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I have bought a used Trek 4300 bike and after a few days I noticed leaking oil from front suspension fork. Oil leaks only from one leg (opposite to disk-brakes). I would like to know more about construction of front-suspension. Is this a serious problem? Is it possible to refill suspension with new oil? What kind of oil is used? How much oil is used?

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2 Answers 2

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While this may in fact be a serious problem which needs attention, it can also be normal. The first thing you need to do is define what you mean by leak.

If it is a thin and light coat of oil on on fork stanchion, this is very likely normal.

If it is a puddle, pool, or drip of oil on the fork leg, this is more than likely a problem. It can be a failed fork seal, or a a damaged damping or lockout cartridge.

Both of these can be caused by over compression of the fork. It could also a be a defect in the fork, which should be replaced. Do you have an LBS you trust? If so, take the bike to them and ask.

Be aware that there are other things that can cause this that may not be damage. How do you store the bike? Do you hang it up on the wall by the wheel or set it upside down on the bars and saddle?

Leaving the fork in a position to have oil draining against the seal can allow it to seep through, but that is normal. I know this is a bit ambiguous, but there is no way to give an answer with certainty without seeing the fork. Post a picture of the leak, and we can be more helpful.

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This. In my experience, the most common cause for leaks is storing the bike upside-down. They get worse as the fork ages as dust and grit will scratch and foul the seals. Often only one leg actually contains oil (moreso for XC forks), which is why you're seeing a leak on one side. You can fix it at home, but it's a fiddly job. I recommend you get advice from your shop. –  Ian Howson Jul 4 '11 at 7:04
    
If it is this, it may need nothing more than to be stored differently. And the more major jobs often present themselves in far more spectacular fashion. A blown damping cartridge isn't something you can mistake for a small seal leak. –  zenbike Jul 4 '11 at 7:16

The level of oil in a fork is very important to its proper function.

When I maintain my fork, I remove the caps for each stanchion using a socket wrench. You may be able to do the same with your fork and examine if there are any obvious problems with it.

Since this is a new bike I recommend taking it directly back to the point of purchase and asking for help, or replacement of a defective product.

This is a serious problem that has to be fixed. Each fork has its own specification for the type and amount of oil to be used. Usually, the specs require the oil level be accurate to a few ml's as well as no air bubbles or other contaminants in the oil. If you want to do it yourself you should be able to find the specs for your bike's fork online. However, because it is leaking, the leak must be found and corrected.

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OP states it's a used bike - may/probably does not have the option to take it back.. –  mattnz Sep 24 '12 at 4:39
    
Of course each fork has its own specification for the type and amount of oil to be used! See, when you drive something simple like a car, you can just put standard stuff like "DOT-3 brake fluid" in it, or "5W-30 motor oil", regardless of whether it's a Honda, BMW or Ford. But, oh no, for bike forks, you have to get special fluid to go with the exact make of the fork! Man, the snake oil vendors of the bicycle world ... LOL! –  Kaz Sep 24 '12 at 5:27

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