Opel GT Braking System

[blancojp]

Part I – The stock Opel GT brake system

In order to see the differences created by different brake system upgrades, we must have a starting point to compare the results. The stock Opel system will be our starting point and from the examples created, you will see how each individual upgrade (we hope) reacts with the remaining stock components.

Detailing individual components, the Opel brake system comprises of the following:
• A brake pedal coupled to an extension rod with a 5:1 pedal ratio. This relates to 50lbs of pressure being applied to the brake booster/master cylinder unit for every 10lbs of foot pressure.
• A single diaphragm brake booster unit. The brake booster purpose is to amplify the amount of pressure being received from the foot pedal using vacuum from the engine. For our examples we will assume a 1:1 ratio to help in our calculating process.
• A dual port master cylinder with a 20mm piston. A dual port unit has two pistons which send fluid and pressure to the front and rear brake systems individually. A 20mm piston has an area of 0.4867 square inches and with 50lbs of applied pressure through the brake pedal assembly, will generate approx. 103psi of hydraulic pressure. The master cylinder also incorporates a residual valve for the rear circuit to maintain a small amount of pressure on the rear lines. This maintained residual pressure is used to partially overcome the force on the top shoe spring therefore limiting the fluid’s gusher effect when you release the brake pedal.
• A front disk system with a solid rotor and 48mm dual piston calipers. The front disk brake assembly comprises of a solid rotor, a fixed twin 48mm piston caliper, a 62mm brake pad and a flexible hose. The 48mm caliper piston has an area of 2.80 square inches each, which the total for the assembly is 5.6 square inches. Using our 103psi example above, the total caliper clamping force on the rotor will be 577lbs or 288.5lbs per caliper side. Pads and rotors will be discussed in detail on Part III of these writings.
• A rear drum system with a 5/8” (15.8mm) wheel cylinder. The rear brake assembly comprises of a brake drum, a 15.8mm wheel cylinder, dual brake shoes and associated hardware with a tension spring. The 15.8mm wheel cylinder piston has an area of 0.3037 square inches. Using our 103psi example above, the total brake shoe force on the drum will be 32lbs per brake shoe or 64lbs total per wheel. Brake shoes will be discussed in detail on Part III of these writings.

Now that we have all of the above figures to work with, we can move to part II and see the reaction of different components and how they would affect the braking system. Also note that all brake systems have a factor called “compliance”, which is the hydraulic fluid volume required to take up all clearances prior to force being applied.

[blancojp]

Now that we have established our starting point on Part I, we can individually see the results of replacing components as they relate to the stock brake system.

The possible upgrades are endless however here are a few:
• Increasing/decreasing pedal ratio – If you increase the pedal ratio, you will decrease the effective pedal effort. If you decrease the pedal ratio, you will increase the effective pedal effort. There is very little gain in performing this change and system compliance is not changed.

• Master cylinder upgrade – The first common mistake is that a larger master cylinder will work better in your stock system. If we were to increase the size of the stock master cylinder piston to 22mm, our new unit will have a piston area of 0.5887 square inches and will generate approx. 85psi of hydraulic pressure. In other words, we reduced pedal travel (compliance) due to the larger piston area but increased the pedal pressure 10lbs just to have the same 103psi from the master cylinder. If we were to decrease the size of the stock master cylinder piston to 18mm, our new unit will have a piston area of 0.3942 square inches and will generate approx. 127psi of hydraulic pressure. In other words, we increased (compliance) pedal travel due to the smaller piston area but decreased the pedal pressure 10lbs just to have the same 103psi from the master cylinder. You can properly change the size of your master cylinder for a specific brake configuration after computing all forces required to stop the vehicle. Another thing to consider is that your new master cylinder might require an external residual valve for the rear circuit, since the Opel unit has it screwed in on the rear output port.

• Front disk brakes – As you can see from our example in Part I, increasing the piston size of the caliper will generate more clamping force since clamping force is equal to the master cylinder pressure(psi) times the area of the piston. However the larger piston will generate additional brake pedal travel which will increase compliance in the system. As an example, a Volvo 4-pot caliper has four 35mm pistons with an area of 1.49 square inches each. Using our example above, the caliper clamping force on the rotor will be 614lbs total, 307lbs per caliper side or 153.5lbs per piston. Overall this is an increase of 0.40 square inches in volume per wheel over the stock caliper unit however, pedal travel increases due to the additional volume required to push the four pistons. Another opposite example would be a Chevy Cavalier 48mm single piston floating caliper, with a piston area of 2.8 square inches. This caliper has the same single piston area of the stock caliper; the clamping force is equal however the compliance factor is less than the stock unit due to the mechanical action of the second pad. Both examples provide you an increase of pad area but require new rotors to operate properly and possibly new rims and tires.

