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Old 05-16-09, 05:59 PM   #1
Unicornz0
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Rock Shox bottoming out on top of the tire, help needed

Hello everyone, I picked up a bike with Rock Shox Magnesium forks, Mag 20. After finally finding that the air fill is the center screw on the dampening dial (three local LBS did not know this even after seeing the bike, thanks to Jesus I found a thread with the Rock Shox service manuals). I'd like to know how much air to add. Air starts to escape from the shock as soon as I remove the air needle, but I've managed to set the screw under a little pressure but I can't tell how much. Not nearly as much as indicated in the Mag 21 manual. Most of the air escapes before I can set the air inlet screw. Must I set the air inlet screw faster or is there some way to actually tell how much air is in the Mag 20 shock?
I'm feeling very tentative about riding this bike after going over the handle bars during my first test ride when the shocks bottomed out on top of the front wheel, causing the front wheel to come to a abrupt stop. I would hate to have the air leak out during a ride and get hurt.
I'm not sure if I have enough air and oil in the shock and certainly don't want another trip over the handle bars. Help this Noob out, and tell me what I need to know about using and maintaining.these shocks.
After adding air the shocks SEEM ok. They do not bottom out and when I put my full weight of 208lbs. on the handle bars. I'm hopping they will not bottom out after taking a bump or a jump hard and fast. The bike has 26" x 2.2" knobby tires.
Does anyone have a link to the Mag 20 User's manual?
After thinking about the Rock Shox bottoming out on top of the tire, this fork appears very dangerous, if the one that I have is installed correctly. If the shock losses pressure the rider can be ejected from the bike, if the fork or fork brace bottoms out on the tire, resulting in serious injury or death. Please see photo.


Thanks in advance for your help,
Uni
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Old 05-16-09, 09:13 PM   #2
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your tire is probably larger than what the fork is rated for. I know when I read the manual to my suspension it says to let all air out and make sure the bridge doesn't contact the tire and if so don't use that tire. It looks old so it's probably designed for 1.95 tires or so or tires with a lower profile than your running.

In other words get another tire before you get hurt.

never seen one with an air neede before so I am probably no help on the shock mechanics.
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Old 05-17-09, 04:07 PM   #3
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You could try lowering the fork legs in the crown. The tops of the stanchions look to be maybe 3mm above the crown clamps. As suggested, installing a tire with a smaller width would be a smart thing to do.

Air Pressure: The Mag 20 has a very small amount of air in each leg to act as the spring. When you inflate each leg to, lets say 30 psi, remove the pump, them insert the pump again, some of that air in the leg must fill the pump and pressure guage. You will always see a significant drop in pressure by checking it with a pump.

Oil Level: You must remove the legs from the crown and remove the top caps to see the oil level. The Mag 20 is very similar to the Mag 21. The biggest difference is the valve body and the addition of a negative spring. You can follow the Mag 21 service manual to check and change the oil in the Mag 20. The procedure is the same.

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Old 05-19-09, 12:49 PM   #4
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You could try lowering the fork legs in the crown. The tops of the stanchions look to be maybe 3mm above the crown clamps. As suggested, installing a tire with a smaller width would be a smart thing to do.

Air Pressure: The Mag 20 has a very small amount of air in each leg to act as the spring. When you inflate each leg to, lets say 30 psi, remove the pump, them insert the pump again, some of that air in the leg must fill the pump and pressure guage. You will always see a significant drop in pressure by checking it with a pump.

Oil Level: You must remove the legs from the crown and remove the top caps to see the oil level. The Mag 20 is very similar to the Mag 21. The biggest difference is the valve body and the addition of a negative spring. You can follow the Mag 21 service manual to check and change the oil in the Mag 20. The procedure is the same.

Thanks Svr, for taking the time to look at the pictures and diagnose my problem. Hopefully lowering the fork legs in the crown will solve the bottoming out problem. After learning a bit about these shocks, I'm told that the crown should not bottom out, on top of the tire, even if they have no air in them at all. I will let you know what happens, after lowering the fork legs, and after removing all of the air.
Thanks so very much,
Uni

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Old 05-19-09, 12:50 PM   #5
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Originally Posted by DirtPedalerB View Post
your tire is probably larger than what the fork is rated for. I know when I read the manual to my suspension it says to let all air out and make sure the bridge doesn't contact the tire and if so don't use that tire. It looks old so it's probably designed for 1.95 tires or so or tires with a lower profile than your running.

In other words get another tire before you get hurt.

never seen one with an air neede before so I am probably no help on the shock mechanics.
Thanks DirtPedalerB.
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Old 05-19-09, 09:14 PM   #6
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no problem, better to spend your money on a tire or a fork than on a hospital bill.. you never want the front wheel to stop and while the fork is fully compressed would be the worst time.
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Old 05-20-09, 03:35 PM   #7
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You could try lowering the fork legs in the crown. The tops of the stanchions look to be maybe 3mm above the crown clamps. As suggested, installing a tire with a smaller width would be a smart thing to do.

Air Pressure: The Mag 20 has a very small amount of air in each leg to act as the spring. When you inflate each leg to, lets say 30 psi, remove the pump, them insert the pump again, some of that air in the leg must fill the pump and pressure guage. You will always see a significant drop in pressure by checking it with a pump.

Oil Level: You must remove the legs from the crown and remove the top caps to see the oil level. The Mag 20 is very similar to the Mag 21. The biggest difference is the valve body and the addition of a negative spring. You can follow the Mag 21 service manual to check and change the oil in the Mag 20. The procedure is the same.
Hi Svr, and thank again for your help. The fork height was the cause of the fork bottoming out. Now the fork legs are flush with the crown and I have 1/8"-1/4" clearance even after removing all the air in the shock, and it does not bottom out
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