Replacing Front Brales on 1980 FXS

[scooper321]

I never said it wasn't and yes you have to ask questions to learn.....

Here is my Original paint 16,000 mile 79 FXS that I totally restored in 2012, Took it totally apart split the cases and all..

Hey - I hope I didn't seem snarky. I didn't mean it that way. I appreciate the comments. So, thanks!

And wow. Nice bike. I see mine in there ... sort of. It's not stock paint, but that's OK. The paint on it is actually in really nice shape so that saves me a few grand $ on the rebuild. And the engine seems to run well. The speedo says 29k, but the speedo cable was broken when I bought it (ha ha). The engine leaks a little, so that's one of the goals of the rebuild. And I hope to God that I don't have to split the cases. That's well outside my comfort level! But I'm optimistic. My bike just needs a little TLC, so I'm going through it, piece by piece, replacing every gasket and anything rubber, cleaning everything, repacking or replacing all races and bearings and making her run safe and well. I'll get into the engine and inspect/replace whatever's needed. I don't plan on putting her back to stock, but she'll look sort of stock when I'm done. Here's what I started with:

[scooper321]

I never said it wasn't and yes you have to ask questions to learn.....

Here is my Original paint 16,000 mile 79 FXS that I totally restored in 2012, Took it totally apart split the cases and all..

By the way, I love that exhaust. I've got my eye out for one. I'd love to go back to the original 2-into-1 on this baby.

[farmall]

Fab Kevin makes mounting brackets for plentiful Tokico calipers to replace the stockers. I have a pair but haven't gotten around to installing them.

[humancertainty]

Fab Kevin makes mounting brackets for plentiful Tokico calipers to replace the stockers. I have a pair but haven't gotten around to installing them.

I didn’t know he made them for the 77-83 forks. He isn’t afraid of odd jobs, though. He made me brackets to run Tokicos on a 39mm dual disc setup with a 70s Ironhead wheel with 10” rotors.

[Tattooo]

Here's what I started with:

[ATTACH=CONFIG]81544[/ATTACH]

That's a damn nice looking bike... I'd just go through it the best you can and enjoy riding it like it is...

The only reason I went through my bike like I did was when I bought it it had been sitting on a dirt floor in shed taken apart for 25 years.. It was a mess but all in all it was all there and still stock... The brake calipers looked like new inside but needed painting... Everything needed painting except the skins they were hanging on the wall...

[scooper321]

I didn’t know he made them for the 77-83 forks. He isn’t afraid of odd jobs, though. He made me brackets to run Tokicos on a 39mm dual disc setup with a 70s Ironhead wheel with 10” rotors.

According to his website, as a stock product, he doesn't. But people on here and some other online forums have shown his work on Superglides and other bikes. I just emailed him to see if he's ever done a bracket for a 35mm shovelhead-era bike or what a custom bracket would cost. I know I can get brackets for my bike from Jaybrake for their calipers, but they are like $120 EACH, just for the brackets!

[scooper321]

That's a damn nice looking bike... I'd just go through it the best you can and enjoy riding it like it is...

The only reason I went through my bike like I did was when I bought it it had been sitting on a dirt floor in shed taken apart for 25 years.. It was a mess but all in all it was all there and still stock... The brake calipers looked like new inside but needed painting... Everything needed painting except the skins they were hanging on the wall...

Thanks. I'd be lying if I said the appearance of the bike didn't have an effect in me buying that one. It was garage kept, but not ridden much and not serviced much (although the wheel bearings had been replaced or repacked at least once, I've found). I rode it home when I bought it, which turned out to be a BAD idea. As I've started tearing into her, I've found all sorts of scary things - like really loose axle and fork nuts. Next classic bike I buy I'm bringing home with with a truck...

[scooper321]

So I made the decision to go ahead with the front brake mod using a FabKevin bracket and the Tokico 2 piston calipers. That way I can stay with my current 35mm forks. From what I’ve read, the SV650(from which these calipers came) used a master cylinder bore size of 5/8”. My 1980 FXS *should* have a 3/4” m/c, if it’s stock. I don’t believe it is stock (it’s chrome and the 80 FXS should have a black m/c). If it’s just a chrome drop in, it will have a 3/4”bore. If it’s a newer (85 or later) it’s likely 11/16”. I’ll have to inspect next time I’m at the bike. But I’m confused as to what impact the bore size has when going from the stock dual single piston calipers to dual 2 piston Tokico’s?

