29er bouncing at speed

Hi Cathy,

I'll have a guess at the bouncing issue. I notice that I bounce a bit
on my coker at my top speed, and I think it's due to tiny adjustments
in the front/back balance with my legs. The adjustments get out of
phase with my smooth spin, and I bounce. If I concentrate on having a
smooth, almost passive balance, the bounce goes away. This is hard
unless the surface you are riding on is smooth and stable. Hope this
make some sense....

--Bob

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thw big apple tire is wide but not for dirt really

try looking at some tires on here,

heres a tire,but theres more on there to look at

http://www.speedgoat.com/product.asp?part=47870&cat=320&b

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musketman
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Bouncing at speed can happen when you are pushing the limits of your
current level of ability. What happens is you think you're pedaling
faster, but you're just pedaling harder!

When you are pedaling as fast/hard as you can, the pedal stroke is
often reduced to a hard downward push rather than a smooth circular
movement.

What then happens is that the entire mass of your pedal, crank, foot,
ankle, shin, knee and thigh moves up and down rather faster than you
can control it. What stops your foot carrying on down towards the
ground is no longer fine motor control in your leg, but the simple fact
that the pedal is in the way!

Add to this effect Newton's famous "equal and opposite reaction" and
you will see that as your pedal, crank, foot, ankle, shin, knee and
thigh move downwards, your body will tend to move upwards. (Yes, I
know that there is a {pedal....thigh} set of approximately equal weight
moving upwards at the same time, but there is still an effect on the
rest of your body.

So, the pedal strokes become jerky, and you start to bounce a little on
your seat, and the combined effect compresses and releases the tyre,
magnifying the effect - if it's a big fat slightly squishy tyre, even
more so. (This is related to why units of soldiers have to break step
as they march across a bridge, to stop the oscillation making the
bridge collapse.)

So, how do we deal with this?

In no particular order:

1) Shorter cranks will mean that the mass of your {pedal...thigh} is
moving less distance up and down. At the same wheel speed, therefore,
there will be less up and down oscillation - the bouncing you were
complaining about.

2) Shorter cranks also change the leverage, so that the wheel has more
control over your legs! Yes, really! It is harder to accelerate or
decelerate the wheel if you have short cranks. This means that the
flywheel effect of the wheel becomes more noticeable, and will help to
smooth out the jerkiness of your pedal strokes.

3) Possibly: adjust the height of your saddle. If you can get into a
position where you are sure that your weight is resting on your saddle
throughout the pedal stroke, that will stop your body mass bouncing up
and down.

4) Possibly, hold the handle or front of the saddle. This will help
you to concentrate on staying still in relation to the seat.

5) Build up to speed. Just going flat out will teach you bad habits.

Do it in stages: ride steadily, then gradually accelerate for 50 metres
or so then back off a bit. Regain your composure, then bring the speed
back up for 50 metres or so. After a few goes at this, bring the speed
up, and keep it up for 100 metres or so, and near to the end of that
100 metre section, accelerate just that little bit faster.

What will happen over time is that your cruising speed will increase,
meaning that your 50 metre burst speed will increase, and your 100
metre burst speed will increase and so on. A high top speed has to be
built on a platform of sustainable fast cruising speed. You have to
build that platform up gradually.

Have fun.

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Mikefule

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Thanks for your replies. I think I'll put the seat up a fraction. But
the cranks are 125s and I find it extremely difficult to freemount with
them as it is. Shorter cranks and I'd never get on.

I thought it might be something to do with the pedalling.

Ok, more practice then re the bouncing.

Cathy

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cathwood

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victims may be the most oppressive - C.S.Lewis

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cathwood said:

Hi
I've just been out for a great ride on my 29er. It showed that I was
completely out of condition but I can work on that. What I did manage
to do was to get up more speed than I am usually comfortable with. The
problem was that when I started to get up some speed I started to
'bounce' on the seat. Is this due to the tyre pressure (too high or
low), incorrect pressure on the pedals or something else?

I'd also like to ask those who take their big wheels off road what
tyres they use. I have noticed that the big apple tyre takes quite a
lot of lumps and bumps, at what stage would you use a more knobbly
tyre?

Thanks for any advice.

Cathy

If you're bouncing on the seat, you might have it set a smidge too
high. Try lowering it half a centimeter. Or, it may just be that
you're ready to try shorter cranks; I always feel like I'm bouncing a
bit when I go back up to 150mm cranks.

I think the Big Apple is a fine tire for off-road use, as long as the
surface isn't too loose. If you're going up or down steep hills on a
loose surface, it's better to have something with a little more bite,
but for anything with a packed surface it's a fine tire.

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Just curious as to what uni you're riding - I bought myself a Nimbus 29"
for Xmas. Specs match yours. I haven't found bouncing so much of a
problem, but it is noticeably harder (for now, anyway) to ride than my
old 20". It's a bit like going back a few months and having to relearn
things.

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If you want a tire for offroad use, or slush and dirt or whatever, I
would try the Kenda Klaw. Its the same one used on the KH 29er. I have
it on my 29er (a bedford XC) and I really like it.

