The most important thing in advance, keep calm, proceed systematically and patiently.
The apparent localization of crackling / creaking noises rarely says anything about the real cause / location of the defect. The noise is subjectively very often felt in the area of the bottom bracket or the pedals, nevertheless, the cause may well be on the saddle or handlebars. You often think you can locate the cause, but that's harder than you think. As you constantly ride on the bike when driving, you load almost all components while driving, even though you think all the power goes to the bottom bracket.
First, it is important to distinguish whether it is a short crackling noise or a sluggish, longer creaking. A sluggish creaking is usually associated with the cleats.
Treten bereits beim Aufsitzen, ohne Belastung der Pedale und ebenso des Lenkers im Stand Geräusche auf, so ist die Wahrscheinlichkeit sehr groß, dass es auf die Sattelstütze, Schnellspanner oder Sattel einzugrenzen ist.
The seat post is quickly removed. It should be wetted with carbon mounting paste if necessary. Going out of the saddle on a test drive - it's gone, so it was the seat post. Never use grease on carbon compounds, only on metal-to-metal joints.
Even the saddle clamp screw can be a cause. When wetting the seat post with carbon mounting paste just turn it out and spend some fat on the screw, the same applies to the clamping ring (if available, but not for the exciting surfaces!). The saddle clamping as a noise source is very rare, but it is not very costly to grease them as well.
To test the stem, handlebar, headset / fork, fix the bike in its stance between your legs and apply front, rear and side loads. The remedy is simple: loosen handlebar clamp and stem, grease (aluminum), use appropriate paste with carbon, reassemble, wipe off excess grease / paste and try again if cracking is gone.
If this does not lead to the goal, then the headset is the probable cause. Basically, the headset should be expanded once a year, cleaned and greased again. When assembling, make sure that the bearing / headset is rotated 90 ° in each position so that the bearing will wear evenly over the years, which may also be a possible cause.
If the crackling and creaking noises are still not eliminated, it will now be a little more elaborate. All screw connections must be systematically checked, lubricated and tightened.
Due to the technological construction in carbon with thin-walled and large-volume cross-sections, even a minimal cracking can be amplified as if by a loudspeaker.
Possible sources are shoes, pedals, pedal axles, crankset, chainring, chain and cassette, the bottom bracket, but also the front derailleur, in MTB Fullys be the rear bearing or even the wheels.
First, the shoes and cleats are to be targeted. If it breaks, cracks while driving, but there is nothing to detect on the standing bike, then the cleats can be the reason. It is best to ride a few meters with sneakers. If the cracking has not disappeared, check the screw connection of the pedal plates and, if necessary, slightly grease and tighten the thread.
The pedals are one of the most common causes of cracking. If grease was not specified during initial assembly, this is one of the most likely causes. Use the open-end wrench (15 mm) to detach both (Please note left is a left-hand thread, right one right-hand thread), grease neatly and screw on again. If this does not lead to success, you should test pedals of another bike, which does not crack. If the error is corrected, you need in most cases new pedals, because almost all models are not removable with respect to the pedal axle - then lacks lubrication in the pedal itself. As a last try, you can spray chain lubricant near the threaded input and hope that this creeps far enough. Low viscosity lubricants (e.g., WD-40) or similar spraying agents only eliminate cracking for a short time, returning more and more. But with spray agent you can regularly keep the click mechanics smooth for a liquid click in and out.
Try to wiggle the crank to make out any play. If only one crank moves, it is not tight enough on the bottom bracket, or the bolt of the Holowtech crank is loosened. When both move, the bottom bracket itself has play and must be tightened if possible, or replaced. Very likely to crack are the cranks on a square. It is recommended not to grease the square cranks, but to apply a thin layer of copper paste instead. Theoretically, a thin film of grease potentially loosens the cranks over time. In practice, this is rather rare. In order to avoid any signs of wear, which can cause cracking, turn each 90 °.
Too dry chainring bolts can also crack, loosen one after another, get some grease and tighten again.
A badly riveted or even broken chain is noticeable by rhythmic noise, interference. This can be dangerous very quickly. A stiff chain link can produce similar noises, the chain can also skip. If the locking nut of the sprocket cassette is insufficient, the rim is not sufficiently fixed, which can lead to crackling noises. The trigger can be a too low tightening torque or too short screw nut.
Disassemble the cranks and turn the bottom bracket by hand. It must be easy to turn without jerking and noticeable resistance. Only very expensive bottom brackets are repairable. The question of whether the bottom bracket cracks / creaks is not clear to examine. In practice, it is best to install another bearing as a test. If the bearing turns really gently, without noticeable jerking and resistance, creaking is unlikely but not impossible.
If metal spokes are installed on the carbon fiber wheel, especially on the rear wheel, the spokes should be greased with a lubricating spray (be careful not to spray on the brake flanks). But hairline cracks in the nipple holes can also cause noise, and they are a serious warning. Either the spoke tension is too high, the rim too soft or there is some kind of overload. Such cracks are first harbingers of bad defects, which then come without warning and often lead to heavy falls, as the carbon wheel in question suddenly jams and blocks. Even when small cracks occur, it should be checked whether the rim can be repaired or the rim needs to be replaced.
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