The Progold Foaming Citrus degreaser is extremely effective, requires minimal scrubbing and has a pleasant citrus smell. It'll damage your tyres (and anything else rubber) if you leave it on too long, however, poses a 'chronic' hazard to aquatic life, and should only be sprayed carefully in well-ventilated areas (think: outdoors).
The nozzle takes an extension tube to let you target parts precisely, which is good for various reasons. You don’t need much for it to work, for a start, which means the 400ml can lasts well. You also don't want overspray taking grease out of any nearby bearings.
More importantly, you don't want too much of it drifting around as, according to the scary horde of warnings on the can, it's highly flammable, extremely bad for your lungs and irritating to skin. Theoretically it's safe anywhere on your bike, though it does warn you not to leave it on rubber too long – it's worth reading the label so you don’t melt your tyres...
As is so often the case with serious solvents and surfactants, it works really well. It’s an absolute doddle to use: spray it on, let it foam and then wash or scrub off. Clean.
While I can't fault this as a saviour of filthy drivetrains, there are plenty of other degreasers that work well with considerably less risk of harm to both you and the environment. It's possible some are simply hiding their nasties more effectively than Progold, but unless your chain and sprockets are absolutely caked in filth, it's hard to recommend this Foaming Citrus Degreaser over a biodegrable one in a hand-pumped bottle.
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