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V85TT bad rear brake
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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: 14 Nov 2022 at 10:28
Not mine, no. The bike isn’t old enough to need an MOT yet. FWIW I find that most disc rear brakes on the bikes I’ve owned are not particularly powerful or progressive. The one on my V7 850 is a bit better than the one on my NC750X. The last really good rear brake on a bike I’ve had was on my 2015 RE UCE Classic. That had a drum rear brake that was great once I’d taken it to bits and reassembled it properly. That said it’s a long time since I’ve owned a cruiser type bike or an old British bike, which usually had far more effective rear brakes than front ones…
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Has it been on an MOT brake tester ?

Cheers
Min
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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: 14 Nov 2022 at 08:15
I doubt that anyone will have the definitive answer to that, but I have found that suppliers can be slow to update the years to which parts they sell apply and that unless the new year model is obviously a significant refresh the existing parts usually fit the previous model. Unless someone knows for certain I suggest you order them and compare to the originals and if they look like they won’t fit, return them. Best to confirm with the supplier beforehand. FWIW the rear brake on my V7 850 is not brilliant but it does work. 
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Originally posted by Mike H Mike H wrote:

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

Hi,

It seems you are right (see the previous post). It seems that the brake is OK, or at least it is what the Guzzi mechanic says. He compared the breaking of my bike with other V85tt he had in the shop and says it is about the same.

So, I am going to change for EBC HH, and about that extension, I cannot use anything that is not from moto guzzi without voiding the warranty, but they have this:


It only increases marginally the size of the lever, but will give the foot more support.

By the way, do you know if the EBC right pad for the rear brake is the EBC FA256?
They say they are for the 2019 and 2020 models. They have changed the rear brake or it is the same for the 2021 and 2022 models?



Edited by Pernao - 14 Nov 2022 at 01:59
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Some news about this problem: I changed the oil fluid, bleed the system and it become slightly better. Took the bike for the first revision, complained about the rear brake, they changed the fluid again, tried the motorcycle, comparing with other V85TT that were in the shop and said to me it was ok. 

Actually it is a bit worse than when I bleed the system (the pedal brakes further down).

So I am changing for EBC brake pads. I found some on Ebay but on the reference  they say that they are suitable for models between 2019 and 2020. They changed the rear brake in the 2021 and 2022 models or are the pads the same for a 2022 model?

"EBC FA256 737.09.50 For Moto Guzzi 850 V85 Tt 2019-2020 
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Again, re V7 850, not specifically V85TT, but service manual says brake bleeding can be done in the conventional manner.


Originally posted by Brian UK Brian UK wrote:

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.


Originally posted by Pernao Pernao wrote:

... have made some hundred kms of mountain roads.
... without sensibility and spongy, going deeper and deeper with prolonged use.


Does sound like should go back to dealer.

Mine did take nearly 100 miles to stiffen up and start workng properly, with new pads of course, but he says hundeds of km.
"Chicken nuggets don't dance on a Tuesday."
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I suppose this isn't much help regarding the original questionSmile. Yep the V85 brakes appear to be soft, I stripped the caliper and lubed everything than needed doing. Its a little better but improves with use. I think its a dealer job to bleed the brakes with the ABS system.
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Yes, most off-highway equipment has multi-plate wet discs.
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Some tractors have multi discs on the axle running in Oil? in a sealed unit.
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Originally posted by Andy M Andy M wrote:


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. 

Andy 



Trains have (well, had, I’ve been out of that business for many years) multiple discs per axle in some cases. 

Typically, medium speed & freight have one disc per wheel (which are fixed to the axle) but high speed trains may have three or four discs per axle.  Diameter isn’t the issue, it’s energy, and how hot the disc (and pads) would get under a full emergency deceleration from Vmax.

The limiting deceleration is, as with motorbikes and cars, determined by the grip between wheel and rail.

Atb
Simon 
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Originally posted by borderer borderer wrote:

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. 

And yet the stresses on the car wheel are far higher.
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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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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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
Bristol UK
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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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