Are these Tokico’s giving me more piston surface area vs the stock single piston calipers, so that a larger than expected (3/4”) bore would need more force on the lever, to create more pressure, making the brakes mushy and stops take more force and more distance?

[Tattooo]

Pressure is caused by your hand, Volume is caused by the MC... You will need to measure your MC and use what size they recommend...

It also has to do with distance that the fluid has to travel...

[TriNortchopz]

"But I’m confused as to what impact the bore size has when going from the stock dual single piston calipers to dual 2 piston Tokico’s?
Are these Tokico’s giving me more piston surface area vs the stock single piston calipers, so that a larger than expected (3/4”) bore would need more force on the lever, to create more pressure, making the brakes mushy and stops take more force and more distance?"

Some research which may be helpful:

The brake caliper is where the multiplying power of hydraulics comes into play. Since pressure applied to a confined liquid is transmitted undiminished and with equal force to all surfaces within the system, the pressure from the master cylinder is exerted uniformly on the much larger area of the caliper pistons, increasing the force many fold. https://www.motorcyclistonline.com/m...m-how-it-works

Consider the fact that the bore of the master cylinder is much smaller than the combined bores of all the pistons in your brake calipers. This means that (made up numbers coming up) for every 15mm you push the master cylinder in, you move maybe 5cc of fluid. But as that 5cc of fluid hits the pistons at the bottom, they don't move 15mm but in total like 0.5mm. This is where you get your hydraulic advantage from, and this is why you don't have to squeeze the brake lever hard enough as if you were actually grabbing the pads with your hand.
If you increase the size of the master cylinder, you will actually be REDUCING the power of the brakes. Yes, with a bigger master cylinder, you will have to squeeze HARDER to get the same braking force at the calipers. This is because you are reducing the hydraulic advantage you have over the calipers. Crazy, right? Maybe but it's still a fact.
If you decrease the master cylinder size, you actually GAIN in braking power due to the increased hydraulic advantage you gain. But, ...you also increase lever travel. In some cases it's so bad that the lever hits the bar, maybe trapping your fingers before you stop hard.
So wait, what fits what now? And how will I work it out?
Ideally, you will know what is standard to your calipers, and either fit the same size master or adjust by the descriptions above. However, of course, that's not always possible. So in that case, you can work it out by Doing Math. What you need to do, is work out the surface area of your slave cylinders' pistons and divide it by the area of your master cylinder piston.
The master cylinder will have (should) the size stamped on it.
Calipers won't have the sizes on them, but you can measure with a vernier gauge or to the nearest mm (they won't likely be in inches) with a tape measure. Measure the OUTside diameters!
Then you need to work out your ratio. Here is the formula, in plain english(?):
(total surface area of pistons on one side of each caliper [so you will get the same result on a 2-piston sliding caliper as on a 4-piston fixed caliper]) x (number of calipers) / (surface area of master clinder piston)
In maths terminology that's:
(((Pi x R^2) + (Pi x R^2))x n) / (Pi x R^2) http://www.customfighters.com/forums...ad.php?t=56704
Okay,got that?? Info is directly related to Nissan calipers, but calculation info is typical for any hydraulic brake.

A chart instead of math;
Front Master Cylinder Ratio Chart
...the importance of master cylinder to wheel cylinder ratios. This critical ratio is of paramount importance in determining "feel". It has been my experience that there is a "sweet spot" in the range. I like ratios in the 27:1 range-2 finger power brakes, feeling some line and/or caliper flex. 23:1 is at the other end of the spectrum-firm. Ratios lower than 20:1 can result a feel so "wooden" as to have a toggle switch effect: nothing happens until the wheel locks.
See chart and more here: http://www.vintagebrake.com/mastercylinder.htm

From V-Twin forum member, Dynatherapy:
It basically comes down to more VOLUME OF FLUID MOVED AND PRESSURIZED WHEN THE LEVER IS SQUEEZED, which is required for adequate brake actuation when more pistons (more volume of fluid) needs to be pressurized.

You may find that a 9/16 bore master will come right to the grip when hard braking is done where a 5/8 bore on the same caliper gives full brakes about 1/2 way through its travel.
Conversely, when replacing a 5/8 designed system with a 9/16, you may not get full braking and the lever may contact the grip before pressure required is achieved. This will become more prevalent when the flexible portions of your hydrolic brake lines age as they will swell slightly with braking, reducing the pressure supplied to the Caliper pistons.

