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Message started by T And T Garage on 01/19/17 at 13:36:32

Title: Rear Disk Brakes
Post by T And T Garage on 01/19/17 at 13:36:32

Has anyone ever done rear disk brakes on a hard tail mod?

Does anyone have any experience with a single foot brake and a proportioning valve to both front and rear?

Title: Re: Rear Disk Brakes
Post by Dave on 01/19/17 at 14:25:11

I would be very cautious about tying the front and rear brake together on the Savage. The conditions that results if the wheel on either end of the bike locks up is scary. I have no idea how to make a system work like you are proposing - where you have the same size caliper and disc front/rear. The pressure reduction to the rear caliper will have to be considerable.

My Honda NT700 has dual discs up front, and each caliper has 4 pistons (2 pairs per caliper). On the left front disc 1 pair of pistons is tied into the foot pedal for the rear brake - so when you apply rear brake you get a small amount of front brake as well. The system seems to work well and I have never had the rear or front brake lock up while braking. When using both the front and rear brake together - you have the lever controlling 3 pairs of front brake pistons, and the pedal controlling 1 pair of pistons in the front, and the rear caliper.

Title: Re: Rear Disk Brakes
Post by batman on 01/19/17 at 17:44:13

TNT , you basically have two paths to go on ,If you want to stay with the disk you need to get the brake master cyl. and reservoir , that matches and make a linkage to activate it and the tail light ,it can be done,I used wheels from a 550 -4 Suzuki on a an old 750-4 Honda. you'll also have to deal with fitting the caliper on the shaft aligned with the disk and anchor the brace to the frame, or drop the disc and use the Suzuki drum hub and get it spoked to an 18" rim .good luck!

Title: Re: Rear Disk Brakes
Post by hotrod on 01/19/17 at 19:59:17

Moto Guzzi uses a system that works very well, but has 2 disks in front and 1 rear. A foot pedal operates one front, and the one rear. The brake lever on the handle bar operates the other disk on front only. IMO, I don't think any Savage needs a rear disc. If you can lock it up, how much more do you need ?

Title: Re: Rear Disk Brakes
Post by Armen on 01/19/17 at 20:37:59

There were some folks racing Guzzis in the late 80's early 90's who used only a front master cylinder for all three brakes. Basically, they tied the front and rear into a brake portioning valve and dialed in as much rear brake as possible until the ass end locked up under braking, then backed it off a bit.
IIRC, it worked pretty well.
The idea was that when the rider was hanging off the bike, it was hard to get a foot on the rear brake and modulate it with any accuracy.

Title: Re: Rear Disk Brakes
Post by hotrod on 01/19/17 at 21:06:35

Some Guzzi racers also went back to the more conventional system, to use, or not use the rear brake. Many racers had rear brakes because the bike came with them, and they had to have them on to get through tech inspection.

Title: Re: Rear Disk Brakes
Post by batman on 01/19/17 at 21:22:55

But having the front and back brakes tied together on a hardtail bike sounds really bad to me. there's no rear suspension ,hit the brakes then a large bump and the rear wheel comes off the road, locks up, and comes back down locked ? I'd think about skipping that.

Title: Re: Rear Disk Brakes
Post by T And T Garage on 01/20/17 at 06:56:25

All great advice from everyone! I think we're going to move forward with our plan of a disk in the rear.

We've got a second front caliper and rotor (a matching set if you will) and it fits beautifully on the new rear rigid frame. We're going to go with a proportioning valve as mentioned above and dial it in so as not to lock the front up.

This may indeed turn out to be a terrible idea - but we want to be unique in our build. We're just looking to see what's possible - what works and what doesn't.

Any recommendations on remote master cylinders?

