[DeTomaso] Brake Upgrade

Fri Nov 11 20:23:22 EST 2011

There area a number of online resources for those who want to theoretically look at their options and play around with rotor to caliper sizing to master cylinder sizing. It's also fairly easy and useful in retrospect to verify an existing brake setup.

I found a great Excel based spreadsheet when I upgraded my GT40 from hung to floor mount pedals and new master cylinders. Tilton  Engineering (the pedal assembly manufacturer) also had an online submission and they came back with similar recommendations, nothing like a second opinion! The effect of the bore of the master cylinder is often overlooked and misunderstood in an upgrade (how many times have you heard of just switching to a large bore Master?), especially where dual or step bore cylinders are used, it's important to match everything to get a system that is ballpark in overall braking efficiency and only rely on an adjustable proportioning valve for fine tuning.

Julian

> From: detomasoregistry at gmail.com
> To: MikeLDrew at aol.com; JDeRyke at aol.com; bjbstewart at yahoo.com; detomaso at realbig.com
> Date: Fri, 11 Nov 2011 15:46:41 -0800
> Subject: Re: [DeTomaso] Brake Upgrade
> 
> I'll chime in here, as I'm one of those types Mike refers to below.
> 
> (Copied from the text below)
> 
> If you elect to try to piece your own system together with various bits 
> and bobs, you can still achieve success.   You'll probably have to resort 
> to some sort of adjustable valve to do so unless you're smart enough to 
> succesfully calculate the necessary piston sizes front and rear to achieve the 
> desired proportioning
> 
> I pseudo-pieced together bits and bobs (Bob Byars to be specific).
> 
> Full details on my brake system (stock calipers and rotors) can be found at the Registry
> website here:
> 
> http://www.provamo.com/Members/TechInfo/BrakeMasterUpgrade.asp
> 
> I tested my stock brakes repeatedly over about 9 months (and two sets of tires -yikes)
> 
> I replaced the brake master with a "stepped-bore" master from Precision Pro-Formance
> (Bobby Byars).  I also upgraded the booster with one about 1" larger in diameter at the
> same time.  I then replaced the stock pressure-limiting (front brake limiting only) with a
> manual proportioning valve, and also scuttled the stock (brass) distribution block (with
> the shuttle.)
> 
> The rotors and calipers stayed the same.  Braking was VASTLY improved.  I could adjust the
> proportioning valve such that the rears locked up first (both unsafe and a scary result),
> just to know what that was like.  Take my word, you don't want the rears locking up first.
> The car swerves like you threw out an anchor.  Where you are eventually pointed even with
> the left and right calipers well matched and balanced is tricky.  But hey us engineers
> just have to push the limits - right?
> 
> Then I dialed the proportioning valve in small increments such that the fronts eventually
> locked up first, and the rears generated very limit heat (or stopping power)  This was
> over about 10-15 increments.  
> 
> In the end I could have not used the proportioning valve at all, as it was sitting at
> neutral when in the best setting.
> 
> Then, having worn out that set of tires (all 4), I moved to my 17" Campy clones with fresh
> Goodyear tires, and repeated a subset of tests.
> 
> The end result is what OTHERS say is an impressive brake system (for stock rotors and
> calipers).  I upgraded to Porterfield pads, and repeated some testing.
> 
> I used to demo the braking system, until I wore out those 17" tires.
> 
> Now I'm back to 15" as I have a Drew-like inventory of 15" tires to consume, before I
> return to the 17"s and new tires.
> 
> Anyway, I enjoyed the experience of making adjustments, testing, and tuning.
> I stopped when it was time to move to the track to evaluate repeated and heavy track
> braking.  Anyone want to fund my tire fund?
> 
> Chuck
> 
> 
> 
> 
> 
> In a message dated 11/11/11 13 15 58, JDeRyke at aol.com writes:
> 
> 
> > 1)- totally remove the stock non-adjustable proportioning valve that 
> > limits
> > the front brake pressure, so you rely mostly on the toy rear calipers.
> > Removing this and re-plumbing for full use at both ends allows the decent 
> > stock
> > front brakes to work to full capacity. This alone improves brake power
> > markedly and is all some guys decide they need.
> > 
> 
> This is a complete misconception fueled by a fundamental misunderstanding 
> of how a braking SYSTEM works.   This modification will improve the 
> performance of the front brakes and screw up the performance of the braking SYSTEM as 
> a whole.
> 
> The original calipers were badly mismatched, front-to-rear.   I have copies 
> of the original braking system DOT certification tests, conducted on 
