Mr. Flibble

Hi all. I've a 2008 Kona Hei Hei supreme with 2008 XTR brakes. I've had the pads replaced, brakes bled and degreased the disks but I'm still getting really poor braking performance with the rear brake. Is there anything else I should look at doing to improve the braking performance?

Panthers007

For one thing, the rear brake really just acts as a drag to stabilize the front brake, which is the brake that does most of the work, and prevents you from going over your handlebars when you pull hard on the front brake. By itself, the rear brake will stop you but more slowly when compared to the front one.

Other candidates for causing a brake to not work as well as it should: Check the rim of the wheel for any oily residue - it's possible to accidentally dribble some lube on the rim and/or the brake-pads. And the brake-pads could be causing this if they are made out of some rock-solid compound that makes them too hard to 'grab' the rim. If so, I would take them off and roughen their surface with some sandpaper. And do check that the brake-cable is firmly (but be careful with alloy bolts - they L*O*V*E to do that) anchored in the brake.

Hope this helps you.

Juha

+1. If you really need to stop, you should be using the front brake, unless you're riding on ice or otherwise low friction surface. Rear brake won't hurt, but will not add much either to maximum front brake application.

--J

krobinson103

Agree with the above advice, but given the brake has been bled it sounds like a hydrolic disc. In that case you could try taking a propane torch to the rotor (briefly) to burn off whatever there is on the surface then go on a good downhill run with fresh pads to bed in the system. If that doesn't work, then a new rotor might help.

Kimmo 

Try pulling out the pads and cleaning off the glaze with a bit of sandpaper, and using brake cleaner on the rotor.

If that doesn't work, new pads should fix it.

Mr. Flibble

Thanks - I'll give the sandpaper a shot and re-cleaning a shot.

I know the front brake does most of the work. It just shouldn't be doing all of the work At the moment I can barely lock the back wheel on gravel. I can however go over the handlebars to hospital due to relying only on the front brake

Kimmo 

If the lever feels like the pads are biting, that says the pads are contaminated IMO. Bit of a risk with hydros and the bleeding thereof.

Monster Pete

The solution, then, is to learn to use the front brake effectively. Find an empty car park or other open space and practice modulating the brake. In the quickest possible stop, the rear wheel is just about to lift off the ground. In this case the rear brake can't contribute at all. Granted, in most cases it never comes to that, but it's a bad idea to rely too heavily on the rear brake. In a panic stop, you might then grab the front brake for some extra, pull it too hard and indeed go over the bars. Your front brake should normally contribute something like 75% of your total braking effort. You should reliably be able to apply the front brake such that the rear wheel will skid with only a small amount of rear brake application.

That said, it does sound like the rear brake is in need of attention.

Mr. Flibble

You'd make a great salesman. I'll work on my skills, but I'd like a rear brake all the same.

Monster Pete

I'm not sure that I would- I'd be forever telling people to use what they've already got rather than buy this shiny new thing!

Kimmo 

I had my brakes American style for 30 years before realising it was a bad idea; I've only been using Euro style a couple of years, and since I switched, it feels like I hardly use my rear brake at all.

So one of my bikes only has a front brake. But I tell you what, I'm sort of nervous on that bike - and I have complete faith in the integrity of the brake; I don't crave redundancy. I think it's that the tiny amount I did use my rear brake counted for quite a lot, particularly in the wet - which suggests to me that it'd count for even more on dirt.

A bit of drag on the rear wheel can add a lot of stability in some situations.

trailangel

Gripmaster Hand Strengthener at REI.com

Mr. Flibble

Better option than losing the girlfriend I guess.

trailangel

Hey, I'm Irish too!

rumrunn6

I don't really use my brakes much, it just occurred to me ...