Title: Re: Rear Disk Brakes
Post by Armen on 01/20/17 at 15:24:54

Some of the XJ750 Yamahas used a cable operated remote front brake master cylinder. The bike had dual front discs, so the master should be close to what you want. The Savage uses a cable for the rear brake, so you might be able to get them to talk to each other ;)

Title: Re: Rear Disk Brakes
Post by batman on 01/20/17 at 15:37:39

Sorry, but it is a terrible idea to link both brakes together !first off a proportioning valve is used to keep the rear brake from locking up not the front,but even so it's at a point that the rear tire leaves the pavement(bump or pothole) it has a chance of locking up ,say when the clutch is used and your shifting,if the bike is leaning the rear tire could brake loose and cause you to go down .You and the bike could sustain serious damage! I would refrain from using the rear brake at any time the bike isn't completely upright.

Title: Re: Rear Disk Brakes
Post by Armen on 01/21/17 at 18:46:38

I know I started out by giving you some info, but the more I think about this plan, the less thrilled I am. Gotta agree with the guys telling you this is a bad idea. When my buds ran linked brakes on their Guzzi racers, they dialed in the proportioning valve to each race day. A change in track or even track condition meant fine tuning the brake balance.
Looking cool and being dead, in the end, is still dead.

Title: Re: Rear Disk Brakes
Post by Gary_in_NJ on 01/21/17 at 20:20:27

Without electronics to manage the system (ABS) linked braking would be a disaster. What is the point of this linked system? Tired of the long reach to the front brake handle?

Best of luck.

Title: Re: Rear Disk Brakes
Post by ohiomoto on 01/22/17 at 06:33:42

All of my motocross bikes from the early 90s on had a rear disc brakes. The rear disc was always substantially smaller than the front. A smaller disc is less powerful and easier to modulate.

Also, the rear brake is critical in many situations. Many in these parts claim it's not needed, but it can help keep you from stepping out the rear wheel in a panic stop (I know a guy who died road racing when he lost his rear brake) or purposely putting the rear into a controlled slide if needed (as I did this past summer to avoid a guy who pulled a U-turn on me).

I say go for the disc if you like just take care to make the system weaker than whatever you use up front.

The idea of linking them with one pedal is just stupid and could result in someone's death. You really need to be able to do different things with the front and rear brakes at different times.

Title: Re: Rear Disk Brakes
Post by Dave on 01/22/17 at 06:50:28

Using the same size disc and caliper on the front/rear is definitely going against logic. The rear does less work, and bike/car/truck builders all use smaller discs and caliper pistons on the rear so the brake is less powerful.

There is no doubt that using the front disc/caliper on the rear will make the rear brake tough to modulate....even with a proportioning valve. Maybe if you can find some really hard compound pads......or a caliper with a really small piston.

Title: Re: Rear Disk Brakes
Post by Kris01 on 01/22/17 at 08:22:18

The rear brake only does ~30% of the braking anyway.

Title: Re: Rear Disk Brakes
Post by justin_o_guy2 on 01/22/17 at 08:40:43

What it does is
Everything the front brake doesn't, minus engine, which Still uses rear tire Traction and aero drag.
The more front brake is applied, the more weight transfer to front, the less traction is there for the rear tire.
So, the harder you are stopping, the less rear brake you can use. Unless you believe that in the fraction of a second prior to making a Stoppie the rear brake is affecting speed.

Title: Re: Rear Disk Brakes
Post by DieselBob on 02/12/17 at 22:02:25

Since I was posting this bike elsewhere on the forum, I'm dropping this pic into the mix. Contrary to recommendations thus far, this lad appears to have a rear disc that's substantially larger than the front. And unless one of us is conversant in Chinese, we'll never know how it worked.
I'm not wild about his brake line routing. And we're going to agree there's another stay on the backside of the shock mount that's simply not visible in this picture.

http://img2.imgtn.bdimg.com/it/u=262387953,3350711850&fm=214&gp=0.jpg

Title: Re: Rear Disk Brakes
Post by batman on 02/12/17 at 22:50:53

DieselBob, The use of a disc brake on that bike may even be as good an idea as that oil line running to the end of the cam shaft,that is to say useless! Looking at the bike in question, Bad exhaust ,rear disc mounted backward ,bad seat cover ,etc.I'd say the builder is clueless.Any mod done to your bike should be safe and functional ,rather than just to be different.