Ask Pinkbike - Rotor Diameter vs. Stopping Power, Dropper Post Length, Broken Chain Mystery, and Tires for UK Winter
Here at Pinkbike we get inundated with all kinds of questions, ranging from the basic "Can I have stickers" to more in-depth, soul searching types of queries like if you should pop the question or what to name your first child. Ask Pinkbike is an occasional column where we'll be hand picking and answering questions that have been keeping readers up at night, although we'll likely steer clear of those last two and keep it more tech oriented.
Bigger Isn't Always Better
Question: Pinkbike user Layman asked this question in the Bikes, Parts, and Gear forum: Why would anybody run anything other than the largest rotor? Other than a weight penalty, which is minimal, I can't see a disadvantage to going with a larger rotor. Am I missing something?
 | A larger diameter rotor will offer more power thanks to the added leverage that it provides, and it will also deal with heat better during long, hard descents like you'd do on a downhill bike, but outright power isn't always the only concern. Most people will cite weight as the reason for going with a smaller rotor, but I'd argue that using rotor size to tune how your brakes feel is the real reason to go down in size - a smaller rotor will offer a more controllable feel at the lever, which is especially important when riding in low-traction conditions. For example, in the summer I'll often use a fast rolling tire on the back of my bike and the trails can be a bit dusty and loose. My Shimano brakes have quite a bit of initial bite to them that can cause me to lock up my wheels unexpectedly when I'm pushing hard, but going from an 8'' rotor to a 6'' rotor gives me back a lot of that control. Yes, there isn't as much power on tap when using the smaller rotor, but the modulation that I get outweighs that loss in my mind. After all, almost everyone's brakes are more than powerful enough so long as you're using them as intended. That said, I'd prefer to run 8'' rotors anytime I'm in a bike park for the power and heat management. The one other thing to keep in mind is that some older forks with quick-release dropouts are not compatible with large rotors. - Mike Levy |
Brakes feeling a bit too grabby? Moving down a rotor size can add more feel and control, especially if you're often riding in wet or dusty conditions.
Choose the Correct Dropper Post
Question: SWolbeck asks in the All-Mountain and Cross-Country Forum: I am looking to surprise my husband with a RockShox Reverb seatpost for Christmas. He has a 2012 Santa Cruz Carbon Nomad. I need help deciding on the size of the seatpost. I know he needs a 30.9 diameter. Can the 2012 Nomad accommodate the 125mm, or is the 100mm safer - and if so, what length would I go with (355mm, 380mm, or 420mm)?
| Droppers are longer than conventional seatposts because they need room for their telescoping innards. Because the seal-head restricts maximum insertion, some longer travel dropper posts cannot be lowered enough to achieve proper saddle height if the frame is tall, or the rider's legs are too short. The 2012 Santa Cruz Nomad has a straight seat tube, so all three travel options of the RockShox Reverb dropper post will fit the frame. You mentioned in a follow-up post that your husband was five feet, nine inches tall. Experience as a custom frame builder tells me that his inseam will be close to 32 inches, which suggests that 120-millimeter option is going to be your choice. Pushed down to the seal-head in the frame, the 150-millimeter option may be too tall at full extension to allow your husband to achieve his correct saddle height. The 100-millimeter option does not give most riders enough drop for proper descents and is typically used when the rider is too short to achieve proper seatpost extension, or when the seat tube design limits the distance that the post can be inserted into the frame. I also have a 32-inch inseam and, while I can ride a 150-millimeter Reverb on most test bikes, the post is usually slammed to maximum insertion. The 120-millimeter Reverb, however, leaves me at least three centimeters of seatpost adjustment to play with below the post's seal-head in the worst scenarios, so the 120 would be the safe bet in your case. - RC |
When choosing a dropper post, get the longest stroke model that you can ride. Before you buy, however, first check that the post can be inserted far enough in your frame to achieve your correct saddle height. If this GT Force's frame was any taller, I would not have been able to lower my saddle enough to ride the bike when the Reverb was fully extended.
Broken Chain Blame
Question: Pinkbike user Poacherx2 asked this question in the Bikes, Parts, and Gear forum: I took my bike to a reputable bike shop for some maintenance. I have a full '07 XTR crank, derailleur and cassette. I asked them to fit an XTR chain but they didn't have one in stock at the time. The lad I spoke to suggested that the SRAM one they had in stock would suffice and that it was a bit cheaper. After 3 or 4 rides to and from work and a ride at Gisburn Forest over the course of 3 months the chain snapped on one side. The chain caught the derailleur and snapped it in half and snapped the hanger too. I took it back to the shop and they put the breakage down to wear and tear. I've had the bike for 5 years, always had XTR chains and never had a problem until this. Do I have grounds to have them replace the broken components free of charge? The chain shouldn't have snapped with as little use as it had.
| I've seen countless iterations of the this scenario during the years I spent working as a wrench. A customer purchases a part, and a few months later comes in with it in pieces, expecting a full refund, since it broke when they were 'just riding along.' It's a tricky scenario - as a shop employee, you want to give the customer the benefit of the doubt, but at the same time, a shop won't remain in business very long by shelling out free parts every time something breaks. In your case, more than likely the chain breaking and the resulting drivetrain destruction was simply an unlucky coincidence, and it's neither the shop nor SRAM's fault. Of course, a chain should last more than a handful of rides, and the majority of the time they do, but it only takes one hard shift to load the chain at an awkward angle and bend it, or one rock strike to accomplish the same thing, and then after a few more miles the weakened link gives up for good. Being willing to compromise is your best bet in a situation like this - you might not be able to get a free derailleur and chain, but the shop may install the replacement parts at no cost, or give you a discount on the new components. If you want to go with the glass half full approach, this might just be the excuse you need to upgrade your drivetrain - maybe a 10 speed cassette, shifter, and clutch-equipped derailleur are in your future? The clutch feature alone makes this a worthy upgrade, it just depends on how much money you want to spend on a seven year old bike. - Mike Kazimer |
Snapped chains happen, something Neko Mulally knows all too well, but it didn't stop him from getting 4th place at the 2014 DH World Championships.
Tire Choice for British slop.
Question: Pinkbike user Titaniumtit asked this question in the Bike, Parts and Gear Forum: After sliding around like a wet fish in a skating bowl, and not knowing too much about tires, I am wondering what 2.2" mud spikes to go with that aren't too draggy when in between trails? Also ones that are not too heavy so what is acceptable weight? Compound wise, I'm thinking the stickier the better, to ride every day XC/hardcore trail and spot of light DH usage on them without being overkill?
| It sounds like you are looking for grip and performance over weight and pedaling speed. For the rear, I would suggest a Specialized Storm in the Control casing, I spent a lot of time on a Storm last year with great success, at 2.0 inches, it is a little narrower than you asked for, but this will help to cut through thick mud and get the tire biting. The medium compound and short, widely spaced spikes clear well, and also roll surprisingly well on road and hard-pack. For the front wheel, Maxxis Shorty's are finally available in the UK in a variety of sizes. For a 27.5" wheel, a 2.3" MaxTerra Exo, weighing 865grams should offer plenty of volume for stability, cushioning and absorption at the business end of your bike. The 3C MaxTerra will be soft on the rocks and roots, and will be spiky enough to dig in to the mud. Of course, there are many different options and opinions about tires, but If you don't want to spent hours banging your frozen knuckles off spokes to swap tires every week for changing conditions, this combination should see you through the multitude of Britain's best winter weather conditions. - Paul Aston |
The Maxxis Shorty should make a great all-rounder in a British Winter.
Have some unresolved tech questions? Jump in the Pinkbike Forum and we'll look to answer it for next time.
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I opted for Intense 2.25" spikes in sticky rubber. Ok ok so theyve added 800 grams combined and they fedl draggy as fook but are quite confident and agressive for changing lines.
as Mike said in his answer, reducing rotor size reduces leverage which increases modulation as the brake has to work harder to slow the bike, generally making the brake more controllable.
Many new road bikes with discs (I'm riding a 2015 model with Shimano hydraulic discs) are only using 140mm to give controllable modulation, but with finned brake pads to help manage heat. Interesting to see what affect a heavy rider brake dragging down a long mountain road in the Alps would do to the road brakes?
ep1.pinkbike.org/p4pb11703549/p4pb11703549.jpg
I love my Saints and can quite happily modulate between slowing and skidding the rear or slowing and REALLY slowing the front. I ride my mates Tech M4s and find myself just not slowing down as much as I expect to when I pull the anchors. It has made me crash his bike off the outside of corners before.
My mate on the other hand loves his M4s He can stop just as fast as I can and has similar control over slowing/skidding. However he rides my bike and skids out the rear too much and ends up pulling stoppies when he's just trying to scrub a little speed.
