought for a long time. But it's not true, the position
of body weight with respect to the vertical line over the axle **is**
critical,

So why do you think the weight distribution when climbing will be
significantly different between a 26" and a 29" bike with the same
chainstay length and bottom bracket height? For descending, a longer
top tube and shorter stem, plus longer head tube or higher stem, puts
the rider's weight further back just like a larger front wheel. 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. A
bigger wheel will be more massive resulting in a greater unsprung mass
on suspension bikes, reducing the effectiveness of the suspension. So
in some respects there are no differences, and in others it's a matter
of swings and roundabouts as is so often the case with this sort of
design issue.

Clive George said:

That's where you're going wrong. You can put longer chainstays on to get the
same effect - no need to change the pedalling position or chainset height.
Frame geometry is about more than just angles.

Been there, done that. My older bike had a wheelbase several
centimetres longer. Point is that agility and handling are important.
Whilst it is not nice to go over the bars, if you can't manage to make
the sharp turn to avoid the tree at the bottom of the slope you are
just as cooked. Last weekend bent my dual control lever box ramming a
tree at the bottom of a slope where I needed my gut on the saddle to
stop going over the bars.

Quoted message said:

People choose not to put the longer chainstays on because I think they tend
to feel that the increased problems climbing steep stuff are offset by the
more agile handling afforded by the short wheelbase.

All the solutions which involve getting the riders moving weight
laterally degrade the handling. Solutions which lowwer the riders
position vertically reduce pedal/chainwheel clearance. If you have the
BB at the same height as a 26" but with 29" wheels you don't degrade
anything.

Quoted message said:

Interesting that you say "many amaturs go down steep slopes with the forks
blocked". I've never even thought of doing that - going down a steep slope
is where I'd definitely be wanting the most suspension action.

Ever heard of the staircase effect? That's what your rebound control is
for, but it's effectiveness varies, especially on forks that cost less
than £500 (most of them). We are talking 60 degree+ drops here....not
long fast downhill sections.

I would love to here some other novel solution, but it is some time I
have been studying this problem. After arriving at the conclusion that
I would be better off with larger wheels I went to look if larger MTB
wheels actually exist, and I found out that not only do they exist but
they are becoming very popular in the states, large riders in
particular seem to love them. It would seem to confirm the case.

Roger came up with the following;:

Quoted message 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

There seems to be some areas where you are a bit off the mark with that
article.

"larger wheels create less powerfull jerks as they ride over obstacles"
They are also a heavier unsprung mass so the suspension has to be that bit
heavier and beefier to cope.

"Telescopic forks are often the cause of grief on the part of less expert
riders" Telescopic forks have been an extremely positive boon to those who
do venture off-road, I know of no-one who has come to grief due to using
them. The main problem ISTM is that forks allow faster speeds and when one
does come to grief, the extra speed might exascerbate any injuries.

"Needless to say, many amaturs go down steep slopes with the forks blocked,"
Absolute nonsense. I know of no-one who goes downhill, as a novice or pro,
with forks locked up. Uphill you often lock them up to stop 'bounce' when
mashing the pedals, but not downhill .. ever. Even cheap telescopic forks,
say < £200, are these days pretty damned good at what they do and certainly
are an improvement over almost any solid fork.

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.

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

Rob Morley said:

So why do you think the weight distribution when climbing will be
significantly different between a 26" and a 29" bike with the same
chainstay length and bottom bracket height?

What counts here is the weight distribution realtive to a vertical line
that transects the axle. Imagine you have the bike balanced stationary
in a wheelie position, the larger the wheel the steeper the angle of
balance.

Quoted message said:

For descending, a longer
top tube and shorter stem, plus longer head tube or higher stem, puts
the rider's weight further back just like a larger front wheel.

This was my thinking, until I actually did a few scale drawings. Agin,
the key issue is that a 29" is not a scaled up 26", the BB can be at
the same height as the 26", so that means the rider can be in a better
control position with same weight position relative to the
perpendicular on the axle.

