Can I add a caliper to the rear brake circuit?

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I've added a sidecar to my st1300 and it needs a brake on the sidecar wheel. I want to add a double banjo bolt to the top of the rear master cylinder, and a line to an additional caliper mounted on the sidecar swingarm.

My thought is that once the system is bled, the sidecar caliper and rear brake caliper will receive the same amount of pressure. I'm worried that the existing master cylinder might not push enough volume to accomplish this. Thoughts?
 
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Great question,,,, and,, I have no idea. But we have some very experienced people here who should be able to offer an opinion. Be sure to post a photo or two of you project,, as it sounds really interesting. Cat'
 

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You may be better off looking into an electronic brake instead. Master cylinders are sized for the caliper they will activate. Adding an additional caliper will double the size and the master will not be able to provide enough volume and/or pressure to work them both properly.
 
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It is a good question, but the caliper piston doesn't take much fluid to move. The master cyl. on the front has enough to move a lot of pistons (5), the rear moves 4. The diameters of the masters are about the same...... I'd give it a shot, but you should match caliper piston sizes on the rear to the same size I would think, or go with a single pot caliper.
 
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playinatwork
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It is a good question, but the caliper piston doesn't take much fluid to move. The master cyl. on the front has enough to move a lot of pistons (5), the rear moves 4. The diameters of the masters are about the same...... I'd give it a shot, but you should match caliper piston sizes on the rear to the same size I would think, or go with a single pot caliper.
I have a single piston caliper from a Yamaha XS650 that I plan to use. I'm putting on the front wheel and rotor off the same bike.
 
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It is a good question, but the caliper piston doesn't take much fluid to move. The master cyl. on the front has enough to move a lot of pistons (5), the rear moves 4. The diameters of the masters are about the same...... I'd give it a shot, but you should match caliper piston sizes on the rear to the same size I would think, or go with a single pot caliper.
I think this question does not have an easy answer, since there are many things to consider. The biggest variable to consider is probably the empty weight of the sidecar compared to the weight with a passenger, which in itself is another variable depending on the weight of the passenger. I don't know if there is a solution for that, the answer to that might have to be a compromise somewhere between the two extremes. Another consideration is if the brake on the side car is activated from the rear master cylinder, the LBS would not apply that brake. Since the weight of the side car will be much less than the weight of the motorcycle, it will require considerably less braking than the rear wheel on the bike. That could easily be accomplished by using a caliper and/or brake pads with proportionally less pad surface and/or combined piston bore area, but it would take a ME to come up with an answer to that potential problem. Whatever is done to the bike/side car combination to best maintain handling stability, there will very likely be a learning curve required to practice the most effective braking techniques. The electronic braking system that Byron suggested may be the most practical approach, especially if it is a system that can be fine tuned to compensate for the variables mentioned.

Sorry for such a long winded reply, I just wanted to reply with all the considerations that came to mind.
 
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From my own messing around with calipers and masters, if you add a caliper to the 13's standard system, you will end up using slightly more pedal travel to apply force to the brakes, and as the ratio between the master cylinder area and the total caliper piston area has reduced, you will have a more brake power for a given amount of foot force.

In a perfect world I'd suggest you need to get a master cylinder with a proportionately bigger piston area to offset that, but I don't think such things are readily available.

The standard rear master cylinder for the ST13 has a 17.46mm i.d. which is already at the upper end for Honda motorcycles. For comparison the non-ACS ST11 uses 12.7mm, and the ABS/TCS model is the same as the 13. Other bikes like a CBR600F4 use a 14mm and that would appear to be the most common size.

This one might be a suck it and see job I think. Given my own (very) limited experience with a sidecar (one word: terrifying!), if you can cope with the assymetrical nature of the forces acting during acceleration, I doubt some odd brake behaviour would be the least bit concerning.
 
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playinatwork
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Thanks Don.
I like the idea of the electronic actuation but from what I've seen I can't find anything that really fits my application. I have the largest car tire I can fit on the back of the bike and the smallest bike tire I could find on the sidecar rim. The difference in contact patch will also have a lot to do with my problem. Equal pressure to the sidecar brake will likely skid the sidecar tire while applying very little pressure to the bike rear tire. I will set the proportioning up for having a passenger in the car as that will be how the rig will be run primarily.

Another consideration is that the bike has a 1 degree lean out from the car so it will run straight down the road. This means that when you grab the front brake the bikes veers left. I want to counteract that with the sidecar brake. I'm also considering not using the bike rear brake but I think that will open another can of worms.

I've already tried running the sidecar without a brake on the car with almost disastrous results. I want this thing as close to right as possible before I ask my wife to set in it again.
 
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playinatwork
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Given my own (very) limited experience with a sidecar (one word: terrifying!), if you can cope with the assymetrical nature of the forces acting during acceleration, I doubt some odd brake behaviour would be the least bit concerning.
So far it's been as close to a death machine as I dare get.
 
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I don't think this is as big a problem as has been suggested. Ural motorcycles have had side cars for years (with brakes, I believe) and there are a lot of Guzzi's out there running side cars. I suggest you google side cars and find a shop that installs them and sets up bikes (or a Ural dealer). Go talk to the mechanics there, these problems that you are discussing are not new.
 
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Just look at the list of problems guys have with the brakes. I would not fool with the system, and i don't like surprises, rather go with something electronic or gravity driven.
 

ST Gui

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If you add a second caliper a larger master cylinder piston will be needed.

I added a second front disc and caliper to my 750 K-4 Honda along with a longer banjo bolt. The brake lever went to the bar with little resistance and no stopping power. But I had another master cylinder assembly at the ready.

It too was from a Yammie which may have had factory dual discs. Bolted right up and may have even accepted the Honda lever but I think I had dog-leg levers anyway.

It worked great. I don't remember exactly how the converted dual disc lever pressure felt compared to the previous single disc set up but it was close and the brakes were awesome.

The LBS will mean the front brake will also help but I don't know how much fluid is moved compared to my 750 conversion.
 
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