brake performance

[jeffc280sl]

I have upgraded my 1970 280SL calipers and front rotors to the Sedan version. The new rotors are vented and the calipers a little bigger but they still use the brake pads as before.

I'm planning to also change out the brake proportioning valve mounted under the car near the gas tank.  I question its performance at this point given its age.  The purpose of the proportioning valve is too make sure the rear brakes do not lock up prior to the front brakes and cause an uncontrolled situation for most drivers. For safety purposes and because of weight distribution of the car the front brakes do most of the work.  As the brakes are applied the weight on the rear tires shifts forward creating a lighter rear end with more potential for early rear brake lockup.  I have located the performance spec for the factory valve in the Mercedes maintenance manual and compared it to a lever type adjustable valve made by Tilton.  The Tilton valve has 7 settings and if I've done my pressure conversion correctly, the factory unit falls right in between setting 2 and 3.  With this valve I will have the option of reducing rear brake pressure by 2 settings and increasing rear brake pressure by 5 settings over the factory proportioning valve. I chose the lever type over the knob version so that I can accurately set the new valve up very close to factory specs and drive the car at/near the same rear/front brake bias as delivered from the factory.  Over time I plan to incrementally adjust these settings and drive the car under many conditions until comfortable with brake safety.  

Other than being very very careful does the group have any thoughts to share.  I plan to keep everyone updated as to my progress.  I ordered the valve today so it will be some weeks before I have any news to report.

Jeff C.
1970 280SL 4-speed

[willsam]

Sounds like you are on the cutting edge of doing the R&D work.  It will be interesting to hear how it goes and see a pic of the new valve.  Is it threaded metric?  Good luck, Will Samples

[jeffc280sl]

The valve is available in a metric size.  

http://www.tiltonracing.com/content.php?page=list2&id=37&m=b

Jeff C.
1970 280SL 4-speed

[George Davis]

Jeff,

this sounds like an interesting project and I look forward to hearing about your results.  I'll throw a couple of thoughts in, if I may:

It will be under hard to emergency braking that the prop valve will make the most difference.  The reason I say this is that it's only under hard braking that you're likely to experience lockup, and keeping the rear brakes from locking before the fronts under all circumstances should be the goal (in my opinion).  Having the rears lock first under hard braking in any car is a good way to get sideways or backwards.  Add to that the fact that these cars can exhibit some rear-end instability under hard braking even when the brakes aren't locked, and it's especially critical to be sure the rears won't lock first.  Last, the optimal bias setting for dry/smooth surfaces may be suboptimal for wet or rough surfaces.

Good luck, and if you do play with increased rear bias, please do it in a place where getting sideways won't hurt.

George Davis
'69 280 SL Euro manual

[jeffc280sl]

George,

Thanks for the advice which I will follow.  I wonder about the effectiveness of the stock 35 year old proportioning valve.  Although I don't know for sure I imagine it is not performing to spec. If failed, I would think its failure mode to be one where the rear braking pressure becomes less rather than more.  If true the front brakes are doing more of the work of stopping the car than originally designed.  This is something owners of this version may want to think about.

Jeff C.
1970 280SL 4-speed

[Cees Klumper]

Experience from the field: I had to do an emergency-braking procedure a couple of months ago. Dry pavement, pretty high speed. The car as expected dove and the tires screeched, but it stayed very straight. So I imagine the valve did its job. I have no information as to its age, but I have not replaced it since I bought the car now 6 years ago.

Cees ("Case") Klumper in Amsterdam
'69 white 280 SL automatic

[jeffc280sl]

Thanks Cees,

Mine would do the same thing in a panic stop.  When I changed the front rotors I changed pads front and rear.  The rear pads were not worn at all.  This may be by design or the brake regulator valve may be faulty and the rear brakes are contributing little or nothing to stopping the car.  I don't know at this point.  After I install the new valve I'll run some informal tests and see if I can feel any difference.

Jeff C.
1970 280SL 4-speed

[George Davis]

Jeff,

interesting article here on prop valves.  Don't know if it applies to the MB prop valves, though.

http://www.babcox.com/editorial/bf/bf50014.htm



George Davis
'69 280 SL Euro manual

[A Dalton]

Quick test for valve function is to have a friend hold valve in his hand as you apply approx 65 lb. foot pressure to pedal at idle [so you have vac  booster in circuit]... as soon as you release the pedal pressure , friend should feel/hear a distinct knock from valve returning to normal open position.
 I do not have line pressure spec conversion for pedal pressure.

[Tom]

My brake proportioner was recently replaced.  I did not fully understand the function, but now that I do, it probably explains 2 events over the past 5 years.

The first was when my brakes locked in a turn on a freeway ramp.  I hit the brakes, they locked and I did a 180.  I did not hit them *that* hard.