Summary: 5/8 may achieve full braking sooner in the lever travel than a 9/16 would.
You CAN go to 5/8 FROM 9/16 stock but do NOT go to 9/16 FROM 5/8 stock.
For discussion reasons we will call the single caliper design a 9/16 and the Dual Caliper design a 5/8

A little Math here:
(100 / 16) X 9 = 56.25 - so 9/16 bore is .562
(100 / 16) X 11 = 68.75 - so 11/16 bore is .687
(100 / 8) X 5 = 62.5 - so 5/8 bore is .625

So you see, you are correct about 11/16 and 9/16 for the years in question, but your post was a question about 5/8 vs 9/16 so I followed suit of your inquiry "for discussion reasons" because I have NO IDEA who manufactured the master cylander in question and it could very well BE 5/8 bore.

You can go from 9/16 to 5/8 and still have a safe margin of lever travel in my experiences.
If you go from 9/16 to 11/16, the brakes come on WAY too hard and too soon, so your hand will be wide open and the brakes will lock suddenly with very little lever pull, making it VERY unsafe and very difficult to apply the brakes smoothly.

DO NOT put an 11/16 bore master on a single disc front brake system. http://www.v-twinforum.com/forums/tw...9-16-bore.html

What matters is not the size of the master cylinder in isolation, but the ratio of the area of the master piston to the area of the combined disc pistons. Remember that the area is proportional to the square of the diameter, so if you go from having two brake pistons to four brake pistons (doubling the area), the diameter of the master cylinder should be multiplied by sqrt(2) = 1.4, not doubled.
Having said that, there will be a range of master cylinder diameters which will work for a given set of brake pistons. A small master piston will be light to pull on, but will feel spongy and won't give full braking before the lever hits the bars. A large master piston will feel wooden, and will only give full braking if you squeeze very hard. Between these two is the optimum where the lever is neither spongy nor wooden, and gives enough to lock the tyre. https://www.reddit.com/r/motorcycles...der_bore_size/

[Tattooo]

^^^^^^ That's some damn good info right there....^^^^^^ That's why you need to use the recommended size MC........

[scooper321]

Great resources above, thanks. I had found a few of them on my own, but not all. The heavy math discussion is interesting because it’s the only one that really seems to explain the difference in going from a single to dual disc system (which I’m not doing, but is intereting). For equivalent bore sizes, is the only diff between and single disc cyl and dual disc cyl the volume of fluid they hold? Seeing a lot of Harley 5/8 bore single disc cylinders but ideally I need a 5/8” dual disc cylinder.

[scooper321]

Then you need to work out your ratio. Here is the formula, in plain english(?):
(total surface area of pistons on one side of each caliper [so you will get the same result on a 2-piston sliding caliper as on a 4-piston fixed caliper]) x (number of calipers) / (surface area of master clinder piston)
In maths terminology that's:
(((Pi x R^2) + (Pi x R^2))x n) / (Pi x R^2)

http://www.customfighters.com/forums/showthread.php?t=56704

Okay,got that??

So I ran the numbers. For a stock configuration, 3/4” bore and a 38mm single piston, the ratio is 8. By all other discussions referenced above, this should be super stiff.

The same 3/4” M/C with a dual piston Tokico caliper, dual 27mm pistons, the ratio is 8.0. Same as stock.

For reference, an 11/16 master would produce a ratio of 11.8, while a 5/8 master would produce 14.3. The 14.3 number agrees with the articles cited about about an ideal “feel” range. But it’s odd that the stock would be so far outside this range. I’m thinking that staying with the 3/4” MC for now might be ok. At least it will the same as stock!

Question: I didn’t own an original shovelhead. Did these bikes have stiff brakes when new?

[Tattooo]

Question: I didn’t own an original shovelhead. Did these bikes have stiff brakes when new?

Yes.........

[farmall]

A small master piston will be light to pull on, but will feel spongy and won't give full braking before the lever hits the bars.

I run braided stainless lines so I don't get spongy brakes (after proper bleeding). I'd rather have the smaller handlebar master cylinder for greater hydraulic advantage but both work acceptably. I've never needed a larger master when adding a second caliper but I don't have any six-piston calipers.

If you delicately pinch most stock bike and car hydraulic hoses you can feel them stiffen when pressure is applied. That's wasted motion.

I didn’t know he made them for the 77-83 forks. He isn’t afraid of odd jobs, though. He made me brackets to run Tokicos on a 39mm dual disc setup with a 70s Ironhead wheel with 10” rotors.

I provided the slider he used to design them. The brackets are good stuff.