2018 FIM Moto GP World Championship

[Andres125sx]

fwiw I imagine .....
front braking potential is not clipped by the rear contact load falling to zero - they're matched at the design stage
Mr Pedrosa's weight limits him via needing more bike lean (for equally competitive combined bike+rider CG angle)
his braking should be competitive and his straightline acceleration greater (or he can use less fuel for equal acceleration)
he should have a shorter wheelbase ie the rear wheel forward ie rider-normalised to same behaviour as Mr Marquez's bike

About the bolded part, I think you´re not considering weight transfer in longitudinal axle. When riders rise to increase drag, they also move their weight backwards, wich IMHO is the real reason they rise. That´s what I did when racing MX (moving as back as possible), to prevent the rear wheel jumping on the bumps. MotoGP is different, lower CoG, longer wheelbase, and no bumps compared to MX, but much stronger braking force wich in the end causes same problem (rear wheel lift), so riders need to act similar to prevent it

Or maybe you think the lower CoG for Pedrosa due to his lighter weight is more beneficial than the handicap of a lower weight transfer to the rear wheel?

[etusch]

This video for bike aero. I couldn't watch. I will watch when I have time
https://www.youtube.com/watch?v=3Zu5_7lXFvQ&t=68s

Thanks for posting again.

OT, but I´m surprised about those drag figures at the beginning of the video. Does a half sphere really have less drag than a full sphere? I´m assuming the flat part of the half sphere is at the back. That does not increase turbulence and drag compared to a complete sphere?

You are welcome. I couldn't entirely watch first one still

[Tommy Cookers]

Andres
agreed the Pedrosa leaning case and braking case are similar ie in braking his wheel load distribution is worse than other riders
so his rear wheel will lift at less deceleration than other riders rear wheels will
yes his braking will be worse

but if P and his bike are 10% lighter than others he needs in braking 10% less grip
ok he has 10% less (or 9.5% less given his worse load distribution) tyre contact load
but the tyre should give more grip relative to contact load and to P's combo weight
so he can potentially develop greater deceleration than others but this is prevented by his more easily lifting the rear wheel
this may make it seem he has more of a problem than he really has

so I was wrong
actually he should move his front wheel forward to get competitive braking
this would reduce his advantage in acceleration
but as I said he can follow others matching their acceleration etc but using less fuel

[Bill_Kar]

With aero, but no wings, the drag vector is potentially below the CG when the rider is crouched and would tend to keep the front wheel down under acceleration particularly at high speed. Any downforce or drag from the wings would help that.

IMO
the drag vector always contributes towards reducing front contact load and increasing rear contact load
because its effect is due to its height above the track not height above/below the CG
ie drag doesn't alleviate the effect on contact loads of acceleration - it adds to the effect

fwiw I imagine .....
front braking potential is not clipped by the rear contact load falling to zero - they're matched at the design stage
Mr Pedrosa's weight limits him via needing more bike lean (for equally competitive combined bike+rider CG angle)
his braking should be competitive and his straightline acceleration greater (or he can use less fuel for equal acceleration)
he should have a shorter wheelbase ie the rear wheel forward ie rider-normalised to same behaviour as Mr Marquez's bike
(EDIT - this braking bit is wrong as Andres later says)

yes this front 'boxkite' aero DF is quite clever - and presumably there's a DF part to internal airflow
though DF from a rear wing (tried by one rider) could easily act well ahead of the wing (this of undesirable in aviation)

So is weight actually a disadvantage in pedrosa's case? Because, my limited knowledge says that riders should be light because of cg reasons.

[J.A.W.]

Again, note: with Moto GP, the technical complexities/functional dynamics overlap - ensure that such simple values will not do..

The manifold dynamic variables involved, well outweigh the basic - 'lighter pilot the better' idea - which is forcing F1 drivers..
into jockey-like weight loss routines..

Moto GP racing places its pilots into assuming much more active movement/posture/agility/power-exertion active roles,
& that of course, includes weight/aero-active - body placement on the machine, & which continually changes over a lap..

A couple of Ducati rider reports from the French G.P. last weekend are indicative..

Jorge Lorenzo reported difficulty with positional ergnonomics, & being fatigued by not being able to brace himself
bodily against braking forces effectively, & having to use excess effort to do so via the muscle-power of his limbs.

&,

Jack Miller reckoned he was lacking top end speed compared to his Pramac team mate, which, if not a power problem,
could be worked on in a wind-tunnel test session, so learning the most efficacious 'tuck-in' posture/fairing aero shapes..

[Tommy Cookers]

[quote=Bill_Kar]
[quote="Tommy Cookers"]fwiw I imagine .....
front braking potential is not clipped by the rear contact load falling to zero - they're matched at the design stage
Mr Pedrosa's weight limits him via needing more bike lean (for equally competitive combined bike+rider CG angle)

So is weight actually a disadvantage in pedrosa's case? Because, my limited knowledge says that riders should be light because of cg reasons.
[/quote]

light weight is a disadvantage in cornering

for braking limited by rear lift (overturning) what matters is angle of the bike/rider CG above the front track contact patch
the angle combines the effects (disadvantageous) of CG height and (advantageous) of CG distance rearward
eg for 1.5g deceleration to be borderline overturning the combined bike/rider CG would be at 34 deg above the horizontal

rider CG is higher but assuming it's enough rearward to give a lower angle than bike CG .... then ....
a heavier and taller (seat more rearward) rider is safe at a braking deceleration where a lighter rider is lifting the rear wheel
ie Pedrosa braking will be worse - he is more limited by wheel lift

but if rider CG isn't in a lower angle position than bike CG Pedrosa has no disadvantage

borderline rear wheel lift is manageable -
as initial overturning rate is limited by the rate of supply of the (potential and rotational) energy required

[J.A.W.]

