Why is it...?

Once a week, I have up to half an hour in a hall in which I can practise basic skills.

After discussions in this forum about tyre pressures, I decided to experiment. Everyone says softer tyres are better for bouncing. A little experimentation showed this to be true, although I was worried about pinching my tube or flattening my rim.

But… inflate a football to full pressure and it will bounce better than a soft one. Squash balls bounce better when they’ve warmed up and the air pressure inside increases. Golf balls are harder than squash balls and bounce much better.

I accept that soft tyres bounce better than hard ones, but it is counterintuitive. Can anyone out there explain? :thinking:

I seem to recall it’s to do with how quickly they regain their original shape. With bouncing balls, anyway… a high pressure ball will try to regain its shape faster, and so launch into the air faster.

A tyre will bounce slightly differently; you’re using it more as a trampoline than just bouncing it.

If you just drop the uni it will bounce faster with a fully inflated tyre than a flat one. However, with it fully inflated you won’t be able to do a pre-hop with it, for example; you’re too heavy, so when you land you’re still going downwards when the hard tyre has reformed and is trying to go up again.

A soft tyre gives you time to get down and spring up again in time with it, so making you jump higher.

I think.

Phil, just me

It’s the rebound rate. A high pressure tire will rebound very very quickly. A low pressure tire will rebound slower. To jump higher you need to time your jump with the rebound of the tire. A high pressure tire rebounds so quickly that no mortal human could time their jump with the rebound of the tire, thus they loose all the benefit of the bounce in the tire.

A lot of it is the shock factor. If you have a slightly flat tire then it will absorb some of the fall. Since the only way to put shocks on a unicycle is on the seat post, that helps a lot. Also if you bounce really high, a hard tire will keep you bouncing for a few hops, making it hard to keep balance.

I like Phil’s trampoline illustration. Along the same line, if you were to think Pogostick, a stiffer spring would not allow as much rebound as a softer spring. Hmm…have to think about this one.


hey folks,John is right here.it is the rebound rate…

i think the key phrase here might well be ‘optimum pressure’ rather than thinking in terms of higher or lower pressures
over inflate that football and i can guarantee u it wont bounce at all
the ‘optimum pressure’ will differ from rider to rider and machine to machine
i dont think it will be possible to create a chart of body weight to tyre pressure

Re: Why is it…?

On Fri, 25 Oct 2002 07:03:12 -0500, GILD
<GILD.d2w3b@timelimit.unicyclist.com> wrote:

>i think the key phrase here might well be ‘optimum pressure’ rather than
>thinking in terms of higher or lower pressures
Agreed. A too soft tyre would absorb energy rather than giving it
back, even if it doesn’t pinch out.

>over inflate that football and i can guarantee u it wont bounce at all
Indeed, if the football bursts. Otherwise, not agreed. Think of a
glass marble. It’s harder than an overinflated football will ever be.
Yet it bounces very well on a superhard surface.

Klaas Bil

I posted only a single copy of this message.