> hand-built preproduction prototype Panteras (chassis number 1007, 1010, etc.), 
> where the need for the proportioning valve was revealed, and then the braking 
> improved once they were fitted.  
> 
> Brakes need to be properly balanced front-to-rear.   Especially in a 
> mid-engined car with extensive weight on the back end, and even more especially in 
> one fitted with aftermarket wheels and tires (where typically the 
> difference in front-to-rear tire width is even greater than stock, and thus the need 
> for rear braking is greater than stock), it's crucial that the back brakes 
> contribute their fair share to the overall stopping scheme.
> 
> The stock proportioning valve is actually a pressure reduction valve.   It 
> is designed to reduce front line pressure, relative to rear line pressure, 
> so that the front and rear brakes are properly balanced.   
> 
> If you remove this valve entirely, as Jack recommends (again and again), 
> what you are left with is overly-sensitive front brakes.   To the ignorant, 
> this is an 'improvement', because for a given amount of leg pressure applied 
> to the pedal, there is a greater apparent braking effect.   As long as you 
> are just driving around normally and not attempting to actually stop the car 
> with any sort of vigor, this can feel like 'good' brakes.   It's quite true 
> that a car so-modified will stop with less effort when coming to a stop sign, 
> etc.
> 
> However, what this modification has done is actually to disable the rear 
> brakes!   That is, the front brakes are now so sensitive, relative to the 
> rears, that when pedal pressure is increased, the front brakes simply lock up 
> and the tires slide, while the rear brakes are hardly doing anything.
> 
> Real-world performance braking (track work etc.) will reveal that cars with 
> such a modification have substantially LESS overall stopping power, because 
> they are relying almost entirely on their front brakes.   This also will 
> lead to rapid overheating of the front brakes and resultant fall-off in 
> braking performance (which is what Brent experienced I believe, as I think a 
> previous owner of his car followed Jack's advice).
> 
> If the stock pressure reduction valve is removed for any reason (on a stock 
> braking system), then it should be replaced with an adjustable 
> proportioning valve.   Initally the valve should be plumbed wide-open (no restriction), 
> then real-world testing should be conducted with progressive adjustment, and 
> repeated hard stops.   A hand-held heat gun (thermometer) can be useful, as 
> rotor temps will give an indication of braking performance--if your front 
> rotors are hot and your rears are not, you don't have enough rear brake bias. 
>   
> 
> Eventually you'll go too far and the rear brakes will lock up first; then 
> back off the valve so that the fronts lock up first BUT the rears are still 
> doing their fair share of the work.   Too much rear brake bias is dangerous; 
> if the rear brakes lock up, the car can easily go out of control.   So the 
> front brakes should always lock up first--but not excessively so.
> 
> Jack's advice regarding brake vacuum is quite valid.   If your power 
> booster is blown out, or if you have insufficient vacuum to operate the power 
> brakes, the brakes won't work well at all.   Some cars can have great braking 
> effect with no power assist (the Ferrari F40 is such an example), but this is 
> achieved through proper engineering--typically the pedal is much longer, so 
> the leg has a much greater mechanical advantage over the master cylinder.
> 
> A properly functioning Pantera brake system, equipped with performance pads 
> but no other upgrades, should be able to lock up all four wheels.   Sadly, 
> many Panteras are suffering from some form of deterioration and this can't 
> be achieved.   Rather than trying to band-aid the system, the responsible 
> thing to do is identify the actual source of the problem so that the stock 
> (which is to say, quite good) brake performance can be restored.
> 
> All bets are off when aftermarket systems are installed.   Ideally, the 
> vendor will have done his homework, and front-to-rear proportioning will be 
> handled via different piston sizes in the calipers (my Wilwood system from 
> Dennis Quella has four calipers that externally are identical, but differ in the 
> sizes of the pistons on the front versus rear calipers for just that 
> reason).   If you elect to try to piece your own system together with various bits 
> and bobs, you can still achieve success.   You'll probably have to resort 
> to some sort of adjustable valve to do so unless you're smart enough to 
> succesfully calculate the necessary piston sizes front and rear to achieve the 
> desired proportioning (which I most emphatically am not!).
> 
> Mike
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