Brake issue: W113 42mm calipers & pressure value vs 108 35mm calipers & no valve

[jameshoward]

I am in the process of doing a rear axle conversion for my (4,08) 230 using a 3,46 from a 108. I will convert from drums to discs at the rear.

From previous posts I understand that when doing the conversion one requires a small valve that sits forward of the rear axle on the underside of the car, the purpose of which is to reduce the pressure in the lines as braking is applied to the rear discs, thus ensuring that the front continues to provide most of the brake force.

However, on a 108 with discs all-round, there is no such valve, just a T-coupling - See this post, Naj's question and the photos: http://www.sl113.org/forums/index.php?topic=10848.0

W113 calipers are 42mm, whereas 108 calipers are 35mm. Is the elimination of the valve achieved by using smaller calipers thereby reducing the braking force applied? It's the only reason I can think of for there being no valve on a 108.

I would like to know whether I can eliminate the need for the pressure reducing valve on my 230SL (which never had such a valve because it has drums) by using the same 35mm calipers used on the 108 (and, no doubt, saving some money into the bargain by not having to shell out on 113 parts and get 108 parts instead).

Any advice/previous experience would be most welcome.

Thanks,

James

[DavidBrough]

Hi James,

I think you should be very careful as correct brake balance is a crucial safety feature and can be difficult to obtain with non matched parts. When I was fitting my axle SLS told me that the 108 3.5 calipers were the same as 113 items externally but had larger pistons inside to cope with the extra weight of a saloon and V8 engine. If you were to fit these more powerful units you would easily lock the rear brakes. In the long run you may well be better to obtain a 113 set complete with standard balancer unit.

[jameshoward]

David,

Thanks for your reply.  I share your view entirely about the need to ensure that brakes are set up properly, indeed, I have a vested interest you might say!

The point of my post is that the calipers on the 108 are SMALLER not larger than those on the 113, so the comment made by the SLS individual seems spurious.

As I said in my post, because the calipers are smaller and because the 108 does not use a pressure value, one might assume that the value is made redundant because the calipers provide less braking force.

My question remains,. I'd be grateful for advice from any with wide experience of Mercedes brakes.

James

[pagoden]

James,

I can't bring to your question the perspective it deserves but would like to further the inquiry as far as my ponderings have taken it. 

A smaller-diameter piston reacting to a given displacement in a closed system must travel further to dissipate that displacement than must a larger-diameter piston.  It's a matter of displaced volume, and the larger piston dissipates a given displacement with less travel, just because it is larger.  So the smaller gets pushed further.  And then, for me, things get more complicated.

In both cases the full extent of piston travel is thwarted by the presence of brake drums or disc rotors, as designed by persons desiring a braking system; piston travel is stopped short by immovable object, forcing a substitution of pressure for travel.   Does this mean that your smaller piston will actually be pushing harder on its brake pad than a larger one would be?  After all, it is 'driven' to go further than its larger sibling.  And since the 'push' energy behind them is pretty consistent, coming from the same master cylinder, it would seem to result in more pounds per square inch expressed at the smaller one just because the avaiilable surface is lesser.  So doesn't it then push harder?  (It 'wants' to go further than the other one after all, but can't.)  This seems contradictory to common sense but is compelling to me when I stop to look closely.

And, to either further muddy or clear the waters, shall we agree for our purposes that the size of the friction pads intervening between the pistons and braking surfaces will be the same in both cases? --- as this has direct bearing on the pounds per square inch bearing on the rotor.  (I propose assuming equal pad size, leaving the possibilities for variation as a possible way to tune a modified system for safe and effective function.)

This is an issue with which I never got fully comfortable, but some rudimentary math guided experimental parts substitutions on much lower-powered cars back in the time when we had more daring, less sense and doubtless stronger bones;  and in some cases we ended up with better performing (pretty much exclusively drum-type) brake systems.   I've pretty much forgotten how to do a lot of that now, but am confident that we have amongst us those who can shine a stronger and more clear light on the issue. 

