Ask PB: Tire Pressure, Ghost Shifting, and Pedals for Bigfoot
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.
Proper PSI
Question: Pinkbike user cryan asked this question in the downhill forum: I'm just wondering if anyone has any good tips when it comes to figuring out the proper pressure to run your tires at when downhilling in different conditions? How much do I run when it's dry? And how much should I run if it's wet and muddy?
 | First, there are all sorts of things that can factor in here, including if you're running tubes or not, what type of rider you are (smooth or smashy), how much you weigh, and what you're trying to accomplish on your bike. Then there's also things like rim width, tire volume and casing rigidity, and even the rubber compound to keep in mind. It's a topic we've covered before, with Vittoria's Ken Avery covering the hows and whys of air pressure in a 'To The Point' article a few years back, but it bears going over again. What I can't do is give you an exact number to go with, but I can explain how to figure it out for yourself. One could easily write a few thousand words about what tire pressures work best in each scenario, but there's a much simpler way to figure it out for yourself: since you didn't specify what kind of tire setup you use, try starting at 30 PSI and riding a section of trail that is typical of your usual terrain. Be sure to include difficult corner as well. Do the section over a few times, letting out 3 - 5 PSI before each test run, until you either feel like the there is too much casing roll (it'll feel vague, and like your bike isn't responding to inputs), or you start to bottom out the tires on the rim over roots and rocks. You might also 'burp' the tire if you're using a tubeless system, which is when the casing rolls over so much that it pulls the tire's bead away from the for a spit second and allows a fart of air and sealant to escape. Add a few PSI until those symptoms go away and you should be within the ballpark, and then you can adjust to your preference from there. As for downhilling in wet an muddy conditions, you'll generally want to run your tire's a little lower. This is the opposite approach that a World Cup-level racer takes, as they add pressure to prevent flats since they can't see the rocks and roots hidden under the mud, but more casual racers and riders should be looking for increased traction when things get slippery. - Mike Levy |
The ideal air pressure allows the tire to conform to the ground better, thereby improving traction.
Phantom Shifting Woes
Question: Jrcd says in a PM: I'm an ex-bike mechanic and have worked on countless bikes at various shops over the years. Recently, I was working on my wife's bike (a Specialized Stumpjumper FSR 29er), and had the rear shifter's indexing correctly adjusted on the stand. Taking the bike for a test ride, the indexing was revealed to not actually be aligned, and I recalled encountering similar situations from my days in the shop. Further cable tension adjustment solved the problem, but my question is this: Why does the weight of the rider cause a discrepancy between shifting on the stand and shifting during a test ride? A similar phenomenon occurs when truing wheels, but it's easier to see the how the rider's weight would affect the tension of the wheel. What gives?
| Your problem occasionally happens to dual-suspension bikes. The standard cable routing on some models will cause a slight change in length as the suspension settles into its sagged position. I had to run down that issue a few times. Test my theory by running a full-length housing and don't attach it to any part of the frame that will cause it to flex. If that fixes the problem, then reroute the derailleur cable and housing on the frame to minimize flexing. - RC |
The 2013 Stumpjumper FSR Expert Carbon EVO 29 has similar under-the-bottom-bracket cable routing as the bike Jrdc was having trouble with.
Pedals for Bigfoot
Question: BigSteve420 asks in the Bikes, Parts and Gear forum: I am curious as to what are some of the biggest pedals available? I have a size 13 foot and the pedals I have seem too small. The Spank Oozy are 110mm x 110mm and the Chromag Scarab are 110mm x 105mm. Are those the biggest or are there some that are biggger?
| Both of the pedals you mention are good options, but for something even bigger I'd recommend the VP Harrier pedals. They measure 120 x 110mm, and are only 12mm thick. When I tested them last year, I found that they were almost too wide for my size 11 feet, but I imagine they would be just about perfect for you, providing plenty of room to avoid having your foot hanging over the edge of the pedal. They're well constructed as well, with an aluminum body that rotates on bushings around a chromoly spindle, and a decent amount of traction provided by the 10 steel pins on each side. - Mike Kazimer |
The VP Harrier has plenty of room to accommodate riders with big feet.
Have some unresolved tech questions? Jump in the Pinkbike Forum and we'll look to answer it for next time.
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look at me here
begging on my knees
look how im rhyming
10/10 timing
i promise im not lying
otherwise ill be dying
amen
follo mwe
I'm Ron Burgundy?
I'm 155 pounds with gear and run 20 psi front/22 psi rear.
That's tubeless on a mix of Maxxis EXO, Shwalbe and Continental TR.
I like my rubber hard, I also ride it very stiff in the front and go hard and slow in the rear, and yes, she did say that
All pressure's are mid-20's and up. Voreis with a super high 38psi F+R. Most WC DH riders are also running high 20's and into the 30's. I think Fairclough stated to be running 30-32ish.
On trail bikes it's abou t4- 5lbs more on the single ply sidewalls, even when tubeless, for the same feel.
It's important to state what application it's for to make any sense of it.. DH and Trail tires are worlds apart.
2. So I will be with BB Cable routing upwards.
3. Size 100X105mm pedal seems the best.
^^*)
The best trick I've learned is just a good few thumb presses on the tire. Once it feels "right" I know I'm good to go. This takes time and practice to master but I've never had a flat in a race run. I invested in a good digital gauge also. I make up for higher pressures by setting my suspension a tad more soft than most. Not super plush but one or two clicks.
As a consumer, I wish the manufacturers would have solved this problem better when DH and freeride pioneers were experimenting in the late 90's and early 00's so that I wouldn't have to immediately start looking for new rims and tires for my first 29er I bought this spring. I had to go from 2.25 to 2.35 tires after my first semi-fast downhill turn, and 21mm rims are falling to pieces and making my PSI go up to hopefully preserve the rims long enough to get some tough, wide rims.
use the "add to cart at their price" feature to get them at the outside outfitter price
boss pedals, better than the spank spikes i've blown up 3x