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 Post subject: Float 80RL fork
PostPosted: Mon Aug 20, 2012 4:43 pm 
retrobike rider
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Location: Hove
Can anybody help me? My 2003 Float 80RL worked fine except that it wasn’t plush enough. The pressure was 65psi, but I only weigh ten stones and the Fox manual has a guideline of 50psi. When I let some air out, the stanchions jerked into the lowers (the axle to crown had been 455mm, but went to 445mm), and I couldn’t pull them back out.

It still worked fine, although with less travel, but I thought it couldn’t be right, so I took it to my trusted LBS. A very experienced mechanic said this was normal, but I’ve never known the axle to crown of an SID or Marzocchi to vary with pressure unless there was almost no pressure, and even then you could pull it back out. This Float got shorter at the recommended pressure.

Anyway they gave it a service and said it was fine, and the axle to crown was back to 455mm. However that was at 80psi, so I let air out again and the same thing happened – now the axle to crown is 450mm. Is this normal as they say, or does it sound as though there’s something wrong that the LBS haven’t spotted?


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PostPosted: Mon Aug 20, 2012 9:42 pm 
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What travel are you getting at 80psi? Are you sure your guage is correct?


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PostPosted: Tue Aug 21, 2012 8:31 am 
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Location: Platsa, Messinias, Greece
The reason for a slightly reduced axle-crown dimension at lower air spring pressures is that the rate of the negative coil spring remains constant whatever air pressure you adjust the forks to and effectively pulls the sliders further back up the stanchions at lower primary air pressures. You should still be able to pull them back out if you hold the wheel down and pull up on the fork crown.

The only real cure (if you want the forks to sit higher in their travel while static) is a lighter negative (top-out) spring instead of the standard one, which is necessarily a compromise as I don't think that Fox offer different negative springs for lighter/heavier riders. This is the advantage of using air as a negative spring, it can be adjusted in balance with the primary pressures.

You can either try and find a lighter rate spring the right size or reduce the rate of the existing spring by grinding down the OD slightly to reduce the cross-sectional area of the spring wire. I have done this to fine tune springs.

If you shorten the spring by removing a coil then the forks will sit higher in their travel but you are actually increasing the rate of a spring by shortening it, which is the exact opposite of what you need.

Any help?


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PostPosted: Tue Aug 21, 2012 5:06 pm 
retrobike rider
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This makes a lot of sense Andy, but the only thing is that I can't extend the fork at all. If I'm only fighting the negative spring, I would think I could - certainly if 20psi in the positive chamber makes this difference, I would expect to be able to compensate for that 20psi in pulling it!

I also think it can't be a very good design if it gets shorter at the recommended pressure for anybody of ten stones. It's not as if it's all that light - most females are lighter, and the lady who came fourth in the Olympics weighs 6-8.

If the negative spring just works to smooth out the ride, I can't work out whether this would actually reduce the effective travel? Say you run 20mm sag, you could say you have 60mm positive travel plus the 20mm negative travel controlled by the negative spring. So maybe now I've got 75mm, made up of the same 60mm positive travel plus 15mm negative?
[Except that I should have more sag than before of course!]


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PostPosted: Wed Aug 22, 2012 8:31 am 
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Anthony wrote:
If I'm only fighting the negative spring, I would think I could - certainly if 20psi in the positive chamber makes this difference, I would expect to be able to compensate for that 20psi in pulling it!


This is the bit that I find puzzling as all that you're trying to do is replicate the effect of setting a higher air pressure - if you bleed all the air out of the forks what happens? Presumably they'll be partially compressed by the negative spring, so will settle into their travel, but can they be fully extended by hand under these conditions?

I've looked at online service guides for these forks and I can't see anything to suggest that there is anything at work here except the balance between air pressure and negative coil spring.
I'd drop a line to technicians@tftunedshox.com and see if they can throw some light on the matter as I'm baffled, to be honest....

Incidentally, while mulling this over yesterday evening, I realised that I've never owned (or therefore worked on) any make of forks that doesn't begin with the letter "M".
So that's Marzocchi, Manitou, Magura and Maverick - strange......


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 Post subject:
PostPosted: Thu Aug 23, 2012 11:41 am 
retrobike rider
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Location: Hove
I might do a bit more experimenting, but I've caused such trouble letting a bit of air out, