Disc brakes performance

omri

I got Deore XT disc brakes on my cargo. With ice-tech coolin system. Was going downhill from sallynoggin roundabout towards dun laoghaire. I would say bike+me+kid was around 130-140kg. We were doing somewhere between 40-50kmph maybe bit more than that, then at the bottom of the hill the lady decided to cross but couldnt make up her mind when she got to the middle of the road. So I had to slam the brakes and to my surprise I wasnt really slowing down as expected. So I slammed even harder pulling levers to the handlebar pretty much. Now my question is - is it what I should expect at this weight and speed or are my brakes not enough ? Maybe theyre not blead properly ? They are new, 2 weeks old at most.

Lumen

New rotors take a while to accumulate enough pad material to brake effectively.

Do a few hard stops, not completely to rest.

Alek

Were you constantly braking as you were coming down? I was descending in Spain with a kid in trailer and no less than 30kgs of gear on top of this (160-170kg total), the descent was a long (10km?) constant 5-7% so had to keep the brakes on at all times... at the end the rear brake was non-functioning at all (possibly boiled the fluid) and the front maybe in 50%.

Front was old XT and rear oldish Deore.

omri

No I was going down hard no brakes, then the lady got in the way and I slammed both brakes, wheels didnt lock but I just got surprised that I wasn't stopping as good as at 40kmph that I sometimes get on my way to Blackrock, same weight and no problems stopping.

'68 Fastback

I have Avid BB7s on my Big Dummy and just yesterday was about to pass a bus that had pulled in at Kilmanham jail but the bike infront(that had shoaled me at the red light) decided to pull back in and stop dead infront of me. I had to slam on and skidded up to his back wheel. I'd say I had a similar weight to Omri but the breaks locked the wheel. Maybe a bleed might sort out the lack of brake power. I would like to think that a new set of XTs would outshine some cable discs.

saccades

Multiple things it could be.

Did you bed the pads in properly? - Warp speed down a hill and brake as hard as you can (without locking), repeat ~10x (you'll know when the pads have bedded in).

With the levers going to the bar I suspect they need a bleed (although I run mine with the bit point close to the bar).

Has the kid being putting mucky fingers on the new rotors?

Did you drag your brakes a lot - it glazes the pads over so they are a lot less effective (not usually a problem on a mtb, but can be for nervous roadies on long descents where they are controlling their speed)

I've XT and I can lock the wheels with 1 finger (I weigh 100kg), ~140kg at 30-40kmph is a lot of kinetic energy to dissipate, I'd still expect to feel a dramatic braking effect.

smacl

Also worth checking to see what type of pads you've got installed. Article on resin vs metallic pads here. AFAIK, most MTBs aer fitted with metallic by default which tend to lock easier at the cost of modulation. My front hydraulic brake isn't great at the moment, even after being down to the LBS for a checkout, so reckon I'll get a bleed kit and new pads myself.

benneca1

Bleed is first thought and oil grease on discs is other

omri

saccades wrote: »

Multiple things it could be.

Did you bed the pads in properly? - Warp speed down a hill and brake as hard as you can (without locking), repeat ~10x (you'll know when the pads have bedded in).

With the levers going to the bar I suspect they need a bleed (although I run mine with the bit point close to the bar).

Has the kid being putting mucky fingers on the new rotors?

Did you drag your brakes a lot - it glazes the pads over so they are a lot less effective (not usually a problem on a mtb, but can be for nervous roadies on long descents where they are controlling their speed)

I've XT and I can lock the wheels with 1 finger (I weigh 100kg), ~140kg at 30-40kmph is a lot of kinetic energy to dissipate, I'd still expect to feel a dramatic braking effect.

Brakes are 3 weeks old. Rotors are probably 2 months old. Mostly used on flat-ish roads with normal braking patterns - red lights etc. I took that downhill road first time. Everything is clean and was not contaminated. They do brake fine but this one time caught me by surprise and it made me wonder if they are enough or not working properly.

saccades

What size rotors?

Mind I have 160's on the road bike, 180's on the Fs and 203's on the hardtail, but bigger rotors stop quicker (more torque on the wheel).

Might be a bit mucked up with road gunk from riding in the wet that you haven't noticed with gentle regular braking.

I'd suggest pumping the brakes on/off and see if they get firmer, if they do they need a bleed. After that I'd do a a set of high speed stoppies and see if that improves them.

After that go back to the shop you got them from and let them have a look, you might need bigger rotors to cope with all that weight at speed.

Don't use car disc brake cleaner - bike brakes don't get hot enough for it.





@68 Fastback - BB7's are the best mech brakes, on a par with most Hydros (just not self adjusting).

'68 Fastback

saccades wrote: »

@68 Fastback - BB7's are the best mech brakes, on a par with most Hydros (just not self adjusting).

I didn't mean to talk them down, they're very decent stoppers but I stumped up for Hopes on my MTB so I'm kind of spoilt

smacl

saccades wrote: »

What size rotors?

Mind I have 160's on the road bike, 180's on the Fs and 203's on the hardtail, but bigger rotors stop quicker (more torque on the wheel).

