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daffyduck1962

Very poor rear brake

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My 2001 VFR800 has always had a very shoddy rear brake. (Had 10k on the clock when I bought it and is now just under 21k) If you want it to hold you stationary on a hill it will do that, but once the wheels are turning it would be more effective to use the soles of my boots!
So I decided to convert to a delinked system with Hel brake lines. It took me all weekend with a the help of a friend. Very involve dwhen it comes to removing all the link pipes.

Anyway the job is finished and the bike is back to it's original glory, so I took it down the road to test the brakes. Front brakes are great, as they always have been, but the back is exactly the same!!!!!!!!!
Plenty of pressure, very little movement in the pedal under foot, but it doesn't slow the bike down any better than before. Rear wheel spins freely when brake released.
I'm guessing there must be a problem with either the rear pistons in the caliper or the rear master cyclinder.
But how do I identify which area to start checking first?

I had always intended to delink the brakes as I thought it was an over complicated system when it came t bleeding the brakes with so many bleed nipples.

The new set up involves a double banjo on the front master cylinder, and a single line to each of the outer piston feeds.
The rear also uses a double banjo and two lines back to both feeds on the rear caliper.
This system is the same as that which is used on the Blackbird, and I spoke to John (he used to run Jaws Motorcycles), and he talked me through it before I began.
Any ideas, anyone?

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Did you try removing the rear caliper and mounting it as high as possible while bleeding?  Don't forget to put something between the pads to take the place of the rotor...

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Things to check/do:


1. Did you pull the pads out of the rear brake and check/clean the general area? Are there any seals protruding around the pistons?
2. Did you use all new crush washers when installing the new line/s to the rear?
3. Have you checked the condition of the rear hydraulic actuator? (cracks, leaks etc...)
4. Have you made sure that the rear pads are properly cleaned and that no grease or brake fluid has contaminated them? (if they were contaminated any more than 'very slightly' you'll want to just replace them).
5. With the rear pads out; Have you confirmed that the pistons move in and out properly and fully, with even motion?
6. Have you checked the condition of the hose linking the res to the actuator? It could be old and just bulging when you hit the brake, thus not transferring pressure fully.

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I have the stock rear caliper, with a CBR600F4 master connected using both lines and a double banjo at the master. My back brake is great.

 

I'd say you need to dismantle the caliper and clean the pistons, and replace the seals. If anything with the stock master you should have a bit of a touchy, lock-prone brake as the master:slave area ratio is now pretty high driving that extra third piston. 

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5 hours ago, daffyduck1962 said:

The new set up involves a double banjo on the front master cylinder, and a single line to each of the outer piston feeds.
The rear also uses a double banjo and two lines back to both feeds on the rear caliper.
This system is the same as that which is used on the Blackbird, and I spoke to John (he used to run Jaws Motorcycles), and he talked me through it before I began.
Any ideas, anyone?

So if I understand you correctly, you have eliminated the proportioning valve out of the system and your setup is now more like a standard brake system common on most bikes? 

Your rear is straight from the master cylinder with the brake lines direct to the calipers? 

 

- If so on the rear, many of us have converted from the delink to standard style brakes, including the rear. -- Specifically for the rear you need to change the master cylinder to a  14mm  -

very common available - RC51, Super hawk, F4i, CBR600. (ok, I dont remember the mm cyl size, but its the most common Honda part.) I just bought a new RC51....  - Change out the M/C and you will get the improvement you need. 

 

Other options are to go with a single line and use a short bridge between the caliper ports. -- check my gallery for photos... 

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17 hours ago, daffyduck1962 said:

The rear also uses a double banjo and two lines back to both feeds on the rear caliper.

Isn't the centre piston a different bore size to the outer two? - Would have thought this would mean it moves at a different rate, either faster or slower than the others and once the pad presses against the disk either the centre or outer pistons will not be able to move further and exert their full force on the pad?

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That's not really the way a hydraulic circuit works. The fluid will push the pistons out evenly until they meet resistance i.e. the pad touches the disc. Then the piston with the biggest diameter will exert the greatest force on that part of the pad. It is normal on front 4-piston calipers for the leading piston to be smaller and trailing larger, as the trailing piston is over the hotter part of the pad and so requires more force to overcome the heating effects and keep pad wear even. 

 

All of the above assumes that the pistons are free to move in their seals, but the location of the VFR caliper (underslung, within the wheel dish) means they are easily forgotten for cleaning; once the pistons trap dirt under the seal and lock that piston up, you are relying on the remaining piston(s) to do all the work. In the case of the VFR caliper, the sliding design also requires free-moving sliding pins to allow the caliper to self-centre on the disc. If these are also frozen, then the inside pad won't be doing much.

 

Moral of the story: pull the wheel off, pull the caliper out and check that the sliders are lubed and free-moving, and that all the pistons extend easily when you pump the brake. If not, then start cleaning.

