wider rear tires
03-05-2014, 03:20 PM
#16
Rennlist Member
same compound..... 275s in the rear, vs 305s on the rear 9" wide vs 12" wide)..... huge difference in longitudinal grip. look at the 9 second hondas..... they dont put pinners up front for grip... they go wider! It is a huge differernce in contact patch with a wider tire, and this gives a higher friction coefficient.
03-05-2014, 03:25 PM
#17
Rennlist Member
Could you explain why? I don't see how the orientation of the contact patch changes the force it can exert.
Narrow tyres are going to need a higher pressure to keep the load deformation reasonable, so comparing at the same pressure doesn't seem useful. Am I misisng something?
Narrow tyres are going to need a higher pressure to keep the load deformation reasonable, so comparing at the same pressure doesn't seem useful. Am I misisng something?
Ive also found a limit of benefit. it seems with the 3000lb 928 racer, that the 315 is the limit of what works, vs the 335, being too wide to get the max use out of. we can see this in tread wear paterns where the inner edge is not even being used (a full 1" of it) vs the outer edge is worn right to the proper spot. taking out camber and the edges are not used on both sides, without major deflating, then the centers of the tires stop getting heat. SO, there is an optimum size for the type, weight and duty of the car in question.
wider is usually better, but to a point.
03-05-2014, 03:59 PM
#19
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Its has been a long time, but i think it has to do with the rubber deformation ,dynamics, or some other factors. there has to be a reason why 275s will spin wildly and 315s seem to be firmly planted on accel in 2nd gear, on dry pavement and straight ahead.
03-05-2014, 04:47 PM
#20
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03-05-2014, 08:27 PM
#22
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03-05-2014, 08:35 PM
#23
Drifting
I've been wondering for some time if the current trend toward 18+ inch diameter wheels with very low profile tires and very open spoked designs are actually increasing rotational inertia, inhibiting acceleration.
Moment of inertia for a hollow cylinder: I = mr^2
Moment of inertia for a solid cylinder: I = (mr^2)/2
To prove this with my physics students we had a race. Let a tire (no rim) and same tire on a steel rim roll down a hill simultaneously.
Student thinking went something like this..... (imagine Wallace Shaw's voice from the Princess Bride movie)
Student A: "Surely the heavier steel wheel will win, for it has more mass and therefore more force of gravity acting acting on it."
Student B: "Ahh...But the tire alone has less mass and therefore less translational inertia...surely it will accelerate more quickly than the more massive steel wheel.
Student C: "Ahhh ha!...But if F=ma, then F is proportional to m and therefore both wheels will naturally accelerate at the same rate!" (this kid's thinking is partially correct, and it 100% correct IF things aren't rotating....thus ignoring air resistance, a mouse and an elephant, if dropped from the same height, will hit the ground at the same time....gravity produces a force proportional to an objects mass, so everything in free fall falls at 9.8 m/s^2, regardless of mass).
Student D: "Bwaahhh...The mass does not matter you fools. It's the distribution of the mass that matters. Since the mass of an unmounted tire is concentrated almost entirely at the outside edge, it will have close to TWICE the rotational inertia of the same tire mounted on a solid steel wheel!" The steel wheel and tire will definitely win!!
Student D is correct. The solid wheels, regardless of mass or material, do indeed always win the rolling down the hill race. I've also noticed dragsters often have solid aluminum disc wheels....good physics.
Now put those manhole covers back on!
Moment of inertia for a hollow cylinder: I = mr^2
Moment of inertia for a solid cylinder: I = (mr^2)/2
To prove this with my physics students we had a race. Let a tire (no rim) and same tire on a steel rim roll down a hill simultaneously.
Student thinking went something like this..... (imagine Wallace Shaw's voice from the Princess Bride movie)
Student A: "Surely the heavier steel wheel will win, for it has more mass and therefore more force of gravity acting acting on it."
Student B: "Ahh...But the tire alone has less mass and therefore less translational inertia...surely it will accelerate more quickly than the more massive steel wheel.
Student C: "Ahhh ha!...But if F=ma, then F is proportional to m and therefore both wheels will naturally accelerate at the same rate!" (this kid's thinking is partially correct, and it 100% correct IF things aren't rotating....thus ignoring air resistance, a mouse and an elephant, if dropped from the same height, will hit the ground at the same time....gravity produces a force proportional to an objects mass, so everything in free fall falls at 9.8 m/s^2, regardless of mass).
Student D: "Bwaahhh...The mass does not matter you fools. It's the distribution of the mass that matters. Since the mass of an unmounted tire is concentrated almost entirely at the outside edge, it will have close to TWICE the rotational inertia of the same tire mounted on a solid steel wheel!" The steel wheel and tire will definitely win!!
Student D is correct. The solid wheels, regardless of mass or material, do indeed always win the rolling down the hill race. I've also noticed dragsters often have solid aluminum disc wheels....good physics.
Now put those manhole covers back on!
03-05-2014, 08:38 PM
#24
Drifting
^^ They both look so cool.(123 Quattro & Tony's)
Been talking with Jim at True Design Wheel about widening my rear phondials.
http://forums.pelicanparts.com/porsc...ification.html
Been talking with Jim at True Design Wheel about widening my rear phondials.
http://forums.pelicanparts.com/porsc...ification.html
03-05-2014, 09:40 PM
#25
Nordschleife Master
03-05-2014, 09:43 PM
#26
Addict extrordinare
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I've been wondering for some time if the current trend toward 18+ inch diameter wheels with very low profile tires and very open spoked designs are actually increasing rotational inertia, inhibiting acceleration.
