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V85TT bad rear brake
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Pernao View Drop Down
Guzzino
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    Posted: 19 Sep 2022 at 12:24
Hi, 

New to the forum. I bought yesterday a new V85TT and I have made some hundred kms of mountain roads. Liked the bike with the exception of the rear brake that is incredibly bad.

Very, very weak, without sensibility and spongy, going deeper and deeper with prolonged use.

This is "normal", I mean do that V85TT has a bad rear brake or the motorcycle was delivered with a problem?

 I know that in the past the Guzzii rear brake also actuated the frontal brakes. It is still the case?
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Mike H View Drop Down
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Mine isn't the 85TT, but the V7 850 - however similarly the rear brake is next to useless. No it is not linked to the front. Guzzi stopped doing that a while back, can't remember when exactly. 

However rear brake has been improved somewhat by changing the pads to sintered, and adding a Harley type brake pedal extender to the pedal, this provides a large flat oval pedal that I can get a good stomp with. Sorry but I've concluded brake pedals that look like gear pedals are just daft. 

Don't know about the spongy thing  - you might want to take the caliper off and pads out for examination - two issues with the V7 850  - the caliper is single piston sliding type and could get hung up on the pins it's meant to slide on (but are seized)  - second issue, pads are retained by 2 pins that screw in with heads like grub screws with Allen key hex holes. - these must be removed and greased ASAP. They are held in with a locking compound (not Loctite) if not removed fairly soon will seize solid and Allen key will just round out the holes. Shocked
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Brian UK View Drop Down
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Originally posted by Mike H Mike H wrote:

Don't know about the spongy thing  - you might want to take the caliper off and pads out for examination - two issues with the V7 850  - the caliper is single piston sliding type and could get hung up on the pins it's meant to slide on (but are seized)  - second issue, pads are retained by 2 pins that screw in with heads like grub screws with Allen key hex holes. - these must be removed and greased ASAP. They are held in with a locking compound (not Loctite) if not removed fairly soon will seize solid and Allen key will just round out the holes. Shocked

If spongy then get it back to the supplying dealer to sort out. Spongy brakes are not right. Possibly just needs bleeding, but it does need sorting out and if you start modifying things on a bike that is only a few days old you void the warranty.
Brian.

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Andy M View Drop Down
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I can't remember the last modern bike I had with a back brake that did anything without putting your full weight on the pedal. Honda NC and CB's, V7-850, Weestrom, all rubbish. Enfield's are better . It's race fashion (along with pointless twin discs), the designers don't think we use them and no one ever walked out of the showroom over it.

I've fitted a pedal extension like Mike H.

Guzzi went down the right lines 50 years ago with linked brakes and plain cast iron discs, but faced with the extra costs and angry toddler journalists rejoined the fashion herd. I've been selling and designing brake systems for 30 years and what racing has made the motorcycle market thinks it wants is honestly disgusting, but as a fashion product there is nothing much to be done. You've only to read all the "wah wah the linked brakes make my tum tum feel squidgy I'm going back to 1915" stuff on every forum related to bikes that have them to see what you are up against. 

Andy 
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Brian UK View Drop Down
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Did they have rear brakes in 1915? Wink
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ranton_rambler View Drop Down
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Originally posted by Brian UK Brian UK wrote:

Did they have rear brakes in 1915? Wink
I recently borrowed a 1923 Matchless - the rear brake was much more useful than the front.
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Speciality View Drop Down
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That was true of most British bikes until the second half of the sixties….
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Speciality View Drop Down
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Points: 194 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Speciality Quote  Post ReplyReply Direct Link To This Post Posted: 20 Sep 2022 at 15:44
The rear brake on my 850 Special is better than most of the bikes I’ve had in the last couple of decades, apart from the drum rear on my glad-I-got-shot-of-it RE C5. It’s not great but I can feel it working, which is something these days. The one on my NC750X is wooden and does very little. Neither brake is spongy. FWIW I wouldn’t expect the brakes on a brand new bike to work brilliantly until scrubbed in a bit. As advised I removed the retaining pins and Coppaslipped them. Mine came out easily enough; there was nothing on the threaded part of the pin by way of loctite or similar. Front brake is a bit spongy but works well.
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Dave P. View Drop Down
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I'm just wondering why twin discs might be "pointless" Andy. I have no opinion on this as my bikes all have drum brakes. I'm just curious.
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I suspect back in the 70’s Guzzi were far ahead of the crowd with linked brakes, my LeMans still has them, and they’re excellent.  I can’t think why anyone would change them, unless they’re racing.  I’m pretty sure that Guzzi's brake engineers know / knew more about it than hobby enthusiasts and journalists.

