Brake Physics 101:

What you gain with larger rotors is more leverage. Think of trying to turn a bolt with a short wrench, then turning the same bolt with a longer wrench. The longer wrench gives you more leverage, increasing the amount of torque (in lb. ft.) for the same amount of force. That is why we reach for a "cheater pipe" sometimes to increase the leverage to get off a stuck bolt.

How does this apply to braking? The amount of force you put into the rotor doesn't change if you don't change the caliper (with bigger pads). But the distance from the pad to the center of the hub increases. Therefore, the amount of leverage you get is better, thus braking power is increased. That is why larger rotors work better. Plus, as has been mentioned, they probably absorb and dissipate heat better due to more surface area (after all, a brake is a device to convert energy to heat - you reduce the forward energy of the vehicle and convert it to bake heat).

Now, if you add bigger calipers you also add bigger pads (bigger calipers only exist to get more brake swept area). If you somehow went to bigger calipers that used stock pads (or even pads that had the same height even if they had more length) the swept area would not increase, so braking would not increase. That is why bigger calipers are also "deeper" and have taller pads. This increases swept area and braking (with more pad area, you need more clamping force, thus the move to multiple pistons). The added pistons increase the amount of clamping force for a given amount of pedal pressure (at the expense of more travel at the pedal for the same travel in the pads).

I really like this conversion. I would love to see it with Corvette calipers too. I wish I lived close so we could see if my stock SS 18" wheels would fit over the conversion! One possibility - add a spacer (not my favorite approach) to move the stock wheel out a bit. Spacers come in a lot of thicknesses.

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Thanks for the explanation and the good words, Jay. We should have the Corvette calipers done soon. When Dan came by the shop the other day when I was there, he was in his SS. I can tell you that there is going to be tons of room in stock SS wheels! I still want to get someone with stock LT wheels to see how they fit, but my aftermarket wheels are the extreme. These will easily fit any stock wheel. A couple of the guys that are interested drive SSs and live in SoCal. Hopefully one of them will make the upgrade soon so you can see how it fits.

 

I can understand this question because I am building a Hot Rod.

With more braking in the front (understood) Was there anything done to correct the front/rear braking ratio that may have changed with this larger brake?

Example: The ratio may have been 60/40 with the stock caliper and with a different size caliper may of changed to be 70/30.

 

For just changing rotors as we have done so far, nothing is needed. The braking percentage front to rear is still the same. The benefit comes from heat loss, and leverage as LewiSS suggested. From where I am sitting in the driver's seat I don't think any changes are going to be needed. We are not trying to add more pressure to the front brakes, we are trying to get them to do more with the same amount of pressure applied.

Now from what I have read here on the forum, the HHR proportioning valve does a pretty good job of handling brake changes that are even more severe than what we have done. The few people who have updated to rear discs have said that there was no need for them to change anything and the existing HHR valve handled it. We haven't done the Corvette calipers yet but I am sure Chris will take this into account when he puts it together... and then again also when we do the rear discs.

 

Songman, not to take issue, but if the front rotors are producing more braking force at the tires with the same amount of pedal pressure then the brake balance has been moved forward (that is, the fronts are doing a little bit more than the old ones compared to the back with the same amount of brake pedal force). If you take this too far you will reach a point where the fronts will lock much easier/quicker than the backs. Now, as you've noted the brake proportioning valve takes care of some of this. Also, the anti-lock will take care of some - it will just have to kick in a bit earlier on the fronts if you press the pedal hard enough to drive the tires to sliding.

None of this is near as bad as having the rears lock first. In that instance, the car really likes to swap ends. You want the balance to be forward to counter this anyway. How much so? If the fronts are going into lock before the rears are even near lock, you are wasting some of the braking force the rears can contribute.

That said, whether it is a car or a motorcycle, a very high percentage of the braking comes from the front, while only a small percentage comes from the back (due to weight transfer and other issues). Motorcycle riders realize this - the rear brakes are more for balance. Trying to stop a bike with only the rear brake is scary!

 

I think we are saying the same thing but you are better at it than I am. The proportioning valve will take car of the "too much brake power" problem for the minor modifications we are making now. Depending on how far we go with this, Chris will have to take into account that other parts might also have to be updated. But for what we have now, and for anything that I see that we have planned, I think the stock proportioning valve can handle it.