Like I say its all about getting used to what your brake can do.
It feels better, because strange as it sounds, I want less modulation on the front and a more positive "hard stop" feel, which I can get used to but I know I have that stopping power when I need it - typically an emergency stop when a motor vehicle does something stupid on the highway!
The original 140mm had great modulation but felt like it was lacking bite
the original rotor was a custom Tektro with aluminium spider, the replacement is a Shimano XT and I got to say the rotor quality feels better with Shimano.
Customer: 'I don't know what you mean! I've never ridden in the rain and I've only ever cleaned it with a damp cloth!'
As predictable as JRA
A chain snapping or breaking cassette within 6 months should be still covered, despite being a wear part. Snapping is not wear. If it is worn down a lot and can be expected to break because of that, it is not covered.
If you love reading -->
europa.eu/youreurope/citizens/shopping/shopping-abroad/guarantees/index_en.htm
www.pinkbike.com/news/Tech-Tuesday--Make-a-Ghetto-Dropper-Post.html
im using huge 224mm hayes v8 rotors, i thought it was a good idea, but when i slightly touch the brakes the rear wheel start to drift everywhere, and even the front wheel in loose gravel. Im going back to 200 front and 180 back.
www.pinkbike.com/photo/11568915
But there is limits:
- your rim has to be strong enough to accept the pressure of the pads.
- If you have "powerful" pads, your aluminium rim won't last long (brake disc are in steel with hardened surface)
- the pads need a long travel to be far of your rim, in case the rim is bending under efforts, or if the rim isn't trued properly, so the brakes have a low rate between lever and pistons, so there is a limited amount of pressure on the pads.
The main downside of rim brakes for me is that they can get unpredictable / lose their brake power when the weather gets bad.
The rim rubbing against the brake pads when it's slightly out of true isn't a bad thing IMO, that just motives me to true it again before it gets worse. This way I take better care of my wheels.
Disc brakes are better dosable though and are easier when you're removing your wheel, especially with fat tyres.
* A rim has flex when it gets squeezed whereas a rotor does not. This means that some of the brake travel goes into flexing the rim inwards.
* A rim brake has to have a bunch of clearance so that the rim doesn't rub the pads. This means that rim brake pads must travel much farther than disc brake pads. There is always a trade-off between amount of travel and ability to apply force. Rim brakes sacrifice force for travel whereas disc brakes can be optimized for force and travel.
* A rotor can be made out of steel because it is small so it can handle hard, high friction pads. A rim needs to be light (generally aluminum) so it needs to be used with soft pads that wear more quickly so that the rim doesn't wear out in a few rides.
Also it will usually last much longer and it is cheaper.
I think that it you only have small and non-steep hills, like the XC trails in The Netherlands, v-brakes are better choice due to the above reasons.
But as soon as you hit something that actually gets technical or where the hill is higher than 30m (so for pretty much all mtb riders here), disc brakes are the best choice.
Being a drivetrain from 07, unless it's not abused, it shouldn't have broken that easily. XTR was built for xc racing (light) so it wouldn't be as strong, especially over that span of time, compared to an XT from the same era.)
good luck!
However,
I have never had a broken chain damage a dérailleur and cassette. Perhaps even the old dérailleur or cassette broke first causing this domino effect. As a shop owner or employee, I would also be wary of being held responsible for a 7 year old drivetrain. As a former shop mechanic, I often heard the phrase,"I had this part for 7 years and never had a problem until I brought it to you."
The customer is so bummed about their old bike breaking right after they finally start investing in it instead of buying a new bike. Now they have to face the reality that super expensive XTR equipped bike won't last forever, and that prices have only gotten higher since 2007 and most of their state of the art drivetrain is considered obsolete.
It also doesn't help when friends and family tell fish stories and exaggerated tales of them breaking down an employee and getting stuff for free. This makes you, as a customer, feel inadequate and cheated.
Jeez, I am rambling and could probably go on all day. This is unfortunate for this person to have their bike left unrideable. I would advise looking for the exact parts they want on eBay or pink bike buy/sell. I would think 2007 brand new parts might be out there and at a really good price.
They installed a SRAM chain, so most likely 99.9% they would have used the master link that came with it.
KMC yep
Sram? No fkng way!
