Bike vs Car

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Post Tue Jun 16, 2015 3:21 am

The old thread has somehow got locked. (It was getting quite a bit sidetracked) And there was no clear answer yet. Please dont delete this thread, as it I believe will contain a lot of informations and be helpfull to lots of people.

From the old thread;

If I could sum up the important bits from the old thread,
Tires dont produce grip just by friction, it also has adhesive grip, meaning the tire interlocks with the surface of the ground, even though it is much smaller than frictional grip. This means that the size of the contact patch matters (a bit), giving car an advantage.



To the bikes advantage, they carry much less weight, even though the tires must be producing the coefficient of friction that the compound can provide, we know this is not the case because of load sensitivity. But this affect is also not so big.


Other than those two subjects, I couldn't think of any reason why wouldn't they be exactly the same cornering speed wise, and even with them, they seem to cancel eachother out, assuming a no downforce car with same tire compounds.


Am I missing a couple of things?

Edit: The old thread was about cornering g's only, so even though we can talk about longitudinal acceleration/deceleration let's not get too distracted
Last edited by Erunanethiel on Tue Jun 16, 2015 11:25 pm, edited 1 time in total.
Erunanethiel
 
Joined: 26 Oct 2013

Post Tue Jun 16, 2015 4:33 pm

The bike has two very small contact patches that are about the size of your palm. The car has 4 contact patches which are all bigger, especially laterally, than the bike tires.

I have driven on a track (the Nurburgring) where they mix bikes and cars. The bikes can accelerate crazy fast once they get out of the corner but they brake much sooner and longer than a car and corner slower. Also, there is of course the fact that on a bike, if you screw up, you are probably going down. In a car, maybe just a slide or a spin.

Long story short: Bike wins on the straights, car wins in the corners and braking zones.
tuj
 
Joined: 15 Jun 2007

Post Tue Jun 16, 2015 5:14 pm

tuj wrote:The bike has two very small contact patches that are about the size of your palm. The car has 4 contact patches which are all bigger, especially laterally, than the bike tires.

I have driven on a track (the Nurburgring) where they mix bikes and cars. The bikes can accelerate crazy fast once they get out of the corner but they brake much sooner and longer than a car and corner slower. Also, there is of course the fact that on a bike, if you screw up, you are probably going down. In a car, maybe just a slide or a spin.

Long story short: Bike wins on the straights, car wins in the corners and braking zones.


Friction is independent of surface area, so having a smaller contact patch doesnt mean anything in terms of mechanical grip. It does make a bit of difference as I have explained in OP, but those are small potatoes, and gets cancelled out by the bikes advantage in terms of tire load sensitivity advantage.

And for Nurburgring, they have some cars with crazy aero which gives cars a huge advantage. And if you are tracking on a bike rather than a car, chances are you are not finacially very well off. I dont think absorbing on the nurburgring is the best way to do this.
Erunanethiel
 
Joined: 26 Oct 2013

Post Tue Jun 16, 2015 5:52 pm

Erunanethiel wrote:Friction is independent of surface area, so having a smaller contact patch doesnt mean anything in terms of mechanical grip. It does make a bit of difference as I have explained in OP


One of the main points I made in the other thread was that surface area does have an influence on grip. The tyre produces grip from 2 mechanisms:
1. Coulomb friction - this is independent of contact patch area
2. Adhesive forces - these are proportional to contact patch area

So if you increase the tyre size, you increase the adhesive component of the total grip.

I've seen the same tenancy on the Nordscleife too. The bikes get in everyone's way in the corners, and then disappear on the straights.
Not the engineer at Force India
Tim.Wright
 
Joined: 13 Feb 2009

Post Tue Jun 16, 2015 6:06 pm

Not tire related, but the track is effectively a good bit wider for a bike.
olefud
 
Joined: 12 Mar 2011
Location: Boulder, Colorado USA

Post Tue Jun 16, 2015 6:24 pm

you are simply wrong about the size of the contact patch area not mattering. If contact patch area didn't matter, race cars would all use skinny tires to reduce rolling resistance.

http://phors.locost7.info/phors19.htm

Oh and as for the Nurburgring? I was driving a rented 190 Kompressor with crap tires and I was passing liter bikes all day long in the corners and under braking. No aero needed.

