Wheels, Tires, and Brakes a size issue

[Dan-MI]

Ok, a friend of mine is being told by various tire and brake places that you cannot run standard disk brakes with oversized wheels and tires. I know from reading posts that a number of you have 15 inch instead of the stock 13s and have stock brakes. Is there a limit to how much larger the wheel size can get before you need to put on bigger rotors or is that just shops trying to drum up business? What are the drawbacks to running stock rotors with larger wheels, if any. This is a question not necessarily limited to Opels. Any thoughts would be appreciated.

[opelwasp]

I'm calling

The stock GT tire diameter is 23.394"

The diameter of 205/50R15 is 23.071"

I fail to see a problem.

Now if you are trying to fit agreegiously large wheels, I guess I could see a problem.

[hrcollinsjr]

The greater the reciprocating mass, the harder it is to stop. Usually the combination is heavier and the greater mass is possibly further away from the center creating even more problems even if the weight is the same.

If I'm incorrect in any part of my explanation I'm sure we'll get the correct answer shortly.

Harold

[namba209]

You're in the ballpark Harold, but it's rotating mass and unsprung weight, not reciprocating mass. The more mass, the more pressure required to slow down and stop it, which translates to faster wear on the caliper pads and disk. As long as you use 13" wheels the stock brakes are kinda sorta sufficient. When you go with larger wheels, then it stands to reason to go with the largest disk and caliper setup that will fit inside the wheels.

[opelwasp]

So if I use stock steel rims and 165/80R13's that weigh about 40 lbs, I'm ok.

But if I put on aluminum rims and 205/50R15's that weigh about 25 lbs that's bad?

I fail to see how. Lighter wheels with the same diameter. Reciprocating mass is a moot point.

[namba209]

Chris, the lighter rotating mass takes less effort to slow down and stop, so the brakes will use less pressure on the brake pedal than heavier wheels. It's the same with a lightened flywheel, the engine will rev up and down faster than a stock one, so less energy is expended to do the deed. Hope this makes sense.

[opelwasp]

So the larger lighter aluminum wheels will be fine then, but larger heavier steel ones will be bad. Right?

[OriginalOpelGTOwner]

Rotating mass is only part of the equation. The rest of the equation is torque in foot/pounds. Is same as using a breaker bar and socket on a bolt. The longer the breaker bar the more torque on the bolt per unit force in pounds. So, if a new tire and wheel assembly has a larger diameter (think of this in terms of the breaker bar length) than the stock tire and wheel assembly the stock brake rotor and caliper will have to stop more torque per unit of forward momentum of the car (think of this in terms of pounds force). If the stock brake assembly cannot handle the increased torque than larger brake rotors/calipers/pads are required.

[hrcollinsjr]

Originally Posted by namba209 You're in the ballpark Harold, but it's rotating mass and unsprung weight, not reciprocating mass.

Ron, Senior moment. Reciprocating mass is up and down.

Thanks,
Harold

[hrcollinsjr]

Originally Posted by opelwasp So if I use stock steel rims and 165/80R13's that weigh about 40 lbs, I'm ok. But if I put on aluminum rims and 205/50R15's that weigh about 25 lbs that's bad? I fail to see how. Lighter wheels with the same diameter. Reciprocating mass is a moot point.

First attempt to explain disappeared.

If the wheel is lighter that's great. If the new wheel/tire combination weighs the same but moves more of the weight outward that's not so great. The further from the center the weight is the more effort that is required to change the speed, this includes acceleration as well as deceleration.

When a racing class sets a minimum weight for a flywheel, racers try to take most of the weight off of the outermost part for the greatest effect.

Harold

[heimue]

All the talk about the rotating mass and inertia of a tire can most probably be dropped, compared with the inertia and energy of the car itself. Nobody would ask to put bigger brakes on because the driver is overweight...
I'll do a calculation sometime later (its 7:45am and I have to go to work) and publish it here.

Dieter

[hrcollinsjr]

Originally Posted by heimue Nobody would ask to put bigger brakes on because the driver is overweight... Dieter

Big person in a little car definitely effects handling and alignment. Weren't some of the Indycar drivers complaining about Danica Patrick because if the cars were equal her weight was enough to give her an advantage?

A friend of mine knew someone who just switched to lighter wheels of the same size. The car suddenly developed the ability to smoke the tires, the same ones that were on the car before the wheel change. Not an Opel!

Harold

[GTJIM]

Braking is all about HEAT - the brakes turn the inertial energy ('forward motion') of the car into heat to dissipate the energy released as the car slows.
'Bigger' tyres (wider, with modern, improved rubber compounds) give better grip on the road - therefore faster braking with quicker slowdown ... and more concentrated heat generation is possible. The front discs have to handle most of this so that is why larger brakes are recommended.

Not a great problem at 60 mph (i.e. under the open road speed limit!) but certainly a consideration if ever slowing down from 100 mph plus.

The original discs were very advanced brakes for their day and much better than the drums fitted to most cars back then. For 'ordinary' daily driving they are still adequate - but more spirited driving, with wider modern compound tyres and the greater repeated use of the brakes can certainly benefit from larger (ventilated!) front discs with greater heat capacity.

[jordan]

The 205/45/16 yokohama avs es 100 tires I have weigh 21 pounds a piece which is a lot heavier than the stock bias ply tire....right where it is the worst place to have the weight circumferential unsprung mass.

[traytodd]

I just weighed my 74 Manta spare at 30 lbs. It is the stock rim with a new 185 70R13 Michelin X tire.

Jim is basically correct. The energy that must be stopped is the kinetic energy of the car's forward velocity and the angular energy stored in the rotating components, mostly the tires. The reciprocating mass has very little to do with as the engine is either disconnected by depressing the clutch or adding to the braking force by its own friction and drag. The kinetic energy fo the car is 0.5*mass*velocity squared. The kinetic energy of the wheels is 4*0.5*moment of inertia*rotational velocity of the wheel squared. At 60 mph for a 1000 kg vehicle (~2200 lbs) the kinetic energy si about 360,000 joules. My guess is the rotational energy in the wheels is only a fraction of the energy of the car. All this energy must be converted to heat by the brakes.

Replacing the wheels with a larger diameter rim but the same OD tire, will have little or no effect on stopping except for the change in the moment of inertia of the tire/wheel combination. (The moment of inertia of a barrel hoop is its mass*radius squared.) A smaller moment of intertia will result in faster stopping ability. Conversely a larger moment of inertia will increase the stopping distance. Larger tire ODs follow the same rules but are more likely to increase the moment of inertia due to the increase in diameter.

Since the brakes are a finite size, they can generate only so much stopping force or torque. The maximum effective stopping force is determined by the friction of the tire on the road. Once sliding motion (skidding) occurs, the stopping force decreases. Newer, wider tires with a larger contact patches and softer compounds will be able to apply a greater stopping force than the stock 165 80R13s. In these cases, larger diameter rotors and larger area pads will provide more stopping ability since the stock brakes are close to maxing out. I find it difficult to lock up my brakes running 185 70R13s. But then I was taught never to lock them up even in an emergency. (Those who have driven in snow and ice know this very well.) I do think the stock brakes on the GT and Mantas could be improved.

I hope this explanation is clear.

Terry