My caliper mounts directly on my frok, no adapter.
says it comes bb so that is figured out. If anything I can upgrade later.
www.jensonusa.com/Shimano-SLX-M675-Crankset
And for brakes, zee are good, but SLX are good brakes and maybe cheaper. with good metallic pads (I use CL brakes VRX), they Rocks. and you save money for other parts.
Don't hesitate to invest in good suspensions and wheels first, (and good brakes so you'll have confidence in your bike), and save money in other parts. A crankset is less important and less expensive to upgrade later.
Just me?
1) Whith smaller discs you need more pressure on the pads to achieve the same braking moment. => faster wear
2) They concentrate heat faster > you need even more braking force. => faster waer
Larger rotors:
+More power
+Less wear (both rotor and pad)
+better modulation when braking hard
-weight
-allignment (larger rotors are more exposed, they usually tend to bend more)
Smaller rotors:
-less power
-More wear (both rotor and pad)
-bad modulation when braking hard
-heat buildup
+better modulation when braking less
+weight
+allignment
+Less exposed
Bigger disc:
- less wear on the disc
- more wear on the pads
Smaller disc:
- more wear on the disc
- less wear on the pads
But this is only with the exact same ammount of pressure coming from the brake pads. With smaller brake pads you will need to apply more pressure for the same stopping power, so that might change the situation.
Interesting statement.
Larger rotors give you more power and more modulation. I could see how someone might feel like a smaller rotor would give them a "more comfortable feel", but that doesn't translate to better modulation.
1 - Force applied at the lever - Let's just assume that's our independent variable
2 - Force applied to the disk - Dependent on force applied at the lever for a given brake system
3 - Torque applied by caliper and rotor combination - Dependent on 2 and radius of rotor
4 - "Transfer" of 3 into friction force on tire - earth interface - dependent on 3, wheel size and coefs. of friction between tire and earth
The added weight of the disc is negligible. For an unlimited range of force 1 and 2, THE ONLY LIMITING FACTOR will be the coefficients of friction between your tires and the ground. The maximum braking force will be the maximum static friction between the tires and the ground.
A larger disk allows you to use that braking force to its maximum with a lower input of f1. But, someone who doesn't have a thermo final in 2 hours correct me if im wrong but i imagine that a larger disk allows you to brake more without locking up the wheel since the larger disk will be spin faster than a smaller one at the braking interface? Then again maybe for a smaller disk the torque of the ground onto the wheel will overpower the caliper's friction... I need to get back to thermodynamics
@tsheep both rotors sizes can reach that point just before lock-up, because both can lock up the wheel. It should just be easier to find and hold with bigger rotors.
@RM396 I'm not sure about that because the added acceleration of the rotor will be offset by the increased diameter of the rotor.
In reality larger discs require less force at the lever to reach that "almost" locking up point, the sweet spot. Less force at the lever means easier modulation so it's easier for you to maintain that sweet spot, but the range of forces available is still the same which is important to realise, probably more so on the road to be honest!
@metaam Ah OK. @RM396 not sure, I'd have thought heat will affect the braking surfaces in some way, after all you can glaze pads, but really don't know by how much when used properly. Now get back to your revision!!
@Gav-B I know how to brake without going over the bar, thank you, and I won't put a 160mm rotor on the front of my downhill bike, I need braking power, heat resistance, and I don't want to change pads every day.
Anybody can lock up the rear wheel at sidewalk speed. get up to Sam Hill speed, though, in a good traction situation, & it's quite possible that you wouldn't be able to lock up the wheel with a lightweight XC brake.
Lastly, total system weight, of which the rider is the biggest part, can have an enourmous effect on the amount of traction you have, & the amount of braking you can apply before break free. That's not a constant, either: wieght shifting & rider movement can magnify their natural weight several times.
How much traction do you have when you're exerting over a ton of downward force? Imagine how much braking force you can apply before breaking that free, especially when not just grabbing a fistfull of back brake?
The only thing mentioned is disc size and stopping power. Get up to Sam Hill speed and grab a fistful of a Saint brake with a 160 rotor and a 203 rotor, the result will be exactly the same.
I'm not saying that bigger discs don't offer many other benefits, they do. I'm also not saying the only way people brake is full on and full off, it's not a debate about braking techniques, I'm no where near good enough to really participate in that.
I'm saying that bigger discs cannot, in themselves, increase stopping power.
As for my "moving the goal posts:" replace what I said with a 160mm rotor instead of an XC brake, & it still holds true. The change is immaterial, as the results are the same.