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Right, I've stripped the rear caliper apart yesterday, and checked the condition of the following parts:-

Pistons - very good,clean no burrs, scratches. slide inside the chambers.

Piston  seals - intact

Slide pins - cleaned and a light covering of copper slip grease applied. No burrs or rough edges.

Rubber boots - intact, no splits.

The spring retainer clip which sits over the pads was slightly bent. straightened out.

There was a collection of oil around where the spring pad clip sits which I think must have come from the scottoiler. Cleaned.

Pads appeared glazed slightly, so roughed them up with a metal file.

Cleaned anti squeal shims.

Caliper body cleaned.

Caliper rebuilt, Hel lines reattached,and system bled. Firm pedal, brakes not binding.

 

Went for a test ride.

Result,slight improvement but it's never going to lock the back wheel up!!!!

 

Could it be the pads; master cylinder or disc?

Stumped, and pissed off.

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44 minutes ago, daffyduck1962 said:

 

Slide pins - cleaned and a light covering of copper slip grease applied. No burrs or rough edges.

 

Don't use copper slip on these pins, it swells the rubber boots they slide in.

Use red rubber grease or similar.

When I replaced the the rubber slider boots on my rear caliper the old ones looked fine until compared to the new ones, I guess the old ones were about 30% bigger.

A combination of copper grease(inside) and various chain lubes(outside) caused the swelling I believe.

 

Maybe we can meet and swap bikes again sometime? My brakes are original(linked) and I can (if I really try) lock the rear wheel with he rear brake

 

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8 hours ago, daffyduck1962 said:

Could it be the pads; master cylinder or disc?

Stumped, and pissed off.

 

Dunno if you caught my previous post, but if you are delinked, the stocker master cylinder doesnt have enough "oomp"  to get it done.  All the clean it work you are doing is great, but is not the fix..... 

- You need to go to to a 14mm master cylinder.... 

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Mello is right (and I was confused on one of my previous posts). The stock master is 17.4mm, so in comparison to a 14mm master the piston area is 50% greater or to look at that another way, you will get 50% more brake pad squeezing force for the same bootful of brake on the 14mm master. 

 

One other aspect of brake performance is the contour of the pads and disc. I know from past experience that a worn disc often develops a dished profile; if you clamp a flat pad onto this, you only get contact at the outer edges (and pretty feeble brake performance) until the pad wears into a matching contour. 

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Here's the simple maths. You need a number between 8-10 for a rear & between 25-35 for a front. Lower the number the stiffer the brake feels, higher the number the softer it feels, but it will produce more controlable power but requires more lever movement.

 

To calculate Caliper Piston area you need to do the following;-

Caliper Piston diameter/2 x result x 3.14 = Area per piston. VFR rear has 2 x 22mm & 1 x 27mm pistons.

So 22/2=11x11x3.14=380mm2 x2 (2 pistons) = 760mm2

And the 3rd piston is 27/2=13.5x13.5x3.14 = 572mm2

Add the two together 760+572 = 1332mm2

 

To calculate the M/c area you do this;-

Piston 17.4/2=8.7x8.7x3.14 = 238mm2

 

Now devide the caliper area by the M/C area, so 1332/238 = 5.59-1  that is the Hydraulic everage ratio, so not much.

 

With a 14mm M/C we get 7x7x3.14 = 154mm2, so 1332/154 = 8.65-1, so just in thw sweet spot for hydraulic lever for a rear brake.

 

Enjoy :)

 

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2 hours ago, Mohawk said:

Here's the simple maths. You need a number between 8-10 for a rear & between 25-35 for a front. Lower the number the stiffer the brake feels, higher the number the softer it feels, but it will produce more controlable power but requires more lever movement.

 

To calculate Caliper Piston area you need to do the following;-

Caliper Piston diameter/2 x result x 3.14 = Area per piston. VFR rear has 2 x 22mm & 1 x 27mm pistons.

So 22/2=11x11x3.14=380mm2 x2 (2 pistons) = 760mm2

And the 3rd piston is 27/2=13.5x13.5x3.14 = 572mm2

Add the two together 760+572 = 1332mm2

 

To calculate the M/c area you do this;-

Piston 17.4/2=8.7x8.7x3.14 = 238mm2

 

Now devide the caliper area by the M/C area, so 1332/238 = 5.59-1  that is the Hydraulic everage ratio, so not much.

 

With a 14mm M/C we get 7x7x3.14 = 154mm2, so 1332/154 = 8.65-1, so just in thw sweet spot for hydraulic lever for a rear brake.

 

Enjoy :)

 

Mohawk,

 

When are you sending the simple math portion of the email??

 

; -)

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On 04/03/2018 at 11:23 PM, Terry said:

Mello is right (and I was confused on one of my previous posts). The stock master is 17.4mm, so in comparison to a 14mm master the piston area is 50% greater or to look at that another way, you will get 50% more brake pad squeezing force for the same bootful of brake on the 14mm master. 

 

That sounds wrong, even if Terry meant it correctly.

