29er bouncing at speed

Unicycles and Equipment
1

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

2

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

3

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

4

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.

5

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

6

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.

7

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.

8

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.

9

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

10

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

11

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

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?:smiley:

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.:slight_smile:

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…

12

Re: 29er bouncing at speed

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.

13

bouncing

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.

14

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