• The rear brake assembly upgrade is a simple one; increase the size of the rear wheel cylinder to ¾”. Just like the calipers, increasing the piston size will increase the total brake shoe force on the drum. The 19mm (3/4”) wheel cylinder piston has an area of 0.4392 square inches. Using our 103psi example from Part I, the total brake shoe force on the drum will be 45lbs per brake shoe or 90lbs total per wheel. As you can see, this is a 40% increase in applied rear brake force; however compliance will increase to compensate the 0.14 square inch area increase. We will not discuss rear disk brake systems since the advantages and compliance issues are self evident.

We have concluded the some of the basic upgrades people consider when increasing the performance of their vehicle. Let us continue to Part III and see a step by step installation of a vented rotor system using both caliper examples above.

[blancojp]

As in everything in life, one must do research in order to accomplish a clear task. My research in any part of an automobile always starts at the U-pick auto salvage yard. If you know what you have and the dimensions you need, looking at a couple of hundred cars often help in determining what parts you can use. Second reason I go to the salvage yard is that I can purchase a presumed usable part for a very small amount, which I can later replace with a new one once functionality has been verified. Then the rest is just using brain cells to put it all together.

I wanted to replace the front brakes on a customer’s GT with a vented rotor system that replacement parts could be obtained anywhere. The car had a modified Opel engine with 120hp and wanted to use the stock 13” wheels without any modifications or spacers. After a week of research I found that a 2000 Chevy metro had a vented unit similar in size as the stock Opel front disk, but also had a 102mm pad area. The caliper was a floating unit and had a 48mm piston. So I got all of the components from the donor vehicle and returned to the shop.

• The rotor is 9 3/32” in diameter and 0.670” thick which is slightly thicker than the stock Opel rotor. The height was 49.5mm which was 6mm less than the stock rotor however the additional clearance could come in handy during installation. The centering hole diameter was 65mm and 73mm were needed to mount on the Opel hub. Since the idea was to be able to just buy and install components, I machined the back of the Opel hub to a uniform 65mm diameter. I then drilled the rotor’s four attachment holes and the rotor mounting process was complete. After assembly, the rotor/hub assembly was installed on the Opel spindle.
• The caliper assembly was a floating design style which requires a bracket to mount the pads and to attach the caliper body to the slider bolts. The extra clearance we gained due to the new rotor’s height will now come into play and allow us to bolt the bracket to the rear of the spindle. The bracket was cleaned, slimmed down and the new attachment bolt positions were drilled and tapped to the original Opel bolt diameter. Checking clearance and adjusting the bracket for proper fit over the rotor, we now had the bracket centered on the rotor. We then assembled the caliper over the rotor and performed final checks on the assembly.
• The Chevy hose for this caliper had the same 10.0x1mm fitting size but was 4 inches longer. We decided to use the hose as is with a few tie-wraps to route and take up the extra length. A 4” section at the end of the hose was left loose due to the floating design.



So let’s sum up what we did and how it affected the performance of our system specified in Part I.
• We replaced the solid rotor with a vented unit for added cooling, 0.170” thicker than the original and with almost the same diameter.
• We trimmed the rear of the hub down to 65mm therefore making it lighter and allowing easily any future replacements of the rotor.
• We used a caliper with the same piston diameter as the stock unit. Compliance is less due to the mechanical actuation of the second pad, but clamping force is the same. The caliper is made out of a lighter metal so decreases unsprung weight.
• We increased the pad size to 102mm from the stock 62mm unit. This part is very important since we now increased the applied force clamping area 62% from stock.
• The stock wheel fits without any modifications to the mounting point so track is not affected.

Just look at the pictures and judge the results.

[markandson]

One slight "flaw" in your design is the fact that the pad area of the new system is larger, but the piston size is the same. As you stated the clamping force is the same, but you are clamping over a larger area and therefore you have reduced the pressure per square unit being applied between the brake pad and the rotor. Depending on the coefficient of friction between the rotor and the pad you may get less braking. A higher coefficient of friction between the new pads and new rotor would be required in order to obtain the same amount of friction (braking force) with the same amount of pedal pressure. Just a quick observation, but I did enjoy reading this thread and your explicit thought process. I am sure the system will work great!

