Brake Rotor Minimum Thickness

[scra99tch]

What is the main driver of limiting a brake rotors thickness?  Could I shim the pads from behind if it was a problem with the piston stroke? 

I started cleaning up my SP 1000 Brake rotor from its surface rust. Set it up in the lathe and used my grinding wheel attachment to slowly bring the surface back to plain metal.  As I have only done one side so far, its right now at the minimum of .228"  I will need to go another .005-.008 to clean up the back side. 

Even at is minimum thickness it still seems like there is plenty of material left for heat dissipation.  I think the linked front rotor will be fine and clean up without much grinding..

BTW this is for a 1979 SP1000 

[YellowDuck]

I'd be very surprised if it's piston stroke that limits it.  I am guessing resistance to warping.

Maybe someone who actually knows something will reply 

[Perazzimx14]

On a modern 5/6mm thick disc even if you wore the disc down to 1/100th  the thickness of a red hair and wore all the brake material off the backing plates the pucks will not stroke enough to come out of the caliper. 

[Frenchfrog]

It will be fine until it isn't and then you are putting yourself and others at risk.
Your call but personally I would not chance that..

[Antietam Classic Cycle]

.228" is 5.79 mm, so you're right at minimum thickness of 5.8 mm already. As YellowDuck mentioned, I think resistance to warping is the main reason to go no thinner.

[RinkRat II]

   Without doing a complete metallurgical analysis on the properties of a 1979 rotor I would surmise that .005" reduction wouldn't matter much unless you plan to go racing.  My $.02 

   Paul B

[n3303j]

Brake pads are on a metal backing plate. That metal backing plate transfers all the rotational thrust through its leading edge which rests against a shelf machined in the caliper. The two pins are primarily to keep the pads in the proper vertical location, not absorb braking thrust.

When the disks get too thin and you have a pad missing most of the friction material there is a very real possibility the metal backing plate could jump the step and jam between the disk and the caliper body.

That's a good reason to run proper thickness disks AND make sure your caliper is properly shimmed so that the disk is dead center between the two caliper working faces.

[ridingron]

How much are you going to ride the bike? How do you ride the bike? Put the rotors on the bike and let the pads deal with the clean up.

I buy used cars and bikes and I've had a couple of rotors that were scared by the pads backing plate, on cars and bikes. A few thousands of an inch? The brakes will never know it and neither will you.

[Wayne Orwig]

I think resistance to warping or even collapsing (vented rotors) is the main concern.

But....

This summer on a long RV trip, my brakes totally failed. Nothing gets your attention like that. The fluid was pumping out about as fast as I poured it in.
It turns out that a rear caliper had stuck enough that it totally burned off one brake pad AND wore out one side of the brake rotor. That wear was enough let the pistons leak the fluid out. All of the pads and rotors were in great shape, but that one.

Short version, a 1/2 worn brake rotor and one worn brake pad allowed fluid to be pumped out.

[nc43bsa]

I think resistance to warping or even collapsing (vented rotors) is the main concern.

Resistance to warping and fracturing.

I had a rotor fracture on one of my race bikes, but that was probably due to a poor drilling job.  And I've seen vented rotors that have worn one side down to the vanes, though not on a bike.

[n3303j]

No one seemed disturbed by the fact that the pad metal backing plate could fit between the caliper body and an undersized disk. Evidently a piece of steel wedged between a caliper body and a rotating part of the front wheel bothers no one??

This is an issue with the stainless disks sold for the T3 by MG Cycle. They are already at (or below) minimum cast iron disk thickness. I have the MG cycle disks. I keep a careful eye on the pad thickness just so I won't end up with the metal plate wedged in between the caliper and the disk.

[Perazzimx14]

No one seemed disturbed by the fact that the pad metal backing plate could fit between the caliper body and an undersized disk. Evidently a piece of steel wedged between a caliper body and a rotating part of the front wheel bothers no one??