The 2nd was a hard straight stop to avoid a car coming into my lane.  Again, another abrupt but not very hard stop.  Brakes locked.

Hopefully I won't be in another difficult braking situation, but I am more comfortable knowing that I have a working brake proportioner.

1971 280sl Tunis Beige Metallic (restored & enhanced)
1971 280sl Tobacco Brown (low mileage stock)
1970 280sl Deep Red (Project Car)

[jeffc280sl]

WOW Tom,

I'm glad you got to the bottom of the problem before you had a serious accident.  This happening twice in 5 years must have made you think the car was possessed in some way.  As I look at the design in the BBB I see it is a spring loaded piston type pressure valve.  If the val;ve were to fail in the open mode, excessive brake pressure would be forced to the rear brakes causing the lockup. Given the serious application I would have thought by design any failure mode would have resulted in a further reduction in pressure to the rear brakes not ever an increase. This is why I thought at first a failure in the regulator valve would have been a degredation over time of a seal or something which reduced rear brake pressure to the point where the rear brakes were ineffective.  I understand from others the failures are few, but if there is a failure it can be unexpected and severe.  How did you get your rear brakes to recover from the lockup?  Your situation while rare is scary.  Thanks for sharing your experience so that the group can learn from it.

Jeff C.
1970 280SL 4-speed

[Tom]

To answer the question on how I recovered from the lock-ups, I really did not recover from the first one.  I did a 180 on the ramp and the car stopped.  While I say a "180", maybe it was really a "90", but in any event, the car spun around and came to a stop without hitting the guard rail.  I recovered quickly from the 2nd event by taking my foot off the brake!

The initial signs that there was a problem was a test drive right before I got my car back.  After the drive, Gernold drained the rear gear oil (unrelated to brakes-rear-end replacement) and noticed that one of the rear rotors was hot.  This suggested the caliper was dragging or the rear brake was somehow being excessively applied.  He could not force fluid through the brake proportioner.  He replaced the brake proportioner, did another test drive and no excessive heat on rotors.

One would think that by design the device would fail such that there are no rear brakes.  I guess this is similar to a thermostat-one would *expect* that it would be designed to fail in the open position, but we all know this is not true for a 1971 design.  Interestingly, I must be jinxed, as I had to replace the same device on my 1992 Land Cruiser.  However, in the case of the Land Cruiser, the braking was poor and we *could not* get the rear brakes to lock-up in a hard stop.  After replacing the proportioner, the braking was significantly improved and I *could* force the  rear brakes to lock.  The 1992 Land Cruiser does not have ABS.  This must be a different design that, at least in my case, failed such that I had no rear brakes.

It's hard to say for sure that in my case losing the rear in the abrupt application of brakes is explained by the failed proportioner.  However, the proportioner did fail and I did have the spin-outs, so it is logical to link the two events.

Best,

Tom

1971 280sl Tunis Beige Metallic (restored & enhanced)
1971 280sl Tobacco Brown (low mileage stock)
1970 280sl Deep Red (Project Car)

[Benz Dr.]

Rear drum and front disc use a different way to limit rear brake lock up.

230SL uses rear drums as well as the sedans and coupes from that time period. These cars have no brake pressure compensator.
 
How did they design this system to produce safe braking even in panic stops? Remember, you CAN lock up rear drum brakes.

Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org

[jeffc280sl]

Finished installing the adjustable proportioning valve.  Installed a metric bubble flare T in place of the old valve.  Made a bracket to mount it on the old bolts and used as is the flex line and main line from the master cylinder.  Made a new hydraulic line for the passenger side rear.  Saved the old one in case I decide to go back to original.  Mounted the new proportioning valve just below the master cylinder and between it and the fender.  Joined the new valve to the old line with a metric coupling.

Set the valve to near factory setting and made a series of "panic" stops.  Car performed as before with nose dive and if pushed hard enough, front wheel lockup.  Rear did not lock.  

Changed the setting from 3 to 6 (7 is max rear pressure) and went through several "panic" stops.  While I have no way to measure as yet, the car seemed to stop faster with less nose dive.  As earlier with added pedal pressure I was able to lock up the front wheels at the end of the braking sequence.  The rear wheels did not lock.  

I have a lot more testing to do before I'll be comfortable with any change.  I have 205/70 series tires and the road was 75 F and dry this evening.  I know tire friction and road surface conditions like rain etc. can make a big difference.  Anyway, I'm pleased with the installation, no leakes, and now I have something to safely experiment with.  May try and setup some cones and do some stopping tests with measurements in the near future.  I'll let the group know of my test results when available.

Jeff C.
1970 280SL 4-speed

[norton]

Jeff Thanks for sharing this test info with us, it's  great stuff.
Are you using stock MB pads?