Image
tuj
 
Joined: 15 Jun 2007

Post Tue Jun 16, 2015 10:43 pm

It's not about grip, if you factor that out, a bike can only brake or accelerate with around 1G because of the high cog. In corners it's a bit more, depending how far you can vr bring the cog down. Marquez has a lean angle maxing out of 61 degrees, so he should get around 1,2-1,3G around a corner, road bikes do around 45degrees max.

Because of the mental weight/power ratio they can maintain maximum acceleration much longer and be fast on the straights.

With cars cog is not so much a factor with braking and accelerating (never saw a "normal" car do wheelies or stoppies).
Jolle
 
Joined: 29 Jan 2014

Post Tue Jun 16, 2015 11:12 pm

Tim.Wright wrote:
Erunanethiel wrote:Friction is independent of surface area, so having a smaller contact patch doesnt mean anything in terms of mechanical grip. It does make a bit of difference as I have explained in OP


One of the main points I made in the other thread was that surface area does have an influence on grip. The tyre produces grip from 2 mechanisms:
1. Coulomb friction - this is independent of contact patch area
2. Adhesive forces - these are proportional to contact patch area

So if you increase the tyre size, you increase the adhesive component of the total grip.

I've seen the same tenancy on the Nordscleife too. The bikes get in everyone's way in the corners, and then disappear on the straights.

Could you please check the second paragraph of OP? I have covered that.
Erunanethiel
 
Joined: 26 Oct 2013

Post Tue Jun 16, 2015 11:17 pm

tuj wrote:you are simply wrong about the size of the contact patch area not mattering. If contact patch area didn't matter, race cars would all use skinny tires to reduce rolling resistance.

http://phors.locost7.info/phors19.htm

Oh and as for the Nurburgring? I was driving a rented 190 Kompressor with crap tires and I was passing liter bikes all day long in the corners and under braking. No aero needed.

http://www.renkucorp.com/jf/euro/IMAG0177.JPG

The bikes have a lot to lose on a track like nurburgring (their lives) so I dont think its a valid comparison since they might not be pushing the limits like you are. If you could be bothered there is a 458 italia vs panigale comparison and since a pro rider and a driver is used, I think panigale even pulled a bit more g's.

The use of wider tires is not for grip, but for better distrubition of heat.
Last edited by Erunanethiel on Tue Jun 16, 2015 11:23 pm, edited 1 time in total.
Erunanethiel
 
Joined: 26 Oct 2013

Post Tue Jun 16, 2015 11:22 pm

Jolle wrote:It's not about grip, if you factor that out, a bike can only brake or accelerate with around 1G because of the high cog. In corners it's a bit more, depending how far you can vr bring the cog down. Marquez has a lean angle maxing out of 61 degrees, so he should get around 1,2-1,3G around a corner, road bikes do around 45degrees max.

Because of the mental weight/power ratio they can maintain maximum acceleration much longer and be fast on the straights.

With cars cog is not so much a factor with braking and accelerating (never saw a "normal" car do wheelies or stoppies).

The old thread was about cornering only, I forgot to meantion that. By the way at 61 degree lean, from TAN(61) you are pulling around 1.8g.
Erunanethiel
 
Joined: 26 Oct 2013

Post Wed Jun 17, 2015 11:01 am

A car is faster around a corner....

If the corner is long enough so both lines have similar radius, because a bike is thinner so if it´s an angle type corner the real line will have a wider radius than the line the car will do

If the corner is not a fast one, because the weight then will play his role

If it´s a single corner, because a chicane (again, if corners are short) can be taken on a more straight line with a bike

So you can compare bike vs cars on a specific corner, but generally speaking there are many parameters wich will make a generic discussion endless

What is not debatable:

A bike is much much faster on straights

A car is much faster on brakings


But even so it may be debatable because.... what bike compared with what car?

