Let's do this Again!!(Brembo-Fourpot Installation)

[Julio A.]

Ok, so I had a little spare time this weekend. Before I do anything potentially dangerous to the bike I always chime in for some advice.

I have measured, measured again and measured a little more until I have the exact number and quantity I have to mill out of those four-pot calipers before I can install them into my dual shock 81' R65.

I need advice on whether to go ahead or not.

I need to drill off the threads then mill out 5mm off both the mounting tabs, as marked by the shiny pencil lead on this picture:


After that, it's then straight bolt-on and smooth sailing.

What do you think? I'm concerned on the structural integrity of the calipers after milling.

[Bob_Roller]

That's my first thought, you are going to remove from the looks of it, 50% of the material .

Just quoting from my aircraft maintenance experience, if you remove more than 10% of the material, as in cleaning up damage, gouges, etc...... the part is unusable .

[tvrla]

Did you dry fit them to the bike and determine they need to be offset that much?

[John M]

Hi Julio, That is a lot of metal to remove. When I installed a similar 4 pot caliper on my 1986 I mounted the caliper without pads installed on the bike an took my measurements from disc to face of piston on each side and then took I/2 of the difference between the two sides to determine how much metal had to be removed ( it was not as much as your photo shows. Perhaps you have already done this, good luck a very good upgrade.

[mrbindc]

what model number are those Brembos?

[DgM]

Julio, fellow bovine wrangler, stop what you are doing, take a deep breath, figure out another solution for increased braking power, congratulate yourself on avoiding disaster.  The lure of the four-pot is strong, but like the song from a siren on a rock, you are being drawn towards destroying any structural integrity of the caliper thus endangering yourself when (not if) braking forces overpower what is left of the four-pot caliper and it fails.  The factory twin disc Brembo setup can lock the front wheel at just about any speed, sourcing parts, rebuilding, installing will be much less than hospital bill from crash.

[Graeme]

I've purchased a set of 4 pot Brembos for the LS fairly cheaply while I thought the getting was good and intended to work on them as I approached the need to replace the existing discs; however, I've thought about it later rather than sooner.

Is there going to be an appreciable increase in useable braking force?

Your hand exerts the force on the Brake Lever which transfers this via the Hydraulic Fluid to the pads.

Regardless of the size or numbers of the pads, this force is not going to change unless there's a booster of some sort.

Therefore the force transmitted is spread as Pressure over a larger area and will do nothing to increase Braking effort. (I.E. Same force, larger area, less pressure per, say, square mm.)

So, theoretically, the Braking effort will remain the same.

I wondered why owners of the LS don't rave about the advantage of the extra disc. I think that now, it's because there isn't any for the above reason.

One would probably get extra life out of the pads.

So my set will be hanging in the shed until I can be sure the effort will be worthwhile & safe.  

John M. did you see an appreciable improvement? There may be a chance that I haven't wasted my money.  

[Julio A.]

Adding another Disk in a otherwise single disk set-up will not increase braking pressure, but rather distribute that same amount of pressure over a larger surface area. That is what creates the stronger braking force.

We should not be looking at the quantity of force since it is a fixed value given by the Master Cylinder but rather look on the surface area the pads have on the rotor. Attaching the four-pot caliper would not increase the force exerted on the pads, but rather distribute all that force into four pistons which would push a larger brake pad with a larger contact area into the rotor.
_________________

My best bet so far is having an adapter plate created to give me the desired offset. This should negate a need to modify both the fork and calipers.

[Graeme]

Adding another Disk in a otherwise single disk set-up will not increase braking pressure, but rather distribute that same amount of pressure over a larger surface area. That is what creates the stronger braking force.

We should not be looking at the quantity of force since it is a fixed value given by the Master Cylinder but rather look on the surface area the pads have on the rotor. Attaching the four-pot caliper would not increase the force exerted on the pads, but rather distribute all that force into four pistons which would push a larger brake pad with a larger contact area into the rotor.
_________________I think that's right Julio. But it will push with the same force, larger area. Therefore less pressure per unit area. I would like to be wrong here as I've already shelled out for the Calipers.

My best bet so far is having an adapter plate created to give me the desired offset. This should negate a need to modify both the fork and calipers. That'd be good  

[tvrla]

Graeme - I don't think you can logically sort through this conundrum. Perhaps there are other factors not being considered in the equation.

The truth of the matter is that replacing a two-pot caliper with a four-pot will give increased braking power. Adding a second disc to the front end will greatly increase stopping power - that's why the sport bikes always had two discs up front.

And comparing a two-pot to a four-pot, the pad surfaces may be comparable, but the four-pot will still stop better. The reason being that the pad is more firmly pressed against the disc with the greater number of pistons.

All that said, I find the R65 dual discs plenty sufficient! A while back I had an experience at seventy mph or above where I was accelerating to pass, then suddenly had to get back in line. The brakes hualed me down very well! It was a very touchy few moments but all went well. I have no complaints at all with my dual two-pot Brembos!

[Graeme]

What caused me doubt was the equation p=F/A. If the force (F) remains the same & you increase the area (A), then the pressure (p) must decrease.

[Barry]

I don't think you need tro be concerned Graeme. Julio has it right. For a given pull on the brake lever the master cylinder produces a fixed pressure in the hydraulic fluid and that won't change regardless of the number of caliper pistons.  The pressure is converted into force at the caliper pistons. If you double the caliper piston area you double the total force on the brake pads therefore all other things being equal double the braking effect. Brake pad area and coefficient of friction are other variables though.

4 pot calipers of the same total area as 2 pot calipers are supposed to better not because they produced more force but because they push on the pads more evenly. For that to matter presumably the pads on 2 pot calipers must distort under braking pressures and not push squarely on the disk.  When I think about this I'm not completely convinced and there must be other benefits of 4 pots.

I can see another way that 4 pots are better.  For any given brake caliper you get better braking with a larger diameter brake disk (or incidentally smaller wheels). The effective diameter of your brake disk is the average of  the brake pad contact area. 4 pot calipers tend to have wider pads of lower height so even for the same overall diameter disk the effective diameter that the pads work on will be slightly larger. 4 pots are working on average nearer to the outside edge of the disk so gain slightly more mechanical advantage.

[Graeme]

OK, Barry, so what you are saying is that the Master Cylinder has "stored" the energy and this can be released to the pistons  which will maintain the same force on the pistons provided that the Brake remains activated at the handle bar?  

Yes, I get it.

Thanks.

Just going back to wirespokes post on the "logical" aspect. That's right to, now that I've considered further. If one was to work it out mathematically, you would find that it did require more force which would be supplied at the handlebar but so small that you wouldn't notice.

 

[Julio A.]

Ah... The Joys of Hydraulics.  

Can anyone confirm if the OEM Brembo's on dual shock R65's are 36 or 38mm? I'm still having that size discrepancy between the one I have and what the internet tells me.

[tvrla]

Julio - wasn't there another thread recently where someone posted a comparison of the ATE and Brembo calipers? Dimensions were included in that write-up.

Another factor to consider in this equation is volume of brake fluid, besides the pistons helping to dissapate heat build up - after all, the brakes work by converting mechanical energy into heat. The more pistons, the larger the heat sink. And with more fluid, it doesn't heat up as fast - but that's just a guess. Brake fluid doesn't work as well the hotter it gets.