Since you seem to have a good handle on how brakes work, I would like your take on the results people are getting from adding disc brakes to the rear but still maintaining the stock proportioning valve. My friend Chris is the expert on all this stuff, but it seems to me that if people are maintaining the stock proportioning valve when adding Cobalt rear discs to the HHR that on any of our planned upgrades the stock valve would be fine. I know it is fine now with just the larger rotors with stock calipers in the front and drums in the rear. It would appear, from the experience of others, that if we put large rotors and stock calipers on the rear that it would also be fine. Now let's say we bump both front and rear up to Corvette calipers, it would seem that all percentages of force would still stay the same. Chris will know all of this stuff but I won't be talking to him until the weekend. What is your take on it?

 

I agree - we're on the same page on this.

If the amount of braking force with the drums happens to be about the same as with the Cobalt (or HHR SS) rear discs, then there would be little change in brake balance with the change from drums to disks. So many variables are involved it is harder to calculate than to just do it and see (size of the cylinders, mechanical advantages, swept areas). There are formulas to do all this, I just don't happen to have the books to look it up any more. One question might be: is the proportioning valve for the drum-brake HHRs the same as for the HHR SS? Easy thing to look up. If so, they're about the same in braking force per amount of pedal applied. If not, you would want to go to the SS valve at the least.

I now have a source for used 'Vette calipers for my own use. Which 'Vette are you going to use as a donor when you design this set-up? Is there a reason you used Viper rotors instead of 'Vette rotors (both have to be re-drilled anyway)? When will I be able to buy a set of rotors and the adapters for 'Vette calipers? I will probasbly stick with my stock SS rears as long as the balance is not an issue.

 

We tried both the Vette and the Viper. Chris uses both depending on what kind of car it is going on. For the HHR we used the Viper because it has a different offset than the Vette. The Vette would have moved the caliper too far forward and it would have hit the wheels. Of course, this is with the stock HHR calipers. The Vette calipers are not as thick so the Vette rotors might just work with Vette calipers. Another thing is that with the Vette rotors, the friction part of the rotor was too wide. There was about 1/4 inch near the inside that wasn't being used. The Viper rotors fit the HHR calipers perfectly.

Hopefully we can get back on the Vette calipers next week. We didn't start anything fresh because we have the Coast Run this weekend and I am taking the HHR so we just wanted to show off what we have. We have already done the measurements and programmed the CNC stuff for the Vette calipers. We just had to move one hole slightly from the HHR/Viper mount. After we get the Run under our belt we'll start again. Hopefully by then someone will tell me how Chris can sign up as a Supporting Vendor so we can tell people that might be interested how to buy these kits.

 

Maybe someone else can chime in. No online auto parts source I can find lists a proportioning valve for any HHR after 2008. Everything listed up till then is all the same.

 

I just spoke to Chris on the phone about the Corvette caliper upgrade that we are doing next. He says it uses C4 calipers. The C5 calipers are much larger and would be difficult to fit while the C4 calipers are near perfect. Next week we wil start on the Corvette calipers and the adjustable front coilovers that we started a couple of years ago but never finished. I ran these for a while trying to dial them in but this was back when his shop was still in L.A. and it was just too far for me to drive.

Also, I found out the kit is going to be cheaper than I thought too. The kit with two Viper rotors drilled to fit HHRs, the custom made mounts with new bolts, and concentric rings is going to go for $375. Everything you need to do the upgrade is included. If you want or need better pads, those are not included and you would need to source those. Chris can probably add the Adaptive One pads like I used for an additional cost. So you can do this upgrade for just $75 more than slotted and drilled rotors. Plus, he said he will do a group buy deal where if he gets 10 buyers he will knock 10% off. Pretty good upgrade for $337.50! Just $38 more than slotted and drilled rotors.

Just so there is no confusion, I am not in business with Chris in any way and do not gain from any of his business. My only connection with him is that he does custom stuff for me and then mass markets it for other people. We hope to eventually find out how he can become a Supporting Vendor so these can be listed for all to see. Otherwise I guess people can just keep PMing me if you are interested or need more info.