Quoted message said:

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. A
bigger wheel will be more massive resulting in a greater unsprung mass
on suspension bikes, reducing the effectiveness of the suspension. So
in some respects there are no differences, and in others it's a matter
of swings and roundabouts as is so often the case with this sort of
design issue.

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.

Paul - censored said:

Roger came up with the following;:

Quoted message 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

There seems to be some areas where you are a bit off the mark with that
article.

"larger wheels create less powerfull jerks as they ride over obstacles"
They are also a heavier unsprung mass so the suspension has to be that bit
heavier and beefier to cope.

It was an empirical observation, note mine but on the part of just
about everybody who has written about thier experience with 29" bikes,
most of which are rigids but it seems to be universally accepted that
they give a smoother ride than 26" rigids.

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.

BTW, why does everybody think that changing the frame geometry is an
"easier" compromise? Making a frame for 29" instead of 26" is no
different from changing the geometry. 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.

In article <[email hidden]>

Roger said:

Rob Morley said:

So why do you think the weight distribution when climbing will be
significantly different between a 26" and a 29" bike with the same
chainstay length and bottom bracket height?

What counts here is the weight distribution realtive to a vertical line
that transects the axle. Imagine you have the bike balanced stationary
in a wheelie position, the larger the wheel the steeper the angle of
balance.

It's the tyre/dirt contact that matters, not the axle position.

Quoted message said:

Quoted message said:

For descending, a longer
top tube and shorter stem, plus longer head tube or higher stem, puts
the rider's weight further back just like a larger front wheel.

This was my thinking, until I actually did a few scale drawings. Agin,
the key issue is that a 29" is not a scaled up 26", the BB can be at
the same height as the 26", so that means the rider can be in a better
control position with same weight position relative to the
perpendicular on the axle.

See above.

Quoted message said:

Quoted message said:

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. A
bigger wheel will be more massive resulting in a greater unsprung mass
on suspension bikes, reducing the effectiveness of the suspension. So
in some respects there are no differences, and in others it's a matter
of swings and roundabouts as is so often the case with this sort of
design issue.

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.

Clear to you, perhaps ....

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

Quoted message said:

Quoted message said:

That's where you're going wrong. You can put longer chainstays on to get
the
same effect - no need to change the pedalling position or chainset
height.
Frame geometry is about more than just angles.

Been there, done that. My older bike had a wheelbase several
centimetres longer. Point is that agility and handling are important.
Whilst it is not nice to go over the bars, if you can't manage to make
the sharp turn to avoid the tree at the bottom of the slope you are
just as cooked. Last weekend bent my dual control lever box ramming a
tree at the bottom of a slope where I needed my gut on the saddle to
stop going over the bars.

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?

Quoted message said:

Quoted message said:

People choose not to put the longer chainstays on because I think they
tend
to feel that the increased problems climbing steep stuff are offset by
the
more agile handling afforded by the short wheelbase.

All the solutions which involve getting the riders moving weight
laterally degrade the handling. Solutions which lowwer the riders
position vertically reduce pedal/chainwheel clearance. If you have the
BB at the same height as a 26" but with 29" wheels you don't degrade
anything.

The converse is true. All you need to consider for the climbing thing is the
position of the various contact points - tyre/ground, feet, hands, seat,
COM. These are independent of wheel size. Why do you think they aren't?

Quoted message said:

I would love to here some other novel solution, but it is some time I
have been studying this problem. After arriving at the conclusion that
I would be better off with larger wheels I went to look if larger MTB
wheels actually exist, and I found out that not only do they exist but
they are becoming very popular in the states, large riders in
particular seem to love them. It would seem to confirm the case.

Your conclusion may be correct for you, but your arguments on that web page
are nonsense.

cheers,
clive

Roger said:

Rob Morley said:

So why do you think the weight distribution when climbing will be
significantly different between a 26" and a 29" bike with the same
chainstay length and bottom bracket height?

What counts here is the weight distribution realtive to a vertical line
that transects the axle. Imagine you have the bike balanced stationary
in a wheelie position, the larger the wheel the steeper the angle of
balance.