[blancojp]

Perhaps the first thread I should have posted was telling the readers not to post anything until all parts were in. That way, you don't unexpectively give away the ending. Part V tells you exactly the reacting force upon the rotor of both braking systems. But in all, this is the simplest way I can explain step by step, using available system examples, what is going on with your brakes without going into major math.

[blancojp]

(A) Solid rotors and 4 piston calipers

A customer brings in a vehicle just to replace the front brakes with bigger rotors and steering rack to a modified mustang II unit. The car had a stock Opel 1.9l engine and had a set of 15” BMW wheels. The owner of this vehicle had several MG vehicles and also had numerous spares and brake articles on conversions and improvements. He decided that the Opel needed a larger rotor to stop better and wanted to use the MG rotor for this application. So he dropped off all of the components from the MG donor vehicle and we started our process.
• The MG rotor is a type “L” (just like the Opel), 10.75” in diameter and 0.350” thick. The height was 44.5mm which was 11mm less than the stock rotor. The centering hole diameter was 73.2mm so it would mount on the Opel hub without modification. The 4 mounting holes on the rotor were already drilled however they had to be enlarged to 10mm making this a simple hub installation.
• The caliper shown on most of the articles the customer brought in were from a BMW 7-series, which would not mount directly to the Opel spindle without modification. There was one article though that had a bracket drawing to mount a Volvo caliper onto the MG spindle which told us a Volvo caliper would bolt right on the Opel spindle without modifications. We gathered all of the parts and started our assembly process.
• We mounted the rotor to the hub, installed the hub onto the spindle and mounted (tried anyway) the caliper. We realized rather quickly that our rotor height would not allow the caliper to be installed since we were off by almost 9mm so we took everything off the spindle and started over.
• A 10mm spacer was added between the rotor and hub therefore increasing the height of the rotor to 54.5mm. The hub/rotor assembly was then installed on the spindle and the caliper was bolted loose in place and centered on the disk for further measurements. After all measuring was concluded, we added a 2mm washer on the caliper mounting area and our caliper was perfectly in place.
• The Opel caliper had a single line for fluid delivery but the Volvo had two so we added a “tee” at the top end and ran two individual braided hoses to the caliper. The system was complete and ready to bleed.

So let’s sum up what we did and how it affected the performance of our system specified in Part I.
• We replaced the sock solid rotor with another solid rotor 10.5” in diameter and 0.080” thinner than the original. The rotor weight is less but has a higher heat index.
• We did not modify the Opel hub so returning the system back to stock was an easy task.
• We used a 4 piston caliper with a 35mm piston diameter. Compliance is more due to the added 0.40 square inch of piston area, but clamping force increased 37lbs per wheel from our stock unit. The caliper is quite heavier than stock so we increased unsprung weight.
• We increased the pad size to 98mm from the stock 62mm unit. This part is very important since we now increased the applied force clamping area 58% from stock.

[markandson]

Never meant to jump in and cause any havoc. One interesting question, that I don't know the answer to. Is there a target force range that designers use for brake pedal pressure? Seems there must be since there is no huge difference between vehicles in general.

[blancojp]

There is no target range per say, you just adjust your system to whatever the torque numbers are and what equipment you have at hand. Most older vehicles, especially with manual brakes, had a large pedal ratio. Newer vehicles do not require major pressure to operate since you have additional electronics and valving to compensate efforts applied.

Stopping a car is a simple job, try to stop some of the vehicles we design which are towing a load of 50-90 tons.

[blancojp]

(B) Vented rotors and 4 piston calipers

The same customer brings the vehicle back for additional suspension work and decided the bigger rotors were not giving him the performance he wanted. He was getting a bit of pulsation when the brake pedal was applied. We looked into the situation and found the rotor slightly warped and recommended that vented rotors be installed. He did not happily agree at the time but returned a few days later and gave us the go ahead on the project.
• The MG rotor was replaced with a vented rotor from a Volvo 240DL which was 68mm in height, 10.30” in diameter and 21mm thick. The rotor was 12.5mm taller than the original so we drilled the 4x100 bolt pattern and slipped the rotor over the hub.
• The solid rotor 4 pot caliper could not be used and were replaced with wider units.
• We installed the hub onto the spindle, slipped the rotor over the hub and mounted the caliper to take our initial measurements. Everything was very close but still needed minor adjustments. After all measuring was concluded, we removed 1mm from the caliper mounting area and our caliper was perfectly in place.
• Since the fluid lines for both 4 pot calipers were the same, we hooked everything back up and we were done.