 

The BIGGER the Master Cylinder piston, the more fluid it moves which means the HARDER the brakes will feel, but the LESS pressure on the pads and so REDUCED actual braking force and vice versa.

 

For the caliper it's the opposite, so the BIGGER the caliper piston, the LESS it moves, the SOFTER the brake will feel and GREATER the pressure applied and hence actual braking power.

 

I'm sure Mohawk's maths were good, but it's the general principle one needs to understand. Too many times I have seen people confused by this and think bigger Master Cylinder means more braking. They feel harder brakes and think that is giving them more braking power when in fact the reverse is true. Don't get me started on the 'Brembo effect'.

 

As with many things, it's all a compromise between hardness of feel and braking power. You can't have both. The reality is though that with hard lines you can achieve mega performance from brakes these days (as long as you junk all that linking and ABS crap). Don't forget, Honda's aims are not necessarily the same as yours. They want (and design) brakes that will provide sufficient stopping power, but are hard to lock up. ABS of course confuses the issue, but their desire to not have too much power is not necessarily what the experienced rider wants. I like a LOT of braking power so I can e.g. dash into mountain hairpins just using 1 or 2 fingers on the front brakes. This is not what Honda are designing for. But happily, there's lots of stuff I can swap to achieve what I want.

 

For the OP, if you have delinked the brakes and the rear 17.5 mm Master Cylinder is directly coupled to the caliper and it feels too hard with not enough braking power, use a smaller M/C. The obvious swap is to a 14 mm one and happily they are not hard to find. Beware though, some FireBlades have the reservoir connection between the mounting lugs, whereas most have it on the opposite side from the lugs which is what the VFR needs so just take care when looking and before you buy, check that it's the right one.

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On 3/6/2018 at 1:17 PM, Mohawk said:

Here's the simple maths. You need a number between 8-10 for a rear & between 25-35 for a front. Lower the number the stiffer the brake feels, higher the number the softer it feels, but it will produce more controlable power but requires more lever movement.

 

To calculate Caliper Piston area you need to do the following;-

Caliper Piston diameter/2 x result x 3.14 = Area per piston. VFR rear has 2 x 22mm & 1 x 27mm pistons.

So 22/2=11x11x3.14=380mm2 x2 (2 pistons) = 760mm2

And the 3rd piston is 27/2=13.5x13.5x3.14 = 572mm2

Add the two together 760+572 = 1332mm2

 

To calculate the M/c area you do this;-

Piston 17.4/2=8.7x8.7x3.14 = 238mm2

 

Now devide the caliper area by the M/C area, so 1332/238 = 5.59-1  that is the Hydraulic everage ratio, so not much.

 

With a 14mm M/C we get 7x7x3.14 = 154mm2, so 1332/154 = 8.65-1, so just in thw sweet spot for hydraulic lever for a rear brake.

 

Enjoy :)

 

 

Just to note that there are different size DCBS/LBS rear caliper pistons; I think the 6th gens are different than the 5th gens, so the ratio would be slightly different.  And if the centre piston is not plumbed into the rear brake hydraulic circuit (by drilling through the back of the centre piston chamber into the main circuit or doing something with hoses externally), the ratio would be different as well.

 

Ciao,

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13 hours ago, BiKenG said:

 

That sounds wrong, even if Terry meant it correctly.

 

 

Yep, I am guilty of bad sentence construction. Smaller master = more power at the caliper. I fitted my 12mm VFR 5G master to my ST1100 in place of the 14mm, and got much better brake power, and with braided lines very good feel as well. With the larger master, the ST brakes felt really wooden.

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Hey Daffyduck1962.

Not wanting to get too technical here but have you tried various brake pad manufacturers and compounds, this can have a big effect on pedal feel and brake grip. Perhaps an organic/resin pad over the sintered type might suit you better. Just a thought.

Cheers.

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Just for a chuckle I removed and cleaned my rear caliper this week, got it all bled nicely but almost no brake action when I went for a ride today. Tried brake cleaner thinking I'd contaminated the pads/disc but no joy. So I pulled the caliper out and spotted that I had one of the pads caught behind the edge of the pad spring, which was holding the pad from making good contact with the disc. Pushed the pad spring down and the pad moved over the top of it, hey presto proper back brakes again.I believe this happened because I had the pistons fully retracted when I refitted the pads.

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14 hours ago, Terry said:

Yep, I am guilty of bad sentence construction. Smaller master = more power at the caliper. I fitted my 12mm VFR 5G master to my ST1100 in place of the 14mm, and got much better brake power, and with braided lines very good feel as well. With the larger master, the ST brakes felt really wooden.

 

Of course, there are limits...  The side-effect of increasing power by decreasing the piston size of the master cylinder is that lever travel increases as well.  This can be a problem on a handlebar, which is a fixed distance away from the master cylinder, but this can possibly be alleviated by using an adjustable brake lever.  All part of the fun of modifying the braking system!

 

Ciao,

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