Mike Halleck
Chesterfield Mi
71 280SL
68 250SL (parts car)
94 E320 Coupe

[jeffc280sl]

Your welcome Mike,

Yes, I'm currently using regular pads.

Jeff C.
1970 280SL 4-speed

[JimVillers]

Jeff .... If you are really trying to reduce your stopping distance, I would suggest experimenting with a high performance brake pad.  While I am not familiar with anyone that manufactures a carbon-kevlar pad for a 113, there are a number of race suppliers that will put the material on a backing plate that the customer supplies.  One supplier is Carbotech Engineering http://www.carbotecheng.com/main.htm.  I use a similar pad on my 2.3-16 and the difference is significant.

Jim Villers
190SL, 230SL 5-Speed, 190E 2.3-16 Kompressor

[jeffc280sl]

Thanks for the input Jim.  

While inspecting my rear brakes I noticed little wear if any on the brake pads.  My focus has been to try and see if over time the proportioniong valve performance declined to a point where the rear brakes were ineffective in stopping the car.  I think that I've determined the old valve was working okay since brake performance is about the same as best I can tell with the new valve setup at original specs.  I'm now just testing at various rear pressure levels to see if rear brake pressure can be increased thereby improving brake performance without needlessly adding a risk factor for early lockup. This will take some time given the need to test under many conditions.  As I do these tests I can say that nose dive  "may"  be improved just a little. I'm doing testing on dry surfaces at setting 6.  I think overall braking distance is shorter and I've not locked up the rear wheels as yet.  I have locked up the front which was part of the test goal. I think I'll stay with the knowns that I have thus far before changing pads and adding another variable.  It is nice to know about this option.  Thanks

Jeff C.
1970 280SL 4-speed

[norton]

I switched to the vented rotor sedan set up on my 71 2 years ago after I installed a set of late model 16 x 7" wheels with 205/55/16 summer performance tires. With the new tires, I could not lock up the front or rear tires. Then I put on EBC brake pads, Improved the stopping quit a bit, still could not lock the fronts, not to much nose dive (car has Koni's and progressive springs)but I was putting enough heat into the solid rotors to warp them, so I went to the vented rotor set up, Good braking, no warped rotors. But after all is said and done, the biggest improvement in braking was from the tires.

Mike Halleck
Chesterfield Mi
71 280SL
68 250SL (parts car)
94 E320 Coupe

[Malc]

Just a couple of comments on brake pad material....

Pad material in itself does not effect stopping distances. pad material differences are mainly based on "hardness" and heat capabilities. Pad have to reach a temperature window to operate at maximum efficiency
In general ordinary pads used everyday are usually quite "soft" this is so they reach thier optimum temperature quickly. For instance in stop start traffic and general driving. If they get too hot they will start to "gas", basically a film of gases between the pad and the disk build up and the float - to the driver thats brake fade!
How do they get too hot?? Track day thrashes, driving with your foot on the brake pedal long decents riding the brake pedal.

However for me who left foot brakes when rallying I have gone for a harder material, drilled and grooved vented disks, racing fluid and this works, disks can get so hot they start to glow! However on a "regular" car this is a waste of time and money.

On my daily hack car (saab 900) I have gone up 2 grades of material as I quite often engage in "spirited" driving, but still have standard disks. Works for me

Finally you can also "over brake" a car by putting bigger brakes etc on it plus the suspension has to be capable of handling the forces imposed by sharp braking and over sized components. If you have worn components the car could pitch and weeve about IE become unstable!!

Malc

[TR]

Mike -- Have you posted photos of your car with those 16" wheels & tires?  I'd like to see that.

Tom in Boise
'71 280SL 4-spd, signal red w/lt. tan interior, restored/enhanced

[norton]

Tom my SL is apart for resto right now, and with my work schedule it may be apart for a while, but I can mount the wheels back on the car next time I'm home and get a pic.  The wheels I'm using are from a 1999 or 2000 C class, 16 X 7 ET 34 (I think, maybe 37)

Download Attachment: mywheels.jpg
11.69 KB
I grabed this pic off the net, they are the same wheels I'm using.
Mike Halleck
Chesterfield Mi and Wilmington DE
71 280SL
68 250SL (parts car)
94 E320 Coupe

[TR]

Mike -- Thanks for that kind offer.  But we can all wait for the pics until your restoration work is done.  Sure sounds interesting though.

Tom in Boise
'71 280SL 4-spd, signal red w/lt. tan interior, restored/enhanced

[jeffc280sl]

Thought I would post an update on my brake bias experiment.  Other than working great there isn't much to report.  I have driven the car maybe 2500 miles since installing the adjustable bias valve.  I've yet to lock up the rear wheels and I've driven in all but snow/sleet.  Next time I need pads I'm going to buy the performance versions and give them a go.

Jeff C.
1970 280SL 4-speed