Deceleration Rate Measurements: OEM vs. 40mm Rear Brake Calipers (Negative "G's")

[Tom, F&L GoR]

Lots of folks have asked about performance measurements of the larger rear brake calipers – I finally have some. This customer car is a stock 1994 with 2-year old Michelin Pilot Sports, OEM Brembo brake pads front and rear, and about 28,000 miles on the odometer. Tests were conducted with driver and passenger on a little used, well shaded public road (so road surface temperature change is minimal) in both directions (to account for wind and slope.) Other than different rear calipers and about 2 hours time, there was no other difference with the car before and after these tests.

Because it was a public road, the measurements I paid attention to were only the deceleration rates, not how many feet between 60 MPH and stop. I didn’t want to mark the road, measure distances or be locked into exact positions. (“No, officer, I’m not going fast, I’m measuring how I slow!”) As you’ll see, using “G” forces still gives a reasonable picture.

The question of “yeah, but how hard were you trying?” can be answered looking at the sudden upward slope in any figure. At this point one of the front tires was finally locking up, slipping, resulting in a poorer vehicle deceleration rate. As the data shows, I am not very good at braking, but by being a poor example, it shows where the room for improvement lies.

This chart shows three of the cleaner runs (stops?) with the OEM brake caliper. It was reasonably easy to sustain 0.80 G’s deceleration, with good feel it looks like one could drop to 0.85 G’s, but touching 0.90 G’s braking results in a tire locking up. Since it wasn’t my car, any time a tire locked I would back way off the brake, and as a result there would be a slight upward spike due to slippage followed by a large upward spike due to release of pedal.



The second chart shows three of the cleaner (!!) events with the 40mm rear caliper installed. Yes, I suck, and now you all know, too. Deceleration of about 1.0 G’s is sustainable, around 1.1 G’s a front tire locks up.


The following chart averaged the three before and three after runs together. With all stops included, it would say the rear caliper change improves braking from 0.8 G’s to 1.0 G’s, with better results possible with a different driver. Certainly stickier tires will also improve the results.



All runs were with the proportioning valve still in place, as per customer request. Most runs I did not come to a complete stop. Feel free to dispute the absolute level of the results, you can tell me that it should have been 0.7 for OEM brakes and 0.9 for the 40mm rears, results will vary with tires, brake pads, and road surface, etc. Bigger images are in my picture gallery (sorry the axis labels turned out so small.) But as far as documenting relative performance, it’s an obvious, measurable, noticeable difference in braking. If you’re near Chuck Tator’s let me know and perhaps we can suction cup this accelerometer to your windshield and try it again.

Questions, comments, or better explanations, please let me know.

[PhoenixGTS]

No wonder I can freak passengers out with hard braking. With clean Phoenix roads, the proportioning valve disabled and stickier tires I might be doing 1.2 Gs or more. Great way to test since distances would not explain much about feel.

[Ron]

Great charts Tom, Thanks! Is there a correlation to stopping distance, i.e. 25% higher G force ~ 25% shorter stopping distance?

Thanks again.....

[Tom, F&L GoR]

Stopping Distance = (original velocity^2)/(2 * deceleration rate)

velocity in MPH times 1.4666 gives velocity in ft/sec
deceleration rate of 1 G = 32.2 ft/sec^2

example for 0.8, 1.0 and 1.2 G
distance from 60 MPH = (1.4666*60)^2/(2*32.2*.8) = 150ft
distance from 60 MPH = (1.4666*60)^2/(2*32.2*1) = 120ft
distance from 60 MPH = (1.4666*60)^2/(2*32.2*1.2) = 100ft


Formula courtesy of Stoptech:
http://www.stoptech.com/technical/Th...%20Systems.pdf

[Ron]

Wow, I'd say that's money well spent! Thanks......

[Steve-Indy]

Nice job, Tom !!!

[GR8_ASP]

Great information Tom. The braking is a small amount less than a stock SRT, which is by no means a small accomplishment. Here is a graph of the SRT-10 braking:

[Tom, F&L GoR]

Gotta pull the prop valve on that car....

Thanks, Ron. Great to compare "real" data.

[ROCKET62]

Gotta pull the prop valve on that car....

Thanks, Ron. Great to compare "real" data.

Looks very interesting. What are the pros and cons to removing the prop valve? What is involved to remove the prop valve?

Thanks.

[PhoenixGTS]

What are the pros and cons to removing the prop valve? What is involved to remove the prop valve?

