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PostPosted: Mon Dec 01, 2008 2:03 am 
Dirt Disciple

Joined: Fri Nov 28, 2008 1:46 am
Posts: 39
GrahamJohnWallace wrote:

Personally, I wouldn’t particularly mind riding a Cleland that had a standard bottom-bracket height as long as the bikes other characteristics weren’t effected. It would more readily ground when riding down ruts, and your pedals and feet might get muddier, but that's no big deal.



Can't shorter cranks be used? The loss in torque be compensated for by lower gearing?

Just read this entire thread, wow.

Geoff,
I can remember reading a magazine article about you & the bike including the B&W photo of you & the Cleland on page two of this thread many years ago, I remember thinking how I'd love one of those.
Can't remember what mag though.

Great bikes. :D


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PostPosted: Mon Dec 01, 2008 1:14 pm 
MacRetro rider
MacRetro rider
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Joined: Mon Nov 10, 2008 10:39 pm
Posts: 170
Location: SE Scotland
Hello REtrouble ~

I would miss the higher bottom bracket, it may be only a couple of inches, but they can be critical.

Mike Burrows has something to say about shorter cranks; he feels that loss of torque, leverage, purchase, call it what you will, is a myth. I credit his argumant with some substance. However, having a decent pair of cranks shortened is not an inexpensive operation and I have more critical things on my shopping list. But, one day . . .

The magazine article appeared in Bicycle Action, issue two (or perhaps three). I have a copy somewhere.

Glad you're enjoying the thread.


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PostPosted: Mon Dec 01, 2008 8:35 pm 
Dirt Disciple

Joined: Fri Nov 28, 2008 1:46 am
Posts: 39
Hi Geoff,

It would be nice if you could dig the magazine article out (just making work for you :wink: ).

Do you know or have an idea of the number of bikes produced?


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PostPosted: Mon Dec 01, 2008 8:44 pm 
retrobike rider
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Joined: Tue Sep 30, 2008 6:25 pm
Posts: 924
Location: Near Wendover Bucks
Here's Mike Burrows' take on both crank length and Biopace...
"Another favourite with inventors. The idea is that, if you could arrange for the crank to be longer than normal on the power stroke, and shorter than normal on the return, you will get more power -but without the problems of leg stretch that a fixed length crank would cause.

Wrong again. You don't get more power with a long crank, sliding or otherwise. You just get a different gear. 'Power' is a function of force and speed combined. And for the devices that change the ratio of power to rest this reduces the muscle's rest time and so lowers its potential for generating thrust. The same effect can be produced by fitting oval chainrings the 'wrong' way, like Biopace, thus avoiding the need for a piece of mechanism that looks like part of a robot dishwasher."


In other words the relationship between the input and the output of any lever can be seen as one of relative speed (velocity ratio), and relative force, (mechanical advantage). The ratio of input and output 'power' will remain the same as no new power is created by the mechanism.

Man Machine interface...The power output of the human leg however, varies with leg extension. Therefore, there must be an optimal stroke length/crank length and cadence for a given person. It's really a question of the obtaining the most efficient transfer of power and finding which crank length provides the most efficient cadence.

Wheel Spin…
The issue of large amounts of mechanical advantage (i.e. a very low gear), is that this can cause wheel spin. To stop this you need a smaller force that is delivered for a longer time.


and Wheelies…
The other problem with large amounts of mechanical advantage, delivered suddenly, is torque reaction. The effect of this is that the front wheel tries to lift off the ground and you need to move your body weight forward to keep the wheel down. This in tern reduces the weight and friction on the rear wheel, that now begins to slip.

The whole thing is a balancing act: with rear wheel friction balanced against front wheel lift and the whole show controlled by the centre of gravity pushing down on things from above. That is why it's important to keep the rear end short, and the riders weight, mobile.

:shock: It’s funny how bicycles are easier to ride, than explain. :shock:

Anyone want to explain how the ‘Bunny Hop’ works?


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 Post subject:
PostPosted: Mon Dec 01, 2008 11:09 pm 
MacRetro rider
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Joined: Mon Nov 10, 2008 10:39 pm
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Location: SE Scotland
I think it's time we had another picture . . .


Attachments:
DingbatActionWeb.jpg
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 Post subject:
PostPosted: Mon Dec 01, 2008 11:43 pm 
Dirt Disciple

Joined: Mon Nov 17, 2008 5:54 pm
Posts: 19
Location: Hyde, Cheshire
Splat :twisted:

Excellent pic 8) Any more :?: :?: :?:


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PostPosted: Tue Dec 02, 2008 12:20 am 
retrobike rider
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Joined: Tue Sep 30, 2008 6:25 pm
Posts: 924
Location: Near Wendover Bucks
Maximum weight over rear wheel time...

(is this the Dingbat shown earlier in the thread???)

Yes. The Dingbat evolved over time:

phase 1= un-braced frame, (standard forks)

phase 2= single brace added, (standard forks)

Phase 3= triangulated and braced forks added


Attachments:
Dingbat1988.jpg
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Last edited by GrahamJohnWallace on Wed Dec 03, 2008 9:36 pm, edited 3 times in total.
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 Post subject:
PostPosted: Tue Dec 02, 2008 12:52 am 
Dirt Disciple

Joined: Mon Nov 17, 2008 5:54 pm
Posts: 19
Location: Hyde, Cheshire
Whaaaa!

Both the pics of this bike are missing the engine :) I don't mean that it should have one o course :oops: :lol:

The newer 24" trialies & bmxs used in the same competitions just look like bicycles being put through their paces (and yes, i've seen the youtube clips of what they can do :twisted: )

The stance of the bike looks very motorcycle trials, as does the riders position on the bike. I know this is stating the obvious, as thats what the design was intending to achieve (is this the Dingbat shown earlier in the thread???), its just that its the first time i've actually seen this in a bicycle...

...and now I want one even more heheh


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PostPosted: Tue Dec 02, 2008 12:56 am 
retrobike rider
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Joined: Tue Sep 30, 2008 6:25 pm
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Location: Near Wendover Bucks
Throw weight back - emergency stop technique...

Best performed at night, on road, whilst using tungsten-carbide studded-tyres. :twisted: (these create nice sparks) :twisted:

Too much front brake however' can result in "lift-off"

There is a technique, that involves vaulting, feet first, over the handlebars and then landing on your feet and running forward.

I've only done this successfully once. When you land however, you do need to remember that there's a flying bike chasing you.


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Last edited by GrahamJohnWallace on Tue Dec 02, 2008 7:14 pm, edited 4 times in total.
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 Post subject:
PostPosted: Tue Dec 02, 2008 1:29 am 
Dirt Disciple

Joined: Mon Nov 17, 2008 5:54 pm
Posts: 19
Location: Hyde, Cheshire
Now I can manage that on my mountain bike at a push if the tarmacs greasy, but not for too long as
Quote:
(this
may create
Quote:
nice sparks)
from the bar ends, cranks or any other exposed bit o metal :shock: :twisted:

You can't beat mud for a softer landing :lol:


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