I caught a bit of Sports Car Revolution on Speed Channel last night. They had just put a new cat-back on a project RSX and were surprised to see that on the chassis dyno they were down 7hp from the stock benchmark! Mystified, they put the stock exhaust back on, and found they were down 12hp (at least the cat-back got them 5hp back).

They did some checking with suppliers and came to the conclusion that the difference came down to the bigger brake rotors they had added earlier. Even though the rotors weighed less than the stock pieces, their larger diameter meant more inertia for the engine to over come. Since they were measuring hp at the wheels, they found the car was actually putting less power to the ground. Add in the increased resistence of wider, stickier tires and you get the 12hp loss. The big BUT - the bigger brakes/tires worked better at the track, so the modified car was clearly quicker than stock when it was put to the test.

I thought that was quite interesting.

 

The same sort of thing happens when you go to larger wheels too. Even though the wheels may be lighter, their is more inertia to overcome and stopping distance and hp suffers.

This is especially bad on the, already poor handling, SUV's who get 20' rims. I saw a report on TV about all the wrecks that are getting blamed on 'dubs' I suppose it doesn't help that the people who get SUV's and 20' wheels are not the sharpest tacks in the box to start with.

 

Spin baby spin. Lets see those wheels spin. I love when they jump out at a light run around the car to each wheel to make them spin.

 

[font=Verdana]From Mini Mania, here is the Formula for determining the impact of a rotating engine component, such as a flywheel. I am not sure how it would apply to a non-engine component, such as a brake disk, but the formula for a classic Mini flywheel works out to be the approximate equivalent of 11.5 to 1, meaning that every pound removed from the flywheel is equal to removing 11.5 pounds of static weight from the inside of the car. Weight taken off the farthest gyration, or “edge” of the rotating mass is most effective. [/font]

[font=Verdana] [/font]

[font=Verdana]{(0.5 x n[/font][font=Times New Roman]²[/font][font=Verdana] x r[/font][font=Times New Roman]²[/font][font=Verdana] + R[/font][font=Times New Roman]²[/font][font=Verdana])} / (R[/font][font=Times New Roman]²[/font][font=Verdana]) [/font]

[size=3][font=Times New Roman] [/font][/size]

[font=Times New Roman][size=3]n = TOTAL gear ratio (gear ratio x diff ratio)[/size][/font]

[font=Times New Roman][size=3]r = Radius of gyration [/size][/font]

[font='Times New Roman']R = Radius of wheel/tire used[/font]

 

i started a thread on this a while ago, try a search for more info

 

Damn, I'd love to see a SUV with twenty foot rims!!!

 

I'm not sure I buy the case that larger rotors will reduce whp on dyno. I agree that more mass (rotors, wheels, tries, etc.) will affect the time to reach a given max speed and time for braking but, once the level is reached, the whp should be the same regardless. By that I mean, if you have 25 lb. S-Lites or 16 lb. Kosie, max whp should be the same once the rpm is reached.

I would think that the more drag created by larger brake pads would be more likely to affect whp.

Now, let me say that I may be wrong ... but thats my .02 and I like to hear what more knowledgeable folks have to say

 

I can attest to the truth of larger parts requiring more power to turn them.

I put a set of 18" wheels and tires on my shop truck. Not only did this slightly alter the gearing due to the 3/4"d difference but the added weight (35lbs PER) made a huge change in how it accelerated and to an extend decelerated.

With brakes the same thing hold true. Larger heavy rotors will require more power to spin up to speed. And when spinning will have an impact on turning too. Huh? Yup, it's a gyro effect of spinning the larger rotors. They don't like to turn as easily.

So, what do you do?? Offer various rotor options which may limit this problem balanced to the needs of the particular application. Huh? Hard track use where you may indeed benefit from 17lb rotors and their thermal capacity are proven to be gross overkill for most street users where the same brake benefits can be exploited from narrower rotors of perhaps 12lbs.

You'll note that this is the basis of not only my new MINI kits but many of the other products I produce. In short; a wider, fatter, heavier rotor will NOT stop your car any better. In the long run at high track temps they will offer improved cooling and thermal capacity however.

I can't speak for the specifics of loss % of bigger brakes as we'd need to do some serious calcs to define the moment of inertia given the mass location, dia, etc etc, but yes it's a physics thing and true.

(BTW; I pulled the 18s off the truck after one road trip pulling the race car trailer, about 1600 miles and put the stock 16s back on. The 18s are for sale)

 

I just caught that same show this morning and agree their find was very interesting. However, they did go on to say that in the hands of a good driver the less horsepower would not make any difference on the track. On the track the farther you can go faster, the faster your lap times will be and there is only one component that allows you to drive faster longer and that's a great set of brakes!:smile: