Rear v-brake has no power!

[Hulk_slogan]

Hello all, been about a year since I signed up and have only just acquired a bike and got the time to ride!

So, the bike is a 2015 Decathlon B'twin Rockrider 340 (£160 new).. and I know I know.. its a cheapie.

I'm having trouble with the rear v-brakes, they have hardly any power at all. I think I've set them up correctly, and I have adjusted the front V-brake's slightly and they work a dream, but the rears, hardly slow me when I'm on the bike.

There is no mount to fit rear discs so that's not an option, and I have a suspicion that the rear seat stays/brake bosses are flexing when the rear brake is pulled, hence there not being much stopping power.

Any ideas? I'm fairly basic when it comes to maintenance.

My original plan to upgrade the rear v-brakes to better shimano v-brakes seems pointless now if indeed the seat stays/brake mounts are flexing.

I have googled old school devices known as brake boosters, and I see that chainreactioncycles sell one, although I'd have no idea how to fit it or if it would even work/be compatible with my bike.

I didn't know whether to ask my LBS to take a look/make adjustments before I spend any cash on components.

My second question is that if I was to replace the rear v-brake anyway if flexing isn't an issue, would I need to also buy and fit a same brand/model brake lever, or would my stock v-brake levers work just as well? Also does anyone know if Shimano V-brakes are direct replacements and will fit B'twin MTB's?

Thanks a lot all, look forward to reading your replies!

 

[Globalti]

Stop worrying about it; this is normal. Most of the braking is done by the front brake anyway. The chainstay is flexing when you are squeezing the lever way harder than you need to lock the brake so don't waste your money on a brake booster, which was another short-lived fad that manufacturers sold us gullible folk back in the 90s.

Just riding along, can you lock the back wheel by braking hard on the back only? That means the brake is working fine. V brakes are the best type of rim brake for power and as long as the pads are correctly aligned, not glazed or filled with embedded grit (seen as shiny bits on the braking surface) and the cable is running freely, there's nothing wrong.

When you brake your weight is transferred to the front wheel meaning that the front brake will do most of the work. The harder you brake at the front the more weight transfer and the less effective the rear brake. Very short vehicles like Land Rover 90s have a pressure limiter on the rear brakes because it's so easy to lock them up and the short wheelbase makes them quite unstable directionally - good off road but not in an emergency stop on tarmac. The same applies to bicycles.

 

[I like Skol]

The back brake of any bike should not be useless, although they are often less effective than the front due to losses from the cable length and routing. As Global says, you only need to be able to lock the back wheel, after that it doesn't matter if the brake doesn't feel as hard or effective as the front brake. If you have only just bought the bike then it may be worth popping back to Decathlon to see if they can spot anything amiss with the set-up, although IME they are normally pretty good at checking over the bikes before sending them out the door.
There are some things that can be done to optimise any brake set-up like lever position and adjustment, cable tension and most importantly, cable routing. Many bikes, especially cheaper ones leave the factory with a stock set of cables fitted and these are often a generic length with the cable outers usually far too long. The best route for the cable is short, gentle curves with just enough slack to allow for component movement (Handlebar rotation, fork travel, etc). The excessive length of some standard cable sets can mean cables have large swoops and almost double back on themselves in places. Removing the excessive bits of cable outer casing can often make a dramatic change to the feel and performance (the same goes for gear indexing too) but you nreally need to sure of what you are doing if you attempt this.
Brake pad alignment is also important. If the pads are not hitting the rim almost flat then you have a reduced contact point and you are then putting most of your effort into distorting the pad rather than into braking.

Hope this is helpful.

 

[bikeman66]

The OP clearly says the back brake hardly slows the bike when applied, let alone lock the wheel. Agreed, anything past locking the wheel is surplus to needs, but you need to have some confidence in both brakes functioning satisfactorily.

A visit to the LBS may well be beneficial if you are sure you have exhausted all other options as far as your knowledge allows.

Seatstay flex shouldn't be so excessive that the brake fails to function to a basic minimum level of requirement, so don't get sucked in by a brake booster. The brakes themselves may well be relatively cheap (although still functional one would hope) so if you think they are set up OK then a small upgrade may be the best way to go. As discs are the way to go now, I'll bet that there are some pretty good V-brakes out there for cheap money. If you can find a pair, some old Shimano XT or XTR will be a good option. You won't necessarily need new levers (my XTR's work perfectly well with the Avid levers on my old Kona).

Another option could be some hydraulic rim brakes. I used to have a set of Magura HS33's and they provided fantastic stopping power (one fingered stoppies were easy) but modulation (the feel of the brake) was something that took a bit of getting used to.

Hope you get sorted soon matey.

 

[raleighnut]

^^^^^+1 Try new pads first, the old ones might be glazed.

 

[Hulk_slogan]

Cheers all for the fast replies!

Thats correct, my back wheel doesn't/cant lock at all when riding. Thats just rear braking alone and with the front brake on too.

I'll check over the set up in a bit just to make sure all of what you've all mentioned about brake pads and cable tension are ok.

The rear wheel does have a little sideways wobble if this matters, ie, the wheel is true and tight in the frame but has slight lateral wobble when grabbed and moved side to side.

As it stands the lever can be pulled around halfway to the handlebars when braking fully, but no joy in locking the rear wheel no matter how hard I pull on it!

Pads currently have a few mm clearence between the rim on each side.

Cheers all appreciate the help!

 

[SpokeyDokey]

Ooh! Forgotten all about brake boosters until I read this.

I had one on some Rockshox Quadras years back to defeat sponginess in my Ritchey Canti's*. Worked well

*Fine brakes but the Quadras, being early tech on the fork front, had giant pencil rubbers in each leg and flexed like a jelly on speed.

 

[sidevalve]

Don't mess around with 'brake boosters' or disc brakes. A V brake if it is working correctly will lock either wheel [yes even the front].
Try getting someone to work the lever while you watch the brake - once the block hit the wheel is there lots of movement still at the lever [ie are the cables compressing]. Is everything moving as it should - you don't have anything seized and hardly moving. Try disconnecting the cables and see if everything moves easily then make sure the cables are free. I suspect one of these might be your problem.

 

[ColinJ]

And ... make sure that your rims are clean. Oily residues on wheel rims can greatly reduce the effectiveness of rim brakes.

 

[maxfox44]

@ColinJ is right. I while ago I stupidly cleaned my rims with a muli-surface wipe and then spent the next week trying to stop the flippin' bike.

 

[Hulk_slogan]

Thanks for all the advice guys.

Had another look this evening, spent a couple of hours adjusting and experimenting, and could definitely see the seat stays flexing upon normal braking while making adjustments with no weight on the bike etc.
Although the brakes will lock the wheel when I've got it all flipped upside down on the patio, as soon as I ride, or theres some weight and momentum such as pushing down on the seat and rolling the bike forward, the seat stays flex way before any real power can be had out of the rear brakes.

Going to email Decathlon and see what can be done! Would definitely feel more confident with a working rear brake.

 

[the_craig]

I see your bike is out from decathlon. I bought decathlon's own brand pads and they weren't very good. And as soon as they got wet they just formed a slick film on the rim.

I would be inclined to change the pads first.

 

[buggi]

Yea change the pads. My rear brakes are pretty shite too but I rarely use them

 

[bikingdad90]

Proper set up of v brakes is as follows:

Windback the barrel adjuster as far as it will go so it is at its loosest. The brake pads should be up against the rim and lock it.

If it doesn't than tighten the cable up without pulling the lever.

If one side is loose then turn the little screw near the spring to change the in/out position on the arm.

Now wind the barrel adjuster back in until the wheel can spin freely and the brakes feel good to you. As the pads wear wind the adjuster in to reset the tension.

Oh and make sure there the pads and rim have no oil or residue on them by rubbing with brake cleaner or sterile alcohol.

Chris

 

[bikingdad90]

Oh and when you pull the levers do the right and left arms both move or just one. It could be a seized arm which needs freeing.