THE FACTS OF FATIGUE
I’m considering buying a Giant from the XTC-C range, but I’ve heard that carbon-fibre frames can become brittle after a couple of years. It’s a lot of wonga to spend on a bike if it needs replacing after only 24 months. What are your thoughts on the ‘structural integrity’ of carbon-fibre bike frames?
Steve Itchy, email
There have been cases of carbon frames suddenly failing after a couple of seasons. Usually, the failure can be traced back to an area on the frame that was damaged in a previous crash or spill, when what was perceived as cosmetic damage was in fact structural.
Composite bike frames are a relatively new development and it has taken a few years for designers and manufacturers to understand its properties fully and to design frames using the appropriate matting, weave, resin and the right amounts of each. In previous years designers ‘pushed the envelope’ with the new wonder material to provide ultra-light frames, but many went too far. The resulting frames were too fragile for extended use, though this was more noticeable in the road market than in the mtb arena.
Excluding cost, carbon-fibre composite frames have the potential to outdo similar offerings built in steel, aluminium or titanium. The strength-to-weight ratio and fatigue life of the carbon-fibre filament is far superior to its metal counterparts, but unless it is woven into the correct matting, pieced and layered up correctly and, most importantly, combined with the correct resin to bond and protect the filament, then its superior qualities go to waste. If you exclude poor design or damage, the fatiguing in carbon frames is really down to the improper use or spec of the bonding resin. The resin needs to be UV resistant or a UV coating needs to be applied. It needs to be applied in just the right amount to prevent delamination of the carbon layers, so that there is enough to ‘web out’ the carbon-fibre matting layers to bond them without air pockets, but not too much that unwanted weight is added, as resin offers minimal structural benefit.
A final note on structural integrity. Carbon-fibre is a petroleum-derived product so the use of solvents, oils and greases over an extended period has the potential to soften and damage a carbon-fibre frame or components.
With regard to your choice of an XTC-C, Giant was one of the first bike companies to mass produce carbon composite frames and has continued to invest heavily in this technology. Giant’s C-tech division is one of the most high-tech composite manufacturing facilities in the world, so if anyone understands the correct application of carbon-fibre in association with a bicycle frame it should be them.
FIGHTING A FREEHUB
I can’t seem to get the freehub off my Cannondale Prophet. I have removed the cassette and axle and tried using a 10mm Allen key as you would expect, but it seems too small. A 12mm Allen key is too big. Any ideas?
Also, are there any other Prophet riders suffering from persistent creaking of the bottom bracket/crank which no amount of regreasing and adjustment seems to cure?
Phil Smith, email
On the freehub issue, a quick check on the internet shows Prophets fitted with Shimano hubs (which I know use a 10mm Allen key), DT 340s (which feature a freehub that just slides off) or Mavic Crossland/Crossmax (which require a 5mm Allen key to remove the axle end caps, then the freehub slides off).
As you appear to be looking at a large Allen key fitting to remove the freehub it sounds like you’re on a generic Taiwanese brand hub. If you have tried a 10mm Allen key that’s too small and a 12mm that is too big, what about trying an 11mm?
Creaking bottom bracket/cranks are normally easy to cure, with modern ‘external bearing’ bottom brackets, it is nearly always down to insufficiently tightened cups or unfaced bottom bracket shells. Check both, but my money is on an unfaced bottom bracket shell. If the bottom bracket and frame seem OK then clean off the axle/crank interface, very lightly regrease and refit the crank arms. Lastly, check the chainring bolts are clean, lightly greased and tight.
About three years ago I bought a Specialized Hardrock Pro and I’ve had no problems with it until about a month ago, when I noticed a grinding noise from my rear brake. I had a look at the disc and saw that there was a big score through the middle of it. My dad took the disc to work and has skimmed it so there is no score. We also fitted new Shimano disc brake pads.
We tried the brakes again but they were still grinding. I’ve now lubed up the brake cable and it is much smoother, but it’s still not that good.
Alex Hall, email
I feel you’ve probably had two separate issues with the Shimano Deore cable brake fitted to your Specialized. The scored disc and grinding noise were most likely caused by a worn-out disc brake pad or by a stone caught between disc and pad. As you have already changed the disc and disc pads, this problem is now cured.
After this, the brakes were still grinding. Matters improved after lubing the brake cable, so I must assume that the grinding you are referring to is ‘feel’ at the lever as opposed to noise at the disc.
The roughness you feel is very likely still to be due to a worn or dirty brake cable. I appreciate you have already relubed the brake cable but I would recommend changing the whole inner and outer. The cost of this is minimal.
Next, check the brake lever for wear, damage or dirt, and clean and lube if necessary. If the brake still doesn’t feel good, disconnect the cable from the caliper and activate the brake caliper arm by hand. Does it operate smoothly? If it doesn’t then maybe the caliper needs a strip-down, clean and lubricate.
This spring I bought a new Giant Rincon, which came fitted with Kenda Kwick tyres, 26 x 1.95 on a pretty standard-looking wheel. I’d been getting a lot of spin-outs and wanted a meatier back tyre. I bought a Maxxis high roller 26 x 2.35. I tried to fit it but it looked huge on the rim. The wheel itself is marked 559 x 20. The tyre says ‘fit HE rim 52 – 559’.
Mick Surf, Cornwall
The 559 x 20 and 52-559 refers to diameter and width in millimetres. In both examples the largest number is the diameter of the rim and the smaller is the width of the rim. The ‘52’ on the side of the tyre is recommending that the tyre can be fitted onto a rim up to 52mm wide. The Giant Rincon wheel is a more standard XC width at 20mm, and although it would be possible to fit your larger (2.35) Maxxis onto this rim, the frame-to-tyre clearance on the rear would be non-existent. You could also experience issues with tyre roll and the tyre bead could become unseated due to the extreme angle of the tyre’s sidewalls.
You could go up in profile width from the standard 26 x 1.95 tyres. Various brands, including Maxxis, provide more aggressive tread patterns than the Kenda Kwicks in a suitable width. I would aim to pick a tyre in the 2.1/2.2 width area or at a push a 2.3 Continental, as these come up pretty narrow for their stated width. Again, the greater width will reduce your tyre and mud clearance around the rear stays.