You raise an excellent and interesting --crucial, even, to those with life and limb involved--  question, James; thanks.  And yes, please, let us now hear from those with wide experience of Mercedes brakes.

Cheers,

Denny

[DavidBrough]

Hi James,

I think I may have mixed the smaller and larger comment from SLS as Dennys post makes sense regarding braking force and piston size as you would assume that the larger heavier vehicle would need stronger brakes. However, making assumptions is not always my strong point and there may be more knowledgeable people that can clarify matters for you. If as it seems you may require a rear balance restrictor there are a lot of racing suppliers such as Revotec and Merlin that stock such things many of which are adjustable.

[jameshoward]

Denny's comment has given me pause for thought.

Here's hoping some experts step in whilst I return to my physics text books....

[Desertpagoda]

Look again, there IS or at least WAS a valve installed on 108-109-111-112 etc. On USA cars, it is mounted below the left/driver side of the rear seat.

[waqas]

Look again, there IS or at least WAS a valve installed on 108-109-111-112 etc. On USA cars, it is mounted below the left/driver side of the rear seat.

Not true for all disc-brake cars.

I once had a USA-spec 1971 280SE sedan (108 chassis) with M130 engine and disc brakes. This car did not have the brake proportioning valve. It only had a simple tee-fitting, similar to what my 230SL has.

As another data point, I currently own a USA-spec 1966 250SE coupe (111 chassis) and a Euro-spec 1967 250SE coupe (both with disc brakes), and they both have the brake proportioning valve in question.

Regarding the original question, I've never measured the size of the calipers on these cars. How does one go about this? Does the caliper and brake pad need to be removed?

[waqas]

As a follow-up to the original question posed, does the lack of proportioning valve have more to do with the rear calipers or the master cylinder?

In other words, is there a difference between brake master cylinders of disc-brake cars with and without proportioning valves?

[jameshoward]

So, to summarise, we know that some disc brake cars did not have proportioning valves, but confusingly, that others did. I haven't got the bottom of the surface area/pressure point made by Denny. I was hoping to see someone who I could ask, but now won't catch up with them for a while.

I am intrigued by Waqas' question about master cylinders. Are these parts interchangable between our cars and others of the era? Perhaps that might help get to the bottom of this.

Still no brake experts out there who want to wade in with an opinion?

JH

[Naj ✝︎]

To answer Waqas's questions:
All the master cylinders, 230 SL, 280 SL and the W108 (at least the ones I checked) are the same at 23.8 mm dia., however the mounting may be different or in the case of the 230, they are not dual circuit as the later cars.
Looking at the W108s, from what I can gather, earlier ones had the 42 mm calipers with proportioning valves. Later 280s and the 3.5s have the smaller pistons and no proportioning valves.

The diameters of the pistons can of course be measured but in this case were obtained by looking at the different part #s of the calipers and then finding out what the differences between them were.

Dennys Point: Hydraulics 101
The pressure in the system will be the same front and rear unless there is a restriction or controller such as the proportioning valve. The braking force is a product of the system pressure and the surface area of the pistons.
The surface area of the 42mm piston is 44% greater than the 35mm piston.

naj

[Bob G ✝︎]

Not quite sure were I heard this or read it. Was there an up grade for our cars front solid disc to a vented rotor and a slightly bigger caliber from a 108 or 109 sedan?

Bob Geco

[pagoden]

Hi Bob,

Yes, I think what you're remembering is the vented rotors/discs from the front ends of large sedans, the 3.5 and 4.5s, etc.  They dump the heat of friction through the venting, so don't get so hot as solid discs do during prolonged heavy use.  I think the upgrade doesn't result in greater stopping power per se so much as it eliminates the 'fade' you get when your brakes heat up, pretty much robbing you of any effective braking until after they have time to cool.  You may have seen this info years ago, or more recently in our Tech Manual, where the posts on this topic were still from the old/other siite last time I looked -- and I think I'm actually sort of quoting Joe A. on this; check it out in Brakes.

Cheers,

Denny