Might be a bit mucked up with road gunk from riding in the wet that you haven't noticed with gentle regular braking.

I'd suggest pumping the brakes on/off and see if they get firmer, if they do they need a bleed. After that I'd do a a set of high speed stoppies and see if that improves them.

After that go back to the shop you got them from and let them have a look, you might need bigger rotors to cope with all that weight at speed.

Don't use car disc brake cleaner - bike brakes don't get hot enough for it.

Is there a cleaner that you would recommend, or should the discs be self cleaning. Always concerned after cleaning the bike and lubing the chain that I might have compromised the discs. Similarly, is there much involved in doing a bleed? Bit of a noob to disc brakes here and I'd like to be able to service them myself. The brakes are BRS-785s on a cross bike, and I've picked up the dealers manual for them, but just wondering is this a job better left to the pros. Not loving the performance from the at the moment tbh.

gambeta_fc

smacl wrote: »

Is there a cleaner that you would recommend, or should the discs be self cleaning. Always concerned after cleaning the bike and lubing the chain that I might have compromised the discs.

For what it's worth I use surgical spirits based on advice from LBS, easy to get in any chemist.

My first set of pads got contaminated, tell tale sign is a squeal/screech when coming to a stop. I was told to remove the pads, sand them down a bit, give them and the rotors a good wipe down with surgical spirits and a clean cloth. That worked for a short while but the squeal returned, the pads seem to be porous and take any contaminant up through them.

I fitted a new set of pads since, use surgical spirits on the rotors after washing the bike and no issues since.

smacl

Thanks for this. Found the following on youtube and given the braking is feeling a bit spongy will give it a go once I get a chance.

https://www.youtube.com/watch?v=SMGl58-tMqg

omri

You can also do it this way. I think it's better than the first video as you're pushing everything (trapped air) up from the caliper to the reservoir and eventually to the little cup. But correct me if I'm wrong.

https://www.youtube.com/watch?v=XfGMGMhsp6U

check_six

I did a bit of work on my disc brakes recently. I got a bleed kit from a crowd called epic bleed solutions. The kit had all the bits required in it along with some clear instructions. The price was decent too. I was pretty happy with how the whole process went as I had been a bit intimidated by the idea of doing the brake bleed previously. I replaced the brake pads for good measure and everything is braking well now.

The kit doesn't come with a bleed block, but there are instructions for making one from old credit cards (or store cards, etc.) which works really well.

A mechanic in the LBS recommended a very light sanding of the brake rotors followed by cleaning with surgical spirits to remove detritus on them.

V-man

Weight (Check, Fully loaded cargo bike)
Speed (Check, 40KM/h+)
Gravity (Check, Downhill)

Personally not surprised that this combination did not stop as fast as you expected, did you ever try to make a emergency stop under normal circumstances? 25KM/h, loaded and on a straight?

omri

Front rotor is 180mm, rear 160mm. Am I correct in thinking that at any speed with load my wheels should lock if I braked hard enough ?

V-man - I never tried emergency stop. I felt that in less extreme situations brakes were sharp and offered enough braking power.

ec18

omri wrote: »

Front rotor is 180mm, rear 160mm. Am I correct in thinking that at any speed with load my wheels should lock if I braked hard enough ?

Not necessarily I find on my bike it varies, If it's on the flat in dry conditions, then yes the wheel will lock. If it's downhill or greasy conditions then the brakes will work it'll take longer to kill the momentum. When you're braking on the flat once you stop pedaling the bikes energy starts to decrease and you'll eventually come to a stop whether you pull the brakes or not. So when you pull the brakes all the stopping power from the brakes goes to taking energy from the system.

When going downhill even if you stop pedaling the bike is gaining kinetic energy rolling down hill. So when when pulling the brakes the stopping power is doing 2 things, first its countering the energy that you're gaining from going downhill and secondly actually slowing the bike down. So the wheel might not lock up as it will on the flat

If you add in a greasy service or worn tyres then the stopping force that the brakes produces will be decreased

omri

Just a quick one. If I'd like to get bigger rotors do I have to use new adaptors on top of the current ones, or just replace my current with the new ones ?

Update: Got sorted via UTUBE.

tigerboon

omri wrote: »

I got Deore XT disc brakes on my cargo. With ice-tech coolin system. Was going downhill from sallynoggin roundabout towards dun laoghaire. I would say bike+me+kid was around 130-140kg. We were doing somewhere between 40-50kmph maybe bit more than that, then at the bottom of the hill the lady decided to cross but couldnt make up her mind when she got to the middle of the road. So I had to slam the brakes and to my surprise I wasnt really slowing down as expected. So I slammed even harder pulling levers to the handlebar pretty much. Now my question is - is it what I should expect at this weight and speed or are my brakes not enough ? Maybe theyre not blead properly ? They are new, 2 weeks old at most.

My guess is you were expecting a bit too much from your brakes. I have discs and find they are very trustworthy but whether they are better than good block brakes is debatable. Downhill at 50kph with a kid on board??