Wheel inertia - what to believe? - Sportbike Forum: Sportbike Motorcycle Forums
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post #1 of 29 (permalink) Old 05-05-2004, 07:46 PM Thread Starter
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Question Wheel inertia - what to believe?

From the MotoGP website:

<<250cc News

New brake system for Sabbatani
News, 05/05/04

After six years in the 125cc World Championship, Max Sabbatani decided to set new goals for 2004 and stepped up to the quarter-litre class to join Taro Sekiguchi in the NC World Trade team, running privateer Yamaha TZM machines. Although the diminutive Italian has yet to score a single point in his 250 adventure, the quest to match the power of the factory Honda and Aprilia machines has led him down interesting new avenues.

The 28 year old is the only rider in the championship to be using an experimental front peripheral brake disc, which was tested last year on several Aprilia 125cc machines. Although the new disc has not been fully developed, Sabbatani claims to find the disc more consistent under hard braking.

Sabbatani also reported that the new disc helps the predictability of the bike in the corners as the weight of the brake disc on the outside of the wheel increases its gyroscopic stability and increases feeling with the front end.>>

So why do manufacturers try to reduce disk size and why is there a market for magnesium and carbon wheels?

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post #2 of 29 (permalink) Old 05-05-2004, 08:08 PM
 
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peripherals?

Are we talking a la Buell?
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post #3 of 29 (permalink) Old 05-05-2004, 08:34 PM Thread Starter
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Yes

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post #4 of 29 (permalink) Old 05-05-2004, 09:11 PM
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I see your point and raise you a......

I'm not keen on taking technology from an overweight ugly pig of a thing like that Buell but......

Wouldn't the extra rotating mass make it harder to stop, especially as it is so much futher out from the centre of the wheel - can't be a good idea in my mind. As I understand, it's all about keeping that mass to a minimum and that's one of the main reasons behind the carbon discs on GP bikes.

Maybe a heavy wheel does help a wobbly old codger who hasn't managed a single championship point....

Clearly a slow news day over at MotoGp.com

Is it weird in here, or is it just me?
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post #5 of 29 (permalink) Old 05-05-2004, 09:14 PM
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In the 125cc class the minimum combined weight of motorcycle and rider (complete with riding gear) is 136 kilograms or 299.2 pounds. In 250cc the minimum weight of 95 kilos or 209 pounds applies only to the motorcycle. Dani Pedrosa weighs a whopping 43kilos (95pounds).

The 125/250cc bikes are so light that I would think any bump would be quite unsettling, and makes them pretty flighty...
Also would guess the larger disk will dissipate the heat more effectively.
I understand the MotoGP 250cc bikes produce about the same amount of HP as my R6. Would be interesting to ride.

When you are going to BIG BIKES I guess we might be talking differently?

This is what you're talking about:
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post #6 of 29 (permalink) Old 05-05-2004, 09:27 PM
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Give or take .2 of a pound

Yeah but, how am I going to clean my rims with those bastids on?????

Anyone else see our jockey sized Casey Stoner throw it away with only a few to go on the weekend in the 125's.... poor bugger he could have walked it in from there

Is it weird in here, or is it just me?
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post #7 of 29 (permalink) Old 05-05-2004, 09:44 PM
 
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Agreed, You have to have some inertia in the wheels to keep the bike upright but this sounds daft!

"Sabbatani claims to find the disc more consistent under hard braking"

I think he is stuck for ideas to describe it - possibly undecided himself!
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post #8 of 29 (permalink) Old 05-05-2004, 10:34 PM Thread Starter
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Beg to differ

You don't need inertia in the wheels to stay upright, there's more than enough in the mass of the bike, your own mass, and the rotational inertia of the engine and gears. Heavy wheels are bad for two reasons: it's unsprung weight, and the rotational inertia resists turning or straightening the bike through the gyroscope effect.

A rim-mounted disk could help reduce both problems. It's further from the axle so the braking effect is increased (another benefit), and this can be traded for a narrower disk surface, saving weight and inertia. I'm surprised they're not more popular.

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post #9 of 29 (permalink) Old 05-05-2004, 10:37 PM
 
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Hated physics...

You lot may as well be speaking Greek to me. Never any good at physics - so will duly back out of this discussion now
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post #10 of 29 (permalink) Old 05-05-2004, 10:39 PM
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Re: I see your point and raise you a......

Quote:
Originally posted by Gixxer_Dave

Wouldn't the extra rotating mass make it harder to stop, especially as it is so much futher out from the centre of the wheel -
Yes it would have more inertia but also more leverage over the wheel. The additional leverage will more than outweigh the additional inertia. As far as the handling improvement, I wouldn't have a clue but wouldn't dismiss it either. It's interesting.

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