And that´s without mentioning what´s the reason to ban cars with downforce, because that´s one of the advantages of cars over bikes, so if you want to compare MotoGP bikes vs production cars I don´t think that´s a fair comparison, but comparing production cars vs production bikes I think those are the main differences
Andres125sx
 
Joined: 13 Aug 2013
Location: Madrid, Spain

Post Wed Jun 17, 2015 2:20 pm

Tire size and contact patch size matter because rubber does not have an infinitely high shear strength. Sure you can get the same theoretical frictional and adhesive forces from contact patches of different sizes, but as contact area goes down, so does the shear area. As this area goes down the shear stress goes up. When the frictional loads surpass the shear strength of the rubber, you lose rubber to the ground as your material cannot physically support the loads demanded of it.
sgth0mas
 
Joined: 18 Mar 2015

Post Thu Jun 18, 2015 5:01 pm

Erunanethiel wrote: ..... By the way at 61 degree lean, from TAN(61) you are pulling around 1.8g.

apparently around but a bit less than 1.8g
some of the 61 deg worth of tipplng moment that is expected to cause 1.8g centripetal acceleration ....
is needed to counteract the 'gyroscopic' moment acting in the direction opposite to the tipping
counteracting this 'costs' the equivalent of 2-4 deg of lean according to Tony Wilson-Jones of Royal Enfield (less in MotoGP though)

the moment comes from the wheels angular velocity (ie the yaw of cornering) whilst rotating
(plus or minus a bit of moment due to the rotating engine and transmission parts)

in a car this effect will slightly increase the lateral load transfer

NB to self .....
the 61 deg angle of lean is probably back-calculated (from accelerometer data) ignoring the above effect and thereby false
if so we do have a true 1.8g but the true angle of lean is more than 61 deg
Last edited by Tommy Cookers on Thu Jun 18, 2015 5:39 pm, edited 1 time in total.
Tommy Cookers
 
Joined: 17 Feb 2012

Post Thu Jun 18, 2015 5:38 pm

Erunanethiel wrote:The bikes have a lot to lose on a track like nurburgring (their lives) so I dont think its a valid comparison since they might not be pushing the limits like you are. If you could be bothered there is a 458 italia vs panigale comparison and since a pro rider and a driver is used, I think panigale even pulled a bit more g's.

The use of wider tires is not for grip, but for better distrubition of heat.


OK, let's try to make some valid comparisons then... The only track I can think of with MotoGP and cars that doesn't alter it's configuration for the bikes is Laguna Seca. In 2012, Jorge Lorenzo set a pole time of 1:20.554 on the Yamaha. In AMA, the best time was a 1:24.XX. The back of the AMA pack was lapping around 1:30.XX. The best time in a Champ car or older F1 car is around 1:05.XX. But obviously those cars have down-force. Randy Pobst did a 1.29.88 in a Porsche 918, so that's probably a fairer comparison. So if we consider a mid-pack AMA racer versus a production super-car, they are basically comparable.

Lastly, I can help but observe that we continue to debate the merits of wider tires. I am baffled by this. Yes, Newtonian physics says the contact patch doesn't matter to overcome static friction. If we do an experiment with an autocross tire and push it sideways on asphalt, we will get a number around 1.7G's equivalent of grip. But show me an autocross production car that generates that much lateral grip....it doesn't happen.

I have heard physics give all kinds of explanations as to why wider tires DO make sense. None of their explanations are satisfactory to me that I have read online. The one thing I can tell you is that more contact patch is better, regardless of bike or car. More contact patch = more grip. Why? Maybe it has to do with micro-heat management and slip angles....I don't profess to know. But I can tell you that it matters.
tuj
 
Joined: 15 Jun 2007

Post Fri Jun 19, 2015 12:50 am

sgth0mas wrote:Tire size and contact patch size matter because rubber does not have an infinitely high shear strength. Sure you can get the same theoretical frictional and adhesive forces from contact patches of different sizes, but as contact area goes down, so does the shear area. As this area goes down the shear stress goes up. When the frictional loads surpass the shear strength of the rubber, you lose rubber to the ground as your material cannot physically support the loads demanded of it.

Read the first post please, this is called load sensitivity.
Last edited by Erunanethiel on Fri Jun 19, 2015 1:53 am, edited 1 time in total.
Erunanethiel
 
Joined: 26 Oct 2013

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