My 2020 BMW also has linked brakes, though all electronic & clever, with ABS etc., and controlled by my right hand rather than foot.  They seem pretty damn good too.

I too spent a few years of my professional life designing & developing disc brakes - big ones, for trains…
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Thanks to all. Perhaps I should have explained that I have not a big experience with modern road bikes. However about 20 years ago I had a Triumph 900 and I didn't found the rear brake particularly bad, neither in other motorcycles that I have used, namely, more than 20 years ago a Africa Twin and more recently, 4 or 5 years ago, a Transalp 700.

But I have raced raid type motorcycles for 15 years, occasionally also in roads, and I do use the rear brake to control the bike, more than for braking and I am truly disappointed with the V85TT rear brake. On the 1000km revision I will try to see if there is something wrong, I will mount better pads and see if the braking gets better. Again, thanks to all. It was relevant information.
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Andy M View Drop Down
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Originally posted by Dave P. Dave P. wrote:

I'm just wondering why twin discs might be "pointless" Andy. I have no opinion on this as my bikes all have drum brakes. I'm just curious.

Lets knock down the marketing guff one bit at a time 
 
The limit of deceleration is set by the tyre. A single disc will lock it on a dry surface, so the second disc is adding nothing. Any disc braked bike that wouldn't benefit from ABS is a missed opportunity and I doubt many exist. Cable operated drums were or course very different. 

The idea of twisting the forks of other means of brake steering is insane, every car, truck etc. has one disc on one side of a wheel. The structure is robust so this doesn't happen. 

A road bike and race bike have very different temperature profiles. The race bike does a warm up lap then multiple decels from high speed in a few miles . This keeps the brake warm. A road vehicle rarely suffers overheating, you ride miles with tiny decels then try an emergency stop, the twins are further below optimum temperature than the single. When I look at ECU data it's rare to see any road vehicle go over 0.4G, yet the occupants will tell you it was at the limit. Our monkey brains limit us except when we've done ten laps in practice. 

Wear can be caused by use or glazing. The latter is caused my making enough heat to soften the lining but not enough force to crumble off the resulting glass before it makes a thick layer that then either doesn't brake well or comes off in lumps. A single worked nearer capacity will glaze less, the underworked twin more. When the pads glaze replacing one pair costs less.

Feel is hard to specify. A twin disc will build deceleration faster so might feel better. You can get the same effect by changing pad materials or upping the size of the master cylinder, so why suffer wear etc. 

The only other vehicles that use multiple discs are off highway types and aircraft where the wheel has a much greater load and is limited diameter. It's still not a preferred solution because running costs are higher as is weight and cost. 

I'd be interested to talk to a race bike designer. The unsprung weight of twin discs is against their primary aim, so either they do still have an overheating problem from the multiple hard decels or the rules or sponsors force them to stick with them. The track isn't the road though. 

Left to my own devices I'd start with something like this



Disc as big as you like to set heat to the application and likewise choose a calliper for performance/feel. Split the calliper plumbing so the rear control gives 1/3 on the front. 

Andy 




Edited by Andy M - 21 Sep 2022 at 11:18
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AdrianW View Drop Down
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Andy,
Thought I recognised the wheel -  here's one I bought earlier (and still have!) ..




One disk is plenty on this... There is a small one at the back too, just in case!!

Cheers,
Adrian
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Brian UK View Drop Down
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I remember when I had my Spada III MOT test done, I heard the tester shout in surprise, the single front disc had acheived over 50% efficiency. It did work well.
Mind you he then went on to fail the "back" brake at 24%. Wouldn't be told to take the front left disc into account.
Brian.

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Its curious why most people will be happy with a single disc per wheel on their car but not on a bike, i remember getting into a bit of an argument regarding the CARC bikes someone insisted that you have to drill and lockwire the rear wheel bolts! he was quite happy with unlockwired bolts on his car. I think that some of the racing guzzies with dr John were running on a linked brake system for all 3 discs. 
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