Just a quick FYI T-C, Brembo report max-braking G-forces in Moto GP - as exceeding 1.5G.. ( & ~8 kg, at the lever).

[Nonserviam85]

....... If the rear lifta it is a combination of the rider overbraking and reaching the dive limits of the front....

so you're saying the rider can't use the maximum braking that is available because that would lift the rear and cause trouble ?
ie the designer is choosing to settle for less deceleration than the tyre could give - what I called clipping

Not exactly, the rider uses the maximum braking that the front suspension settings and brake discs allow him to use.

[Tommy Cookers]

agreed dive brings overturning closer (especially with conventional forks)

but dive can be eliminated by the designer - sometimes this has been done even in Moto GP
though most attempts at this eg so-called hub-centre steering have steering geometry unsuitable for racing

[henry]

...

The manifold dynamic variables involved, well outweigh the basic - 'lighter pilot the better' idea - which is forcing F1 drivers..
into jockey-like weight loss routines..

...

Funnily enough the BMI of top MotoGP riders an F1 drivers are quite similar, in the low 20s, which is itself low for a muscled athlete.

Average BMI current F1 grid, 21.3, 8 top MotoGP, 22.2.

[henry]

Just a quick FYI T-C, Brembo report max-braking G-forces in Moto GP - as exceeding 1.5G.. ( & ~8 kg, at the lever).

Is that total G force, braking + aero drag, or only braking?

[J.A.W.]

Henry, its braking force - but a rider sitting up & using his torso as an airbrake - would tend to reduce the apparent G, no?

See here: http://www.brembo.com/en/company/news/m ... mbo-brakes

[J.A.W.]

Interesting, I had never considered drag vector influence while braking, not sure if accurate, but to me it makes perfect sense

Sorry mate, from your comments it is obvious that you have never ridden a superbike motorcycle

We´re talking about MotoGP, not superbike. Did you notice for how long they keep the front brake lever fully applied before releasing it a bit for corner entry?


Anycase, as I´ve stated previously, I was asking for sources to learn about the subject, my reply was to JAW who is very fast criticizing people for not being technical, while he barely add any technical bit. He´s very technical saying people how ignorants they are and how wise he is, that´s all, but meanwhile he even misses some basics about motorcycling

Yeah, Superbikes have to run Fe brake discs, & like-wise the 'irony' is strong in Andres' remarks..

Clearly, as N-85 noted, a grasp of the "basics" - of how to operate hi-performance tarmac motorcycle braking - is lacking..

Andres missed the essential point made.. the front brake is 'King', (the rear brake is for corner balance/manoeuvres),
since the whole chassis is pivoting on the front tyre patch.. hence 4T 'engine braking' - is an unwanted factor.. & dialed out..

[henry]

From the Brembo link from @JAW, thanks, I have tried to put some numbers on straight line braking.

For the biggest stop Brembo quote a top speed of 201 mph, 90 m/s. I estimate the aerodynamic drag to be around 1200 N. I took the overall bike weight as 240 kg, so if the rider shuts the throttle the deceleration will be around 0.5 g. (As an aside the bike is probably still accelerating at around 0.25 g at that point).

If the rider sits up I estimate another 600 N drag, so total drag 1800 N and deceleration from drag 0.75 g. Brembo quote 1.5g max braking so wheel brakes contribute 0.75g.

The additional drag vector is higher than the CG, and will make a couple with the bike inertia (which acts through the CG) and that will tend to reduce the front wheel load. That is obviously undesirable and to counter it a rider could move forward, @nonserviam85 suggests that this is in fact what happens.

In terms of the forces on the rider at this point they are subject to 1.5 g forward, say 90 kg, and 60 kg rearwards from the drag, for a resultant loss of 30 kg. So sitting up reduces the strength required. As has been suggested I don’t ride so I don’t know how the rider applies the loads to the bike, arms to handlebars is obvious but I guess some interaction between the lower body and bodywork must take place.

As the stop progresses the aero drag will reduce and will only contribute about 0.15g at the end of the stop. The rider may move rearward during the stop to minimise weight transfer. It seems likely that the mechanical contribution is fairly constant at 0.75g. Since it is the mechanical force that causes the forward weight transfer it would appear that even given the increase in CG height due to the rider sitting up the transfer is probably not 100%, and the rear wheel lift is a consequence of road forces.

The basic physics of bikes seems trickier than, say, F1 cars not least because a major mass, the rider, moves around so much.

[nacho]

Tony Foale has an article on aerodynamics:

https://motochassis.com/Articles/Aerodynamics/AERO.htm