Question on brakes - thinking of using calipers with larger pistons

[MrNomad]

This question is for those builders who are experienced with various braking systems.

We're building a 6A and ended up with new calipers that have a single piston that's approx 20% larger than the standard caliper piston used on Van's airplanes. These calipers are Cleveland model number 30-75X.

If we use the standard flange (see attached pic, flange to the rite), these calipers will work after some minor trimming of the pad. The goal is to enhance the braking via the oversize piston.

The question is, has anyone used 30-75X calipers and what was the result?

[IowaRV9Dreamer]

HI Barry - this in an interesting idea. Going to a bigger piston will generate more force, but will it allow more braking? I thought our brakes were limted by the amount of energy (heat) they can dissipate without overheating. Maybe you'll also add bigger rotors?

[MrNomad]

Originally Posted by IowaRV9Dreamer

HI Barry - this in an interesting idea. Going to a bigger piston will generate more force, but will it allow more braking? I thought our brakes were limited by the amount of energy (heat) they can dissipate without overheating. Maybe you'll also add bigger rotors?

Thank you Dave. The reason I posted the question was to get quality feedback such as yours. The present plan is to use the standard rotors but if the extra force warps the rotors, we'll nix the idea altogether. Hopefully, additional feedback will come in along with actual experience.

[DanH]

The critical issue with the standard brakes is disk mass, ie the ability to store heat.

Compared to stock, a caliper with a larger piston diameter will increase pad clamp force for the same applied force at the master cylinder. It will make it easy to lock the wheel (zero benefit), but it adds no additional energy storage capacity. To do that you need a larger, thicker disk.

The 30-75X caliper is in fact used with a 6" wheel, 1.5" axle, and 164-04000 disk. The larger wheel diameter makes it harder to lock. The big disk makes the kinetic energy rating 200,000 ft lbs per wheel. Sounds like an RV-10?

The kinetic energy rating per wheel is 117,500 ft lbs for the stock RV-6 brakes. The best upgrade is a Cleveland 199-93 thicker disk kit used with the stock 30-9 calipers....kinetic energy per wheel becomes 155,500 ft-lbs.

Consult Cleveland drawing 50-76 for general brake package details.

Now a true story......many moons ago a certain younger, dumber fellow decided his roadracing bike needed more front brake. Being poor but inventive, he cleverly borrowed a smaller diameter master cylinder from his pit bike. The warmup laps were wonderful....the brake now had lots of power with only a two-finger squeeze. All remained good until the first 10/10's lap and the end of the main straightaway, WOT to the last foot then maximum braking....which lasted only a few seconds before reverting to no brake at all.

I had exceeded the kinetic energy storage capacity of the stock, stamped steel disk, which had instantly warped and knocked the pucks back well beyond any distance I could pull with the lever.

Don't think I'd put those big calipers on those little disks if I were you

[terrykohler]

Barry:
The standard brakes are more than adequate. Using the oversize piston probably won't hurt, but honestly, the brakes are mostly for turning. Even on short fields, it's really about proper approach speed and landing on your target - if you have to depend on your brakes for rollout, you may be in a field that you can't fly out of. Practice flight at MCA. Practice flying a stabilized approach. Much more important than big brakes.
Terry, CFI
RV-9A N323TP

[647jc]

May also want to consider the nose gear. The harder you brake, the more weight transfers to the nose gear.

[kenmorris]

Brakes have been discussed a lot.

http://www.vansairforce.com/communit...ead.php?t=9267

[IowaRV9Dreamer]

Originally Posted by 647jc

May also want to consider the nose gear. The harder you brake, the more weight transfers to the nose gear.

That is a VERY good point. I was planning on doing the mod that Cleveland has for additional kinetic energy (thicker disc, and I think different pad material).... but Raiz's model has given me a lot to think about. Sure wouldn't want to brake hard - hit a bump, and fold my NLG

[Beancounter]

Like Dan I too used to roadrace bikes. I suspect Dan did it back when bikes had solid front discs so he has a different experience than I. I think that the primary performance restriction in RV brakes is the ability of the disk to expand radially. Modern bike brakes can absorb tremendous heat while remaining flat because they have floating rotors. I believe warping in aircraft brakes occurs because the braking surface is rigidly attached to the rest of the hub. When heated it cant grow radially and so it warps. Going to a thicker disk is not addressing the root problem. If you could just get a machinist to create some floating rotors or even cross drill the rotors, I believe you could put a lot more heat into the brakes. Then you could explore ways to improve braking power.

My RV's brakes have no modulation or feel. Its like stepping on a brick. I would like to change the caliper piston size just so I get more pedal movement. Within reasonable economics I want the best brakes I can get. This sounds a lot like the RV9 320 vrs 360 or the FP C/S prop debate. You dont use it all the time but excess brakes(power) is nice to have when you are in a bind (pun intended). Is there a history of warping, I would be surprised unless someone was doing repeated stops like during hi speed taxi testing. Don't let the naysayers stop your experimenting. Mr Nomad please let us know how it works out.

Matt

[MrNomad]

Special thanks to Dan for his in-depth reply. That kind of intelligent analysis was precisely what we were looking for as we build this 6A. I build classic Chevys and the difference between power brakes and standard is what we hope to accomplish via the larger piston. Less foot pressure should deliver the same clamping force.

We all (obviously) agree that less braking is desirable for many reasons, not least of which is the load on the front wheel. And we all agree that a rotating wheel provides direction whereas a skidding wheel facilitates an incident. Finally, my front wheel NEVER touches the ground until lift is gone.

But there has to be a theoretical limit to how much load is transferred to the front wheel (for an airplane on level, dry pavement, no bumps) and that would seem to occur just before brake lockup occurs. An increased piston size should require less foot pressure to achieve the same front wheel load.
One factor that was listed elsewhere dealt with piston travel which we will closely monitor. We appreciate Ken?s links to other posts.
If we pursue this option, I?ll report back so everyone can share our experience. What prompted the idea was accurately stated by Matt: ?My RV's brakes have no modulation or feel. It?s like stepping on a brick?.
Exactly!