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ntappin
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Hi Cathwood,

I have a Nanoraptor on my 29er it's not as wide as a big apple, but it
has a good tread and rides nicely. I run it a 60lbs on road and off.
Mikefule has it right with regards to the bounceing.

Cheers Dave

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mark williamson said:

Just curious as to what uni you're riding - I bought myself a Nimbus 29"
for Xmas. Specs match yours. I haven't found bouncing so much of a
problem, but it is noticeably harder (for now, anyway) to ride than my
old 20". It's a bit like going back a few months and having to relearn
things.

I'm also riding the Nimbus 29" (with Big Apple tyre).

Bouncing hasn't been a problem until now, when I have increased the
pace a bit.
I also regulary ride a 20" freestyle and a 24" muni. Swapping between
unicycles is a skill like any other I think. It used to take me a
couple of minutes or so to adjust. I'm OK now though.

My idea for the future is to get a coker for distance riding and to fit
the 29er up for riding off road. Or to get another 29er for off road. I
can't decide at the moment.

Cathy

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cathwood

'Of all tyrannies a tyranny sincerely exercised for the good of its
victims may be the most oppressive - C.S.Lewis

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cathwood said:

My idea for the future is to get a coker for distance riding and to fit
the 29er up for riding off road. Or to get another 29er for off road. I
can't decide at the moment.

That way madness lies. Madness and poverty. Madness, poverty and
clutter.:eek:

Also, a Coker is pretty competent off road too. And now you can get a
road-biased 36" tyre, so why not have two Cokers?😄

For simple practicality, the 700c (28/29) wheel has more options for
rim, spokes, tubes and tyres. It's lighter, cheaper, more durable,
easier to store or transport, and safer in traffic or crowds. And you
bang your head less on low branches.

Changing a wheelset takes only a few minutes. You could have two
wheels: one set up with long cranks and a fat knobbly, and one set up
with short(er) cranks and a smoother tyre. That means only one frame,
seatpost and seat.

But still, a Coker is a Coker.🙂

As you know, I have a Coker, a 26 and a 28. I have the complete
freedom to choose which to ride. I find the Coker gets used less and
less. The pattern seems to be 28 for summer, 26 for winter. I have
toyed with the idea of getting a winter tyre for the 700c.

I might even sell the Coker. It deserves to be used more, and I can
just see it spending its retirement cruising gently along the sea
front...

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Your leg and foot on either side have to move up and down through each
pedal revolution. That means that all the time there is a varying
vertical acceleration of that mass going on (ignore horizontal
acceleration for now). For your two legs, this acceleration is not
symmetrical. That means that the two legs and feet together are
effectively a mass that bobs up and down through two cycles per one
wheel revolution.

If you ride relatively slowly, this bobbing goes on quite slowly too,
and hence is 'stable'. If you ride faster, you enter into a region
where the frequency of the bobbing up and down starts to 'resonate'
with tyre springiness, body springiness and other 'springy' components
in the system. Through resonation, the excursion of the up and down
motion gets amplified, and that's the bouncing you experience.

What can you do about it?

1. Ride slower. That's not what you want.
2. Mount shorter cranks. That may be for later.
3. Concentrate on a smooth circular 'drive' of your pedals, rather
than just up and down.
4. Ankling! That's where you use the motion range of your ankle partly
to propel the pedals (in addition to the usual leg movement). Any
ankling you do reduces the amplitude of your (heavier) legs. Have your
ball of your feet on the pedal (as opposed to the arch) to make
ankling more effective. You may find it hard to move the ankles as
part of your unicycling pedal motion. What helped me in that respect
was to think that the thighs should move as little as possible.

As to the tyres: the BA is wide but not very suited to off-road. In
some conditions it can do fine. But grip in wet conditions is
marginal. Also for off-road you would mostly prefer low pressure, but
the BA likes high pressure or it gets squirrely.

An old roadie would guess the high cadence has you spinning squares
instead of circles. If you can, ride a fixed gear or spinning bike to
smooth out your pedel stroke. Riding any old "10speed" on a portable
trainer at high cadence (100 rpm or higher) will help.Try not to use
clipless or toe strap pedals if possible.

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Quoted message said:

What can you do about it?

1. Ride slower. That's not what you want.
2. Mount shorter cranks. That may be for later.
3. Concentrate on a smooth circular 'drive' of your pedals, rather
than just up and down.
4. Ankling! That's where you use the motion range of your ankle partly
to propel the pedals (in addition to the usual leg movement). Any
ankling you do reduces the amplitude of your (heavier) legs. Have your
ball of your feet on the pedal (as opposed to the arch) to make
ankling more effective. You may find it hard to move the ankles as
part of your unicycling pedal motion. What helped me in that respect
was to think that the thighs should move as little as possible.

Thanks Klaas Bill.
I suspected the answer would lie along these lines and thinking about
it, I had the pedals in the middle of my foot as it was off road and I
had been riding up hill. I had just turned around.

Cathy

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Disclaimer: Everything I post (unless otherwise stated) is my own
opinion, based on stuff that I've learned.

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