This is an issue with the stainless disks sold for the T3 by MG Cycle. They are already at (or below) minimum cast iron disk thickness. I have the MG cycle disks. I keep a careful eye on the pad thickness just so I won't end up with the metal plate wedged in between the caliper and the disk.

Does it bother me.....hold my beer.... I say drill and tap the caliper piston and thread in an elevator bolt so I can make adjustable length pistons 

[n3303j]

Does it bother me.....hold my beer.... I say drill and tap the caliper piston and thread in an elevator bolt so I can make adjustable length pistons 

BUT ITS NOT THE PISTON COMING OUT OF ITS BORE AND LOOSING BRAKE FLUID THAT GENERATES MY CONCERN.

It's the steel backing plate on the brake pad moving beyond the machined support channel in the caliper body and getting wedged in between the caliper body and the disk. There is a reason the caliper has a pad plate support track machined into its body!

Easy enough fix if you remachine both caliper faces to bring them closer together. (Or run a disk that matches the original engineering specifications.)

[scra99tch]

Well that does seem like a cause for concern that the stainless disks are at or below minimum thickness. I take it also that these pads are not pinned into the caliper and just float.  My biggest concern was the extended piston and warping issues caused by less material on the disk.  Other than that its probably a fairly safe bet that I can use these for some time with new pads just to get on the road and at some point maybe get a floating set of disks and possibly upgrade the calipers. 

What I had aimed to achieve was not having to buy a new disk while I repaint and restore the other front disk.


Thanks for your replies.
 

[scra99tch]

Just read through this post.

https://wildguzzi.com/forum/index.php?topic=106546.0

[n3303j]

Yes the Pads are pinned into the caliper.

But think about the grade, number and diameter of the bolts that mount the rotor. Then consider the same features of the bolts that mount the caliper and those that hold it together. Those fasteners are designed to absorb braking energy from a 800# machine being brought from top speed to a full stop in the shortest amount of time.

The pins holding the pads are ONLY to locate the pad into the frame of the caliper. There is no way they would absorb maximum braking force without deforming or failing.

You can run a slightly undersized disk. Just keep an eye on the pads to be sure the metal backing plate is always supported by the caliper body when the brakes are applied. That is what I am doing with the stainless replacement disks on the T3.

[Perazzimx14]

Yes the Pads are pinned into the caliper.

But think about the grade, number and diameter of the bolts that mount the rotor. Then consider the same features of the bolts that mount the caliper and those that hold it together. Those fasteners are designed to absorb braking energy from a 800# machine being brought from top speed to a full stop in the shortest amount of time.

The pins holding the pads are ONLY to locate the pad into the frame of the caliper. There is no way they would absorb maximum braking force without deforming or failing.

You can run a slightly undersized disk. Just keep an eye on the pads to be sure the metal backing plate is always supported by the caliper body when the brakes are applied. That is what I am doing with the stainless replacement disks on the T3.

I would imagine the tire would slide long before even one disc bolt bolt got anywhere near is shear point let alone 6 of them.

[n3303j]

That's the point.
But if the pads were depending on the two locator pins as an anchor I bet the pins would fail. Especially in a hard stop with a high friction brake pad using a high traction tire on chip seal.
Have to engineer for the most severe application and then a bit beyond.

But the real issue here is just being sure that the brake pad backing is always supported by its track in the caliper body. Not just hanging on the pins.

[GuzziNZ]

Take from this what you will and I know I will get grief for this but here's my 2 cents.
Disc on my T is all but new and measures 6.10.
A friend with a collection of Guzzi's including a very quick racing sidecar said all his discs are sitting around 5.25. He rides hard and said he has never had a issue.
As I said make of this what you will its only one person's experience.

[TN Mark]

If it’s ‘ok’ to run brake rotors below their MIN thickness it should be equally ‘ok’ to run tires past the wear bars.

Come on people, these aren’t difficult situations to figure out.

