Sportbike World banner

1 - 20 of 29 Posts

·
Registered
Joined
·
315 Posts
Discussion Starter #1
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?
 

·
Registered
Joined
·
1,518 Posts
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 :twofinger
 

·
Registered
Joined
·
560 Posts
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.:D :D

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

This is what you're talking about:
 

Attachments

·
Registered
Joined
·
1,518 Posts
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 :(
 

·
Registered
Joined
·
374 Posts
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!
 

·
Registered
Joined
·
315 Posts
Discussion Starter #8
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.
 

·
Registered
Joined
·
726 Posts
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:D
 

·
Registered
Joined
·
4,230 Posts
Re: I see your point and raise you a......

Gixxer_Dave said:

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.
 

·
Registered
Joined
·
1,518 Posts
Sounds like bullshit to me and I'm an expert

Trying to avoid a Dad and Dave type thing here, but here goes.......

I would like to hear an explanation of this so called 'leverage over the wheel' cause I think it's rubbish, if there were significant benefits of running the discs towards the outter edge (or anywhere else for that matter) it would have been tried and tested years ago on two/four wheels and now be in use by Rossi and whomever else can actually go fast.

A narrower disc surface yes, but the thing has a circumference of getting on 17inches and would outweigh conventional discs easy!!!! More bollocks I say!!

This guy or at least his team are doing a Paris Hilton for some attention and he and this product will not be heard of again :twofinger
 

·
Registered
Joined
·
4,230 Posts
I think the leverage is self evident. It's a dramatically larger radius, therefore the leverage. As far as the handling, my reference was to his claim for the stability on the really light bike, light wheels, etc. As I said, "I'm not sure but find it interesting". I can imagine the claim without having experience to support or refute it. It will be interesting to see if anybody else trys it or likes it.:dunno:
 

·
Registered
Joined
·
315 Posts
Discussion Starter #13 (Edited)
‘Leverage over the wheel’ – I assume this means braking torque - as you say, self-evident. The pads would be about 50% further away from the axle so braking torque would increase 50% too. Pretty useful!

Some of this could be traded for lower effort on the brake lever, or less lever travel, or if they stay the same, a narrower disk and smaller pads. A standard disk has width of 3cm so this could reduce to 2cm for the same area and weight (sorry Dave), with 50% more braking; unfortunately its rotational inertia would more than double.

Or a width of 13mm would have the same braking as a standard disk and weigh a third less, but it might get a bit too hot and its rotational inertia would still be 50% up.

Although this is what Sabbatani seems to want, it’s probably why they haven’t used for racing while for road bikes there are other issues like cleaning the rims and changing tyres. There’s no question they’d work well though!
 

·
Registered
Joined
·
1,518 Posts
I still wouldn't bet on it

Nice work on the theory there Einsteins......

I still say the proof is in the pudding and it's more gimick that the latest Brake through (get it) technology :D
 

·
Registered
Joined
·
315 Posts
Discussion Starter #17
What about servos?

On reflection you're absolutely right Dave. The only useful bit of the disk is the bit under the pads, the rest is at best a heat-sink, otherwise unsprung weight and unwanted inertia. So the smaller the the disk the better. The only limiting factor is insufficient braking torque, so why don't bikes have vacuum servos like cars? They're light and simple, just a box with a tube to the inlet tract, a diaphragm and a bit of hydraulics. No doubt somebody will tell me it's been tried.
 

·
Registered
Joined
·
4,230 Posts
That study, as I gather it from a quick perusal, only discusses the steering effort to initiate a turn and then maintain it once in. I grasp that concept and always did without knowing what the numbers actually were. What it doesn't identify is what is acceptable to a rider and how it effects his feel. I wouldn't want that effort to be zero, nor would I want it to require a Herculean effort to turn at all. Somewhere between is the balance and as the range narrows, I'm sure rider preference starts to enter the picture.

The potential for reduced braking effort I believe is agreed on, yes? If so, then the discussion is in weight and its advantages or disadvantages as it effects steering input. I guess what I'm not seeing is more than what I am seeing in the provided information. That is the assembly weight of the wheel as it relates to wheels I am familiar with, and where, from center outward, is that weight placed. All things that effect the steering feel as so eloquently described in that study. That is where my inexperience with the typical 125 wheel assembly weight as well as the details of the perimeter brake design leave me at a partial disadvantage to pass judgement without actually trying it or hearing the reviews of several who have.

I also identify several "what ifs" that aren't detailed in the perimeter brake description. As 1-4 described, with the additional leverage over the wheel (torque advantage), the rotor could be physically smaller, less area, than the typical ones I am familiar with. It also might survive at a thinner cross section due to lower heat from a longer time spent cooling, not under the pad, and it doesn't require a seperate center, although that center means less to the gyroscopic effect as its diameter is reduced. I guess what I'm trying to spell out here is my musings that at one moment, can identify a problem and then, in the next moment, can see possibilities that may reduce the identified problem. The rotating weight of the perimeter assembly may not be as high or as bad as a linear upscaling of current brake design would suggest. Especially if the additional leverage allowed the use of one rotor instead of two. When I add to that my speculation that the front assembly as well as the total bike weight on a 125 may be so light that the feel of the whole package becomes less than optimum, I conclude as I identified, that "I don't know" and "it would be interesting".

What would be interesting data to have would be what steering effort by the rider is optimum or at what effort does it become troublesome. That could then be compared to the above study to accurately determine on paper what the above information suggests.

BTW, my inclination is that it won't prove advantageous but I'm not so quick to totally dismiss it with the information and experience I have or what the above information proves to me. Hell, I'd have never invented the steering damper because I'd have just left more caster in the front, knowing that too little aggravates head shake.;) It will be interesting. I don't think it will take long to prove itself either way.
 

·
Registered
Joined
·
1,518 Posts
Yeah but

'.......where is the average yaw velocity and are the minimum and maximum radius of the steering pad respectively.'

The one in italian actually made more sense..... :(


Both docs prove the need to counter steer, which we all knew or at least use already right :rolleyes:

Back to the drawing boards leverage lovers :twofinger
 
1 - 20 of 29 Posts
Top