Kimmo 

Take em off and see how you go. That would fulfill the statement in your sig...

cyccommute 

You both are misunderstanding the issue at hand. Mr. Flibble's Kona Hei Hei is a dual suspension mountain bike. I assume that he's for its intended purpose, i.e. as an off-road bike. While it is true that you can get by with sloppy technique and rely only on the front brake on asphalt, doing so off-road is a recipe for disaster. In many downhill instances, the bike already has the rear wheel higher than you would normally encounter on pavement. This puts the rider closer to the point where they are going to go over the bars from the start. The trail surface isn't as grippy as asphalt either. Both brakes are essential to the control of a mountain bike...not just the front one. In fact, a maxim in mountain biking is to back off the front brake when the rear wheel starts to skid. This puts the rear wheel back in contact with the ground. You sacrifice some deceleration for control.

The front brake actually contributes 80% to the deceleration of a bike on level ground with a rider seated in the "normal" position. However, relying too heavily on the front brake results in throwing away that 20% of deceleration up to the point where the rear wheel leaves the ground. Even on-road, it's a good idea not to skid the rear wheel because a sliding wheel is harder to control than a rolling one. Off-road, that's even more important.

Relying too much on the front brake has other related handling problems. The maximum possible deceleration is a mathematical oddity. It doesn't occur when the rear wheel leaves the ground...you can still squeeze more deceleration out of the bike at that point. Maximum possible deceleration occurs just before the point where you are falling forward of the contact patch. In other words, it is a nose wheelie gone wrong. The handling problem occurs when you are from the point where the rear wheel has left the ground to that nose wheelie. You are relying on the front wheel for complete control of the bike. You are on a freewheeling unicycle with a pivot point thrown in for good measure. It's a very unstable system. On asphalt, you can get away with it because the surface is smooth but off-road the surface varies so much that it doesn't take much to knock the wheel off-line and make you crash.

I'm not saying that you should avoid the front brake nor am I saying that you should rely to much on the rear brake. You should learn to use both judiciously.

cyccommute 

It sounds like the bleed job wasn't done properly. They may not have degassed the brake oil sufficiently and you may still have some air in the line. I'll preface this by saying that I've only bleed Avid brakes. From the videos that I've watched of bleeding your XTR brakes, I think Shimano leaves out a crucial step in the bleed method. Avid has you put the brake fluid (DOT instead of mineral oil) in a couple of syringes that can be sealed. You then pull a vacuum on the syringe several times to get the air out of the fluid. The videos I've seen on Shimano have you just inject the fluid into the caliper and then "flick" the hoses to get air out. The degassing method from Avid would be more effective at removing air. Avid also has you put some pressure on the system at the end of the bleed but I don't know if the Shimano system would tolerate that.

Another issue might be seals in the lever have deteriorated so your bleed job does no good. Air may be infiltrating back into the system. I'd do the bleed job again with degassing of the fluid and if that doesn't work, redo the seals in the lever.

On the other hand, you said you've had this work done so I assume you had a shop do it. The best course of action is to take the bike back and have them do the job properly.

Mr. Flibble

Thanks. Yes, I got a shop to do it. I haven't bled brakes myself. If the other suggestions don't work I'll do it / get it done again.

MileHighMark

Does it feel like you're running out of travel at the lever, or are the pads just not biting?

Mr. Flibble

It feels like they aren't biting. I'm putting a lot of pressure on the lever and not running out of travel.

MileHighMark

Ahh, OK. That sounds like you may have contaminated pads. I would recommend cleaning (and possibly lightly sanding) the rotors, and replacing the pads. Before you do so, however, clean the caliper with isopropyl alcohol and confirm that it's not leaking fluid.

Wilfred Laurier

why in the world do you keep spewing this garbage

it is simply not true

you have proven over and over again that you are misunderstanding some text
and presenting it here as fact

edit

everything else you said is true though

cyccommute 

I agree. If you can put plenty of force on the lever without it bottoming out and the brake pads have no friction, the pads are contaminated.

Considering that the work was done by a shop, I'd take it back to them and have them replace the pads. New pads shouldn't be contaminated out of the box. It sounds like they may have dripped brake fluid on them.

cyccommute 

I keep saying it because it is true. Get a copy of "Bicycle Science" and do the math. I have. It makes sense when you understand the math behind the principle. I'm not talking about the maximum practical braking which occurs at about the time the rear wheel leaves the ground. I'm talking about the maximum possible deceleration before the rider is thrown over the bars which is the way that Wilson puts it.