So let’s sum up what we did and how it affected the performance of our system specified in Part I.
• We replaced the sock solid rotor with a larger vented rotor 10.3” in diameter and 0.325 thicker and heavier than the original.
• We did not modify the Opel hub.
• We used a heavier and wider 4 piston caliper with a 35mm piston diameter. Compliance is more due to the added 0.40 square inch of piston area, but clamping force increased 37lbs from our stock unit. The caliper is heavier than stock so we increased unsprung weight.
• We increased the pad size to 98mm from the stock 62mm unit. This part is very important since we now increased the applied force clamping area 58% from stock.

[blancojp]

Replacing the front and rear brakes on the GT can be accomplished quite easily, using components that are already in production from other cars. Just by looking at a BCA catalog can tell you how many vehicles have the same A1 and A4 wheel bearings. But the main object should be to stop the vehicle in an efficient manner without changing the geometry and balance of the existing suspension components. So let’s take a look at the stock and upgraded brake systems and how it affects the stopping of our vehicle:
• The stock system rotor is 9 1/8” in diameter and the clamping pressure is 577lbs. The standard stock pad has a coefficient of friction of 0.33 which makes our net clamping force equal to 190lbs. Using the rotor diameter and the net clamping force; we can calculate the generated brake torque, which in this case is 56.5 ft/lbs.
• The Chevy system rotor is 9 3/32” in diameter and the clamping pressure is 577lbs. The net clamping force equals to 190lbs and the generated brake torque is 56.15 ft/lbs. We gained nothing but the ability to run our brakes cooler.
• The MG system rotor is 10.3” in diameter and the clamping pressure is 614lbs. The net clamping force equals to 202lbs and the generated brake torque is 69.9 ft/lbs. We gained 23.8% in stopping power but still have the same heat issues as stock.
• The Volvo system rotor is 10.3” in diameter and the clamping pressure is 614lbs. The net clamping force equals to 202lbs and the generated brake torque is 69.9 ft/lbs. We gained 23.8% in stopping power and the ability to run our brakes cooler.

From the above we can see clearly that both systems using the Volvo 4 piston calipers and stock pads generate approx. 24% more brake torque than the stock brake unit. However a performance pad with a Cf of 0.45 or greater used on the stock system will generate 85.5 ft/lbs of brake torque which is 23% greater braking torque and far less expensive than a several hundred dollar upgrade.

The best and cost effective brake upgrade still is:
• Replace the front rotors with radius drilled units.
• Rebuilt calipers with green stuff pads.
• ¾” rear wheel cylinders and performance rear shoes.

[spamseptictank]

Just by looking at a BCA catalog can tell you how many vehicles have the same A1 and A4 wheel bearings.

JB,
This may be a stupid question (and I searched for it first), but what's a "BCA catalog"? Thanks.
Grace and Peace,
Dave

[blancojp]

BCA is the company that makes wheel bearings. Opel uses BCA # A1(outer) and A4(inner) which are about $4.00 each at any auto part store. There are quite a few cars that use these bearings, so use your imagination but remember the Opel spindle is not made to handle large amounts of torque.

[Manta Rallier]

Hi JB,

Very interesting stuff; particularly as we are looking into brake upgrades for our Manta rally car. Your detail is very good and appreciated. The note from others about the pad pressure being lower with larger pads is true, but this is equalized by the larger pad area; the net brake (clamp) force stays constant with constant pressure.

A couple of things to add:

- One benefit of the larger front rotors is cooler rotors since the heat load is spread over a larger area. It's not that important for normal street driving but can be very beneficial for any race or auto corss applications.

- If you increase braking power at either end of the car, everyone needs to be aware that this will change the brake balance from stock. This can cause tricky driving under heavy braking or in slick conditions. One change I would caution others about is the increase of rear wheel cylinder diameters. This increases rear braking by over 40%, but really moves the brake bias towards the rear. We do this in rally cars, but handling that is 'loose under braking' is desired for rallying. It is NOT desired if you send your spouse or teenage offspring out on the road on a rainy or snowy day! Under hard braking the rears will lock and the car will spin out of control.

- Increased front braking of 20% or so is of less concern, but it could still lead to more easily locked front brakes in slick conditions, so be conscious of this change.

- Note that increased braking power front or rear, and the subsequent brake bias change, will come from changing brake pad coefficient of friction. So be aware of this when you change pads to a more agressive pad. CoF info can be had by researching the net or calling the brake companies. If you do the 3/4" rear cylinder mod for street driving, please think seriously about upgrading your front pads for a CoF of .4 or greater to try to equlize this to some degree.

- And finally, for all, note that drilled rotors tend to eat pads in a hurry if you are using them hard at all.