Depending on your tires and car, removing the prop valve has the theoretical effect of allowing the rears to lock up before the fronts which could cause the rear to come around and would not be safe. Not an issue on my car as I have street compound fronts and sticky rear tires (and used to have tall 19" rears). You have to test once you have it out to see on your own car.

You can either remove the spring or the seal to disable it. I took the seal out. In any event you have to "open" the rear of the system to do this and must re-bleed afterward.

[Tom, F&L GoR]

As part of the Federal Motor Vehicle Safety Standards, all cars in the US are required to have a proportioning valve. The prop valve is actually a pressure limiting valve designed to prevent brake line pressure in the rear from continuing to build at the same rate as the front in maximum braking events. The idea is that with more weight shifting forward in a braking event, the rear end gets light and cannot accept as much braking; therefore the rear might lock up and the vehicle loses direction stability. All this is what PhoenixGTS just said.

The "problem" with Vipers is that the brake balance of four piston Brembos up front and one-piston Brembos (same brake design as a 1988-92 Renault Alliance) in back was never close enough to cause rear wheel lock even without the pressure limiting valve!

The change to 40mm rear brakes a large difference, part of which is eliminating that predictable and terminal plowing feeling. Because it is "different" I recommend that folks first leave the proportioning valve in until they get used to this, then experiment with high braking and the valve disabled. The slight increase in rear brakes gets the front-rear balance that much closer, will again change the "feel" of the car, and if the driver likes how it "feels", can perhaps slow down quicker. While I know many have removed it, myself included, the difference between the prop valve in or out is a driver preference issue; different racer types have it in or out and different street drivers have it in or out. In ~three years (wow), I've not heard anyone that did disable the prop valve go back and reinstall the spring or seal.

[GTS Bruce]

Interesting formula.The 40mm rear caliper on early gen 2 would make the brakes just about as effective as the 01-02 with abs. GTS Bruce

[Andrew/USPWR]

That accelerometer is a very cool devise. How much are they?
I’d love to see some charts of 0-100-0 on some Vipers. Stock and otherwise.

And thanks for the info

[ruckdr]

All,
As I said before, the difference in braking is breathtaking (see link below), now Tom has put numbers to what we experience in the car with the 40mm brakes.
If you don't have them - just do it , you will be pleasantly surprised, and have a car that WILL really stop.
As the numbers show, the stock brakes need about 150 feet to stop, which is not poor, it is aweful; and the new rear brakes provide superb braking.
(I am still looking for a way to post the video file-a Windows Media Audio/video file) Any Ideas??

http://vca2.viperclub.org/forums/sho...rue#Post557485

Later

[ROCKET62]

Tom, thanks for the info and advice. If/when I change out the rear calipers - it would seem to make sense to change rotors and pads as well as mine are now 13 years old and have almost 20,000 miles on them. Any recommendations? Would like to minimize dust without sacrificing performance if that is possible. Street driving only with Pilot Sports.

[AviP]

Wonderful data Tom. Great job as usual. I will buy your rears eventually when I move to a house, probably this winter. Apartment living sucks.

[Cop Magnet]

Thanks for taking the time to collect and put some objective data up! I have an ABS car so not applicable, but very interesting nonetheless. What were the differences in the rear brake pads on these runs (type, thickness, etc) beyond the surface area difference?

[Tom, F&L GoR]

for GTS Bruce:
Two years at the Rendevous I was ~3 seconds quicker in my Gen 1 than the yellow SRTs. A lot of the reason was the stiffer suspension I have, but I certainly didn't feel my brakes were a handicap. However, I have to brake in a straight line; with ABS you can jam on it anytime you want. That's cool.

for Andrew:
http://www.caranddriver.com/article....;page_number=1
I used the VC3000.

for ruckdr:
Your commission check is in the mail.

for Rocket62:
for low dust I've only heard of the EBC Greens. For me, when they first came out, they didn't seem to "bite" as quickly as I wanted for Autocross, so I went to dusty trusty Brakeman#3s. But for street use, Greens are fine.

for AviP:
It'll take you two hours to swap them in a friends driveway... or at Tator's - he's installed a few already. Including his own car.

for Cop Magnet:
None. The modified brakes are the same casting as the OEM calipers but with the bore enlarged to 40mm. The piston is sleeved, as done in automotive restoration work, out to 40mm. Consequently the hydraulic force is more than 20% greater, but pushes on the same brake pad. That's an advantage - use all your existing stuff, no surprise extra costs.

