Ask Pinkbike - DH Bikes, Bottom Brackets and Drivetrain Rumbles
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.
DH Bike Dilemma
Question: Pinkbike user norcodher11 asked this question in the Downhill Forum: I'm looking to change bikes and have narrowed it down to either the Scott Gambler or the Specialized Demo 8. Do you have any comments about the positive and negative attributes of these two? I know there are a lot of Demos around, which must mean that they are good bikes, but the Gamblers look pretty interesting. Let me know what you think.
 | To be honest, there's not much between a lot of downhill bikes on the market, but the two that you're considering really are at opposite ends of the spectrum in terms of what they each excel at. Let's look at the Demo first: it's a nimble bike that is going to take less effort to ride and race on tracks that require a bit of pedalling or are less fall line in their layout, and it's a load of fun on smoother, jump filled trails that really allow its playful side to come out. Depending on the terrain, it's also one of the best cornering downhill bikes on the market, at least in my mind, thanks to its relatively short rear end and low bottom bracket height. If the Demo is the gazelle of downhill bikes, the Gambler has to be the lion. The Scott has a much larger presence on the trail, and while a solid rider is going to get it down any course in good time, it feels most at home on steep, chunky and fast terrain. It also sports adjustable geometry that allows you to tailor it to exactly those sorts of tracks, letting the rider slacken it out much more than what's possible with the Demo, and really feels at home on the gnarliest of trails. Where it doesn't excel, at least in my experience, is on tamer terrain that requires you to use some ponies to keep your momentum up. It's a serious bike that performs best on serious trails. I'd recommend that you make an honest evaluation of where and how you ride before laying out cash for either. I'd tell a downhiller who rides and races on more average terrain to go with the Demo, but the Gambler may be more up your alley if the only pedalling that's required on your trails is the twenty feet from the truck to the trailhead, which really isn't how it usually is. Sometimes the gazelle is fastest, and sometimes it's the lion. - Mike Levy |
The Gambler isn't the best bike for "flow trails" but it's certainly one of the most confidence inspiring on serious terrain.
One-By Drivetrain Rumble
Question: KEEPitSTUPID writes in the All-Mountain, Enduro & Cross-Country forum:A few months back, I changed my 3x10 to a 1x10 setup, with a Shimano XT clutch rear mech and a 32-tooth thick-thin chainring by Raceface, and it ran faultlessly. Then, a few weeks back, I began to get a rumble when pedaling backwards. No changes were made. I have since swapped out the chain and cassette with a KMC chain and a Shimano Deore XT cassette with the same ratio as before. Now, when I pedal hard I get a rumble - and I cannot see a problem with my drivetrain. Any clues would be appreciated.
| Take the chain off the front sprocket and spin the crank. If it spins smoothly, you can eliminate the possibility that you blew up your bottom bracket bearings. Next, view the rear derailleur from the back of the bike to see if it is visibly bent inwards. If not, the most probable reason for your rumble is that the rear derailleur's B-tension screw is not turned in enough to keep the upper derailleur pulley from running into the cogs. The B-tension screw contacts the hooked tooth of the frame-mounted derailleur hanger. Turning it in (clockwise) rocks the derailleur body away from the cassette sprockets. Another problem that is specific to one-by transmissions is running the chain too short, which draws the lower cage pulley so far forward, that the angle the chain creates between the pulley and the chainring is greatly exaggerated when the transmission is shifted to the extreme ends of the cassette. This causes rumbling at the chainring as the links attempt to engage the sprocket teeth from an odd angle. The new chain and cassette may have tighter tolerances and thus would naturally make more noise. Check the B-tension screw first and then set your chain up as long as possible. Start by stringing it through the rear mech with the derailleur shifted into the smallest cog. Set the chain length so that the pulley cage just begins to angle downward - that's the longest that the chain should be. Finally, you did not mention whether you were using a short, mid or long-cage rear mech. If you are using a mid or short-cage derailleur, it may not have enough chain takeup to cover your gear range, in addition to the amount of chain growth built into your rear suspension. A drivetrain with a perfectly tuned chain length on the bike stand may be pressed to the breaking point when the suspension is sagged or compressed while climbing. You may need to bump up to the next longer pulley cage if this is the case. - RC |
The B-tension screw (left) keeps the upper derailleur pulley from running into the cassette cogs, which is the cause of most rumbling drivetrains. KEEPitSTUPID's narrow-wide Raceface chainring is probably not causing the noise.
What type of bottom bracket do I have?
Question: thekwp2011 asked the following in the Beginners Forum: I want to order either a spider or direct mount ring form North Shore Billet for my SRAM X9 crankset. I need to specify if I have a BB30 or GXP BB. How can I tell which one I have?
| Bottom bracket 'standards' can be confusing, since the number of different options continues to grow. If your cranks are already removed, the easiest way to tell what you have is by measuring the spindle diameter. As the name suggests, a BB30 or PF30 crank will have a 30mm spindle diameter, while a GXP crank's spindle will measure 24mm with a 22mm splined portion on the end. If you look at your bike, the bearings for a BB30 setup are typically found inside the frame, while the bearings for a GXP would be housed externally in cups that thread into the frame. Another way to find out without doing any measuring or crank removal is to go to your bike manufacturer's website - there should be a section where all of the frame specs, including bottom bracket type are listed. - Mike Kazimer |
The bearing for a PF30 bottom bracket are housed inside the frame, as opposed to being housed in external cups that are threaded in.
Have some unresolved tech questions? Jump in the Pinkbike Forum and we'll look to answer it for next time.
Author Info:
Must Read This Week
Sign Up for the Pinkbike Newsletter - All the Biggest, Most Interesting Stories in your Inbox
PB Newsletter Signup
Of course I also wonder when I will win my state's lottery. Unfortunately, I have never "played" the lottery but I suspect that both events are just about as likely.
For examples of that ridiculous behavior by pinkbike members, i suggest you start following the posts of Amanda Batty, as a female DH racer she gets all the immature sexist abuse this site is known for.
www.pinkbike.com/u/ambatt
www.pinkbike.com/news/first-look-rocky-mountains-carbon-thunderbolt-msl.html
@deeeight - I don't know if you work in the bike industry and get given a lot of gear so you either don't ride the same bike for long enough to see these problems, or you don't care if it breaks because you didn't pay for it, or maybe you just don't ride that hard, but I KNOW from experience that these new press fit BBs suck.
As to the way comments are propped on pinkbike... never mistake immaturity with knowledge. Bike designers long ago gave up taking this site's member opinions seriously because the vast majority who bother to complain about something don't represent anything but the whacko fringe of the sport.
Frames usually don't "get" out of alignment during usage without a crash being involved but they often come that way brand new out of the shipping box. The epoxy specialized's dealer used to re-install the BB was a bandaid solution to avoid having to warranty the frame.
deeight, do you work in a shop? If not, then you clearly have no idea about how many customers are actually affected by this. It really sucks. What the bike industry is doing is obviously saving tons of money on a cheaper BB construction and weighing this against how many come back for repair and/or claimed as a warranty. When this design was first pioneered in the road bike industry, it worked pretty ok because it's obviously a different type of riding with less abuse on the products involved. Now that this design has been brought to the mountain bike world, it really sucks. And personally, I am avoiding buying frames that come with press fit. It simply has no advantage for the end user and is far less reliable than the BSA threaded external BB.
Sorry for the bad english
Specialised and Giant are global brands with big import distribution capabilities the Evil empire wins!
I ride a Mondraker Summun the best DH bike ever by a long margin, Ive ridden Giants and Demos, It kicks arse, do like the Gambler though, not ridden one in battle, I would have thought the 27.5" model would be a better match against the 26" Demo in pedalling department.
The newer Demo 27.5 lefty sided frame looks like a pure racer, so Levys above comments may not be as big a factor if comparing the new model, also I think if you're honest with your ability for less skilled riders the bigger bike while maybe less flickable sprightly on smoother trails will help you on improving speed stability and safety riding DH rather than being out of control riding stuff slowly that puts others as well as themselves at risk, something I see quite allot these days on DH tracks and bike parks, not everyone is an expert in fact most are not, just not honest in they're abilities.
I didn't think the Demo pedalled all that well, my DHR and V10 and Mondraker way superior incl the Giant in that too. Its not bad just not better, does corner nice, better than the Giant and the V10c imo/e but not as nice as a DHR and esp not the Mondraker which is the best cornering DH bike Ive ever thrown a leg over by a long margin.
Just my added 2c
All chainrings will wear out eventually. I'll take replacing a narrow/wide chainring every 1000 miles over having to deal with constantly broken chain guide parts if it means I get a smooth, drop-free drivetrain.
Honestly though, my XX1 drivetrain started making that noise about 1000 miles in. I checked the B-tension screw and then replaced the front chainring: problem solved.
vimeo.com/80191950
But problem is that until frame manufacturers are not under thumb of gearing companies forget it being easily available
I think that scott pedals 300000000 times better than a demo
Do you guys have some opinion about that?
To the OP, the best way to know which DM ring or Spider to order is to measure your spindle. If you have a 24mm spindle you should order the GXP model. If you have a 30mm spindle, you may STILL need the "GXP" model - you'll know which version to get by measuring the spacer found on the non-drive side of your spindle. If that spacer is ~15mm, you'll still likely need the GXP model. If it is ~9mm, the BB30 model will suit you better.