[Perazzimx14]

If it’s ‘ok’ to run brake rotors below their MIN thickness it should be equally ‘ok’ to run tires past the wear bars.

Come on people, these aren’t difficult situations to figure out.

I get what you are saying but not a linear comparison. Tire wear bars are more related to the tires ability to channel water than the safe running thickness of the tire.

[TN Mark]

True.

And a brake rotor thickness, in great part, determines effectiveness at stopping the vehicle safely and without warping etc.

[GuzziNZ]

Anyone think 0.30mm is an insanely small margin of wear for a cast iron disc rotor?
Funny story about a brand new disc rotor the friend who races Guzzi's brought.
He set it up and started to drill cross holes until he hit a hard spot. On close inspection he could see a cavity and when he chipped away at it found a small intact ball bearing that had been cast into the disc.

[wirespokes]

If it’s ‘ok’ to run brake rotors below their MIN thickness it should be equally ‘ok’ to run tires past the wear bars.

Perhaps you don't remember Bert Munroe setting a speed record of 200 mph on race tires he 'made' himself by shaving off all the tread?

Just because a minimum thickness is specified, doesn't necessarily mean it's unsafe to use undersize. Who knows? Maybe the spec was modified by the lawyers? Did the factory determine catastrophic failure is definitely imminent below that spec? We don't know how that limit was determined, and the reason for this thread.

The engineers aren't always right. For instance, when disc brakes entered the motorcycle world, the master cylinders were way too large. I know that's true for Guzzis, and BMWs too. Eventually they got it right, but it took many years. So far, we haven't heard of any catastrophic failures from running thinner discs than spec'd. But it is a good point to pay attention to the fit of the pad backing plate to the caliper.

[Antietam Classic Cycle]

The engineers aren't always right. For instance, when disc brakes entered the motorcycle world, the master cylinders were way too large. I know that's true for Guzzis, and BMWs too.

The V7 Sport/750S had a properly sized master cylinder (PS15 for two F08 calipers), but the Eldo and 850-T still had the same master cylinder with a single F08. I think Guzzi (DeTomaso) may have been pinching pennies, figured owners would upgrade to a second disk anyway, the mounting lugs were on the fork and the parts to do so were available.

BMW saddled themselves with the ATE caliper(s), which seemed to be most of their issue.

[wirespokes]

The V7 Sport/750S had a properly sized master cylinder (PS15 for two F08 calipers), but the Eldo and 850-T still had the same master cylinder with a single F08. I think Guzzi (DeTomaso) may have been pinching pennies, figured owners would upgrade to a second disk anyway, the mounting lugs were on the fork and the parts to do so were available.

BMW saddled themselves with the ATE caliper(s), which seemed to be most of their issue.

I know DeTomaso was cutting corners, but don't think he had anything to do with the master cylinder size. And I don't see how a different size master would cost more or less. But BMW went with German-made ATE calipers (understandable), which, when adjusted and in good nick, work pretty well. But the Italian Brembos are definitely better and less fiddly, and won out. In the case of the airhead brakes, it was more the overly-large master that was the issue even after switching to Brembos in 78.

[Antietam Classic Cycle]

I know DeTomaso was cutting corners, but don't think he had anything to do with the master cylinder size. And I don't see how a different size master would cost more or less. But BMW went with German-made ATE calipers (understandable), which, when adjusted and in good nick, work pretty well. But the Italian Brembos are definitely better and less fiddly, and won out. In the case of the airhead brakes, it was more the overly-large master that was the issue even after switching to Brembos in 78.

I guess I wasn't clear enough - I surmise that the bean counters at Guzzi didn't want to include the second disk and caliper as standard in order to keep the cost of the Eldo and 850-T down. Nothing to do with the cost of the master cylinder. As I wrote, they likely kept the PS15 master cylinder because they "figured owners would upgrade to a second disk anyway, the mounting lugs were on the fork and the parts to do so were available."