[Cal Cobra]

Great info guys, thanks a lot for taking the time to post it and break it down into a perspective we can all appreciate.

Just another great example of why I bought a Viper after lurking/learning on the VCA forums for the better part of a year!

Cal

[ViperRay]

Great data Tom!

It's nice to be able to quantitate the "seat of the pants" feeling those of us who have done this mod have felt.

I'd like to see how it does without the prop valve like in my setup... also vs. the 4-wheel Stoptechs if that opportunity ever arises.

[GTS Dean]

I dealt with flatspotted fronts for years. I disabled the proportioning section of the combination valve years ago, which helped quite a bit. I run StopTech slots up front (on the track) with OEM rotors out back. I put off doing Tom's 40mm kit for a long time, but I'm here to tell you that they work FABULOUSLY. They will not stop front lockup, but they will *greatly* reduce it, along with your stopping distance.

Such a deal!

[Joseph Dell]

What he said. I'd been putting off doing tom's rear brake upgrade. I too have the stoptech Front kit.

As lilo and stich put it: do it. Do it now. Do it.

Actually, it was a different dynamic duo. But do it. It is the best cheap mod you can do to a genII... And the benefits include safety.

JD

[Joseph Dell]

Hey Tom - please post the ratio/graphs that you did for the Stoptech vs. OEM calculation. I bet everyone would like to see that!

[Tom, F&L GoR]

Joseph is referring to a diagram I plaguarized from Stoptech's site because it did such a good job explaining the idea - otherwise it is explained with a jumble of numbers.

Their version was this:


Joseph has a somewhat unique combination of the Stoptech front kit (larger rotor, caliper spaced out, but smaller pistons) and it was helpful to demonstrate where I thought adding 40mm rear calipers would end up. Happily, it seems to have hit a sweet spot.

My version of the diagram is here:


In this diagram, "Stoptech calipers" means only the swap of the OEM calipers for the 36/40mm Stoptech calipers. "Stoptech kit" denotes the 36/40mm piston calipers and their larger rotor (i.e. the caliper is spaced out.) The kit is theoretically not as effective as the calipers alone because spacing the caliper out gives it more leverage and moves toward front bias again. The 38mm rear caliper conversion (with OEM fronts) gives the same balance as the caliper-only. The 40mm rear point is with the OEM front caliper. Joseph has the kit on the front and I wanted to make sure adding 40mm calipers didn't move him too far to the right.

This graph wasn't entirely made up. I plugges all the numbers of piston sizes, rotor diameters, number of pistons, etc, into the formula for clamping force that Stoptech provides on their site. That let me compare the different setups. From nearly 100 customers of the 40mm calipers and a much smaller number of 38mm customers, I am very sure of the left side and center points as far as driver comfort during braking. From some very limited information from Gen 2 ABS cars that disabled the ABS, I projected the point on the right. (The ABS rear caliper is 43mm diameter and I believe if just over the very brave or hairy edge of best performance.)

[Catwood]

Here is a quick print of my brake force last week at the track at the heaviest braking zone. This is with brakeman #3 and Tom's 40mm calipers. I also have Dave's adj prop valve for the rear. I put down all the laps in an overlay, even the bad ones where I coasted through the corner.

I will do this again with a different pads soon. I'm not sure if the Brakeman #3 are my favorite. Data will tell

Carl

[Knight Viper]

Awesome write up guys, Thank you. Now I know my next mod!

[viper spray]

My Viper tech is putting them on my 97 GTS tomorrow . (Tom , the cores will be sent back in a couple days)

[Catwood]

As good as they are I think you can wring out a bit more with Racecar's Prop valve.

Dave and I are considering a side by side test using my new G2X. Tom...you going to be in San Diego any time soon to witness.

[GTS Dean]

Originally Posted by Catwood

Here is a quick print of my brake force last week at the track at the heaviest braking zone. This is with brakeman #3 and Tom's 40mm calipers. I also have Dave's adj prop valve for the rear. I put down all the laps in an overlay, even the bad ones where I coasted through the corner.

I will do this again with a different pads soon. I'm not sure if the Brakeman #3 are my favorite. Data will tell

Carl

Carl,

What are your tire sizes - 305/335-18? I'm also assuming Hoosiers?

BM #3 are OK for rears, but lack the bite and release characteristics of PFC. BM also wear LOTS faster.

Dean

[KepRght]

Tom, where would gen3 brakes fall on this chart?

http://i97.photobucket.com/albums/l2...pTechViper.jpg