Quoted message said:

For descending, a longer
top tube and shorter stem, plus longer head tube or higher stem, puts
the rider's weight further back just like a larger front wheel.

This was my thinking, until I actually did a few scale drawings. Agin,
the key issue is that a 29" is not a scaled up 26", the BB can be at
the same height as the 26", so that means the rider can be in a better
control position with same weight position relative to the
perpendicular on the axle.

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?

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.
--
Tim.

[email hidden]

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 are thinking of a 29" as being a scaled up 26". In reality the BB
is placed at the same height as it would be on a 26", so that means in
an ideal cycling position the riders weight is closer to the axles.

Quoted message said:

The converse is true. All you need to consider for the climbing thing is the
position of the various contact points - tyre/ground, feet, hands, seat,
COM. These are independent of wheel size. Why do you think they aren't?

Think about balancing the bike in a wheelie position.

Quoted message said:

Your conclusion may be correct for you, but your arguments on that web page
are nonsense.

I have seen nothing here that contradicts them, perhaps I should point
out they way a 29" is not a scaled up 26".

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, but the basic concept is, as you say,
well-established, though 'cross tyres aren't quite as chunky as those
used on 29" MTBs; 35mm is usually the widest available, with a slightly
less knobbly tread pattern to give the bike enough "zip" on smooth
grassy areas, tarmac sections, etc.

David Belcher

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

Quoted message said:

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

Nobody's saying that. All I'm saying is that it has the same effect -
there's no intrinsic advantage to the bigger wheels for the climbing case
you mention.

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.

Sort of. The rim size has. Cyclocrossers don't use the 2" tyres the 29"
people use - 700x30 is a fairly normal cyclocross tyre. The reason they use
the bigger wheels is merely because they're road bikes - nothing to do with
700s being better at off road than anything else. 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.

cheers,
clive

"Roger" <[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 are thinking of a 29" as being a scaled up 26". In reality the BB
is placed at the same height as it would be on a 26", so that means in
an ideal cycling position the riders weight is closer to the axles.

That'll be where you're going wrong. No, I'm not thinking of a 29" as being
a scaled up 26".

Draw the contact points - tyre/ground, pedals, saddle. Then put the wheel on
and the downtube. Then all that happens to get the same geometry with
different sized wheels is different length/angle chainstays and seatstays.

cheers,
clive

In article <[email hidden]>

Roger said:

Paul - censored said:

Roger came up with the following;:

Quoted message 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

There seems to be some areas where you are a bit off the mark with that
article.

"larger wheels create less powerfull jerks as they ride over obstacles"
They are also a heavier unsprung mass so the suspension has to be that bit
heavier and beefier to cope.

It was an empirical observation, note mine but on the part of just
about everybody who has written about thier experience with 29" bikes,
most of which are rigids but it seems to be universally accepted that
they give a smoother ride than 26" rigids.

How many people ride 26" rigid off-road these days? My time-trial
wheels are faster than my tandem wheels, but it wouldn't be a good idea
to swap them around even if they did fit.

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.

BTW, why does everybody think that changing the frame geometry is an
"easier" compromise? Making a frame for 29" instead of 26" is no
different from changing the geometry.

Designing a frame for 29" wheels /is/ changing the geometry - and
restricting the choice of components that will fit.

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.

Long before mountain bikes and hybrids were "invented" the Off Road
Fellowship were getting down and dirty in the UK with their modified
touring bikes.

Rob Morley said:

How many people ride 26" rigid off-road these days?

Those of us that bought our MTBs when suspension of any sort cost Real
Money, and don't do enough off-road to justify an upgrade! ;-/

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/

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.

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 :-(

Quoted message said:

--
Tim.

[email hidden]

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? I'm not
really convinced about the rest of the arguments though. Aside from
anything else, rather than my wheel size being the main barrier to going
down a 60 degree slope as per the diagram, I think the lack of a
lobotomy scar on my head is going to be the greater preventative...

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/