Moment of inertia for a hollow cylinder: I = mr^2
Moment of inertia for a solid cylinder: I = (mr^2)/2
To prove this with my physics students we had a race. Let a tire (no rim) and same tire on a steel rim roll down a hill simultaneously.
Student thinking went something like this..... (imagine Wallace Shaw's voice from the Princess Bride movie)
Student A: "Surely the heavier steel wheel will win, for it has more mass and therefore more force of gravity acting acting on it."
Student B: "Ahh...But the tire alone has less mass and therefore less translational inertia...surely it will accelerate more quickly than the more massive steel wheel.
Student C: "Ahhh ha!...But if F=ma, then F is proportional to m and therefore both wheels will naturally accelerate at the same rate!" (this kid's thinking is partially correct, and it 100% correct IF things aren't rotating....thus ignoring air resistance, a mouse and an elephant, if dropped from the same height, will hit the ground at the same time....gravity produces a force proportional to an objects mass, so everything in free fall falls at 9.8 m/s^2, regardless of mass).
Student D: "Bwaahhh...The mass does not matter you fools. It's the distribution of the mass that matters. Since the mass of an unmounted tire is concentrated almost entirely at the outside edge, it will have close to TWICE the rotational inertia of the same tire mounted on a solid steel wheel!" The steel wheel and tire will definitely win!!
Student D is correct. The solid wheels, regardless of mass or material, do indeed always win the rolling down the hill race. I've also noticed dragsters often have solid aluminum disc wheels....good physics.
Now put those manhole covers back on!
Moment of inertia for a hollow cylinder: I = mr^2
Moment of inertia for a solid cylinder: I = (mr^2)/2
To prove this with my physics students we had a race. Let a tire (no rim) and same tire on a steel rim roll down a hill simultaneously.
Student thinking went something like this..... (imagine Wallace Shaw's voice from the Princess Bride movie)
Student A: "Surely the heavier steel wheel will win, for it has more mass and therefore more force of gravity acting acting on it."
Student B: "Ahh...But the tire alone has less mass and therefore less translational inertia...surely it will accelerate more quickly than the more massive steel wheel.
Student C: "Ahhh ha!...But if F=ma, then F is proportional to m and therefore both wheels will naturally accelerate at the same rate!" (this kid's thinking is partially correct, and it 100% correct IF things aren't rotating....thus ignoring air resistance, a mouse and an elephant, if dropped from the same height, will hit the ground at the same time....gravity produces a force proportional to an objects mass, so everything in free fall falls at 9.8 m/s^2, regardless of mass).
Student D: "Bwaahhh...The mass does not matter you fools. It's the distribution of the mass that matters. Since the mass of an unmounted tire is concentrated almost entirely at the outside edge, it will have close to TWICE the rotational inertia of the same tire mounted on a solid steel wheel!" The steel wheel and tire will definitely win!!
Student D is correct. The solid wheels, regardless of mass or material, do indeed always win the rolling down the hill race. I've also noticed dragsters often have solid aluminum disc wheels....good physics.
Now put those manhole covers back on!
03-05-2014, 09:51 PM
#27
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good quesitons.
Ive also found a limit of benefit. it seems with the 3000lb 928 racer, that the 315 is the limit of what works, vs the 335, being too wide to get the max use out of. we can see this in tread wear paterns where the inner edge is not even being used (a full 1" of it) vs the outer edge is worn right to the proper spot. taking out camber and the edges are not used on both sides, without major deflating, then the centers of the tires stop getting heat. SO, there is an optimum size for the type, weight and duty of the car in question.
wider is usually better, but to a point.
Ive also found a limit of benefit. it seems with the 3000lb 928 racer, that the 315 is the limit of what works, vs the 335, being too wide to get the max use out of. we can see this in tread wear paterns where the inner edge is not even being used (a full 1" of it) vs the outer edge is worn right to the proper spot. taking out camber and the edges are not used on both sides, without major deflating, then the centers of the tires stop getting heat. SO, there is an optimum size for the type, weight and duty of the car in question.
wider is usually better, but to a point.
03-05-2014, 09:53 PM
#28
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^^ They both look so cool.(123 Quattro & Tony's)
Been talking with Jim at True Design Wheel about widening my rear phondials.
http://forums.pelicanparts.com/porsc...ification.html
Been talking with Jim at True Design Wheel about widening my rear phondials.
http://forums.pelicanparts.com/porsc...ification.html
03-05-2014, 10:32 PM
#29
Burning Brakes
Lower rotational inertia would be better for them, although strength and aerodynamics are going to be major concerns.
03-05-2014, 11:55 PM
#30
Rennlist Member
Most Porsches were designed with staggered sizes. So perhaps you may be adding over-steer by going big in the front? I know a lot of 951 racers like square setups. But in a 928, I think I recall the suspension geometry in the front utilizes a negative scrub radius that is very sensitive to too much positive offset. Going to a fatter rim cheaply, usually mans you are going down that path.
But then, I am a newb.
But then, I am a newb.