Edit: (And BTW, doesn't a complete brake analysis on the power to stop the mass of the car have to include the wheel radius as well as the rotor radius? It's a form of lever system after all....)

Regards, and thanks for the excellent info, JB,
Mark B.

[blancojp]

I tried to make it simple since most people have no clue when it comes to calculating areas and so on. The tires I left out because the rotor is mechanically coupled to the wheel and so on. If you look at it from that perspective, any change will change the applied torque value and therefore changes the braking as well.

But I am still trying to find the optimal master cylinder for my car. Either I have too much brakes or very little, can't seem to get it to a happy point.
But you would be surprised how good is the small vented rotor upgrade with high CoF pads, totaly awsome and you get to keep the original wheels.

[opelwasp]

You have any pics of the machining you had to do to the Metro caliper and rotor? I am very interested in this now that I work at a welding shop and have access to all kinds of neat tools.

[blancojp]

You need to machine as follows:
1) Machine the rear of the Opel hub to the same diameter as the rotor. Then install the rotor, mark and drill the four attachement holes and secure the rotor to the hub using the original 4 bolts.
2) Clean up and machine the caliper bracket to fit between the rotor and spindle. Make sure the bracket is centered on the rotor.
3) Drill the spindle caliper mounting holes such that the bolt goes through.
4) Drill and tap mounting holes on the caliper bracket.

Match the hoses and you are done.

Now if this is too much work, you can buy this as a kit from us and will take you an hour or so to install.

[opelwasp]

Thanks, that kind of work is right up my alley.

[Manta Rallier]

But I am still trying to find the optimal master cylinder for my car. Either I have too much brakes or very little, can't seem to get it to a happy point.
But you would be surprised how good is the small vented rotor upgrade with high CoF pads, totaly awsome and you get to keep the original wheels.

Waht diameters are you trying? And have you considered moving the pivot point of the MC shaft up or down on brake pedal a bit for 'fine tuning'?

And good on the improvement; somehow the idea that Metro brakes being better than fine German-engineered brakes does not quite compute ............ but I realize that the laws of physics overrides all of our emotions!

BTW, on the Volvo rotor upgrade, you mention going to a wider caliper, but I don't see any ID of the actual caliper. Can you share what caliper is used in that set-up? This set-up is interesting for our Manta rally car project, where we plan to go to 14" wheels.

Also, have you thought about aftermarket aluminum multipot calipers? The closest I have found for mounting hole distance is 88.9mm (3.5") vs. the stock 75 mm so that would take an adpater, but am curious if you have looked into this and found anyting better. (I have only done minimal research on the the web, so something may be out there.)

Regards,
Mark B.

[blancojp]

I have tried just about everything on this car but it is always the same, either I hit my face on the steering wheel or the car coasts to a stop. I have over 200 master cylinders on the second floor of my shop and I have used from a 5/8" bore to a 1 1/4" with different results. I have also adjusted the rod several times with almost the same results.

My problem is that I have Howe GM III front calipers on my Opel and they not only require volume but also pressure to operate properly. According to my calcs, I should be able to use anything between 19-21mm master cylinder with a 2.25" pedal travel. That is not the case, the pedal travel is over 4" to a fully locked position and that is too close to the floor for my taste.

On the Volvo caliper side, I can guide you through (or post) a complete installation since I am setting up several kits for a customer. You can re-use the stock hose and hose bracket without having to go into the "Y" type installation and dual hoses. This makes the installation tight and a bit more profesional.

[Manta Rallier]

Hey JB,

As for the Volvo calipers, I would like to know the type/model/application, and any mod to the mounting tabs on the steering knuckles. I am assuming that the hole-to-hole mount spacing is 75 mm like stock? As for lines, this is for rally racing so we'll use braided teflon lines, but knowing how to use only one line to the caliper is good; there are a lot of rocks flying around the wheels in a rally and fewer things to tear up is good!

Hmm interesting problem with your brakes. I'll make a guess that you've got too much of a volume mismatch for a single diameter MC to handle. Can the extra travel be accounted for by the rear fluid volume that needs to be moved also? (Are the rear brake adjusters set so the shoes are real close to the drums? That will reduce pedal travel.) If this is a large volume mismatch F to R, have you thought about a stepped diameter MC like was used in some GM cars in the 70's? I never really analysed them but assumed they were there to compensate to some degree for the volume mismatch between large front calipers and rear wheel cylinders. The mount size would probably be whacky for a stock Opel brake booster.

And have you tried putting 3/4" rear Chevette whell cylinders in the car, to try to balance F-R sopping power as well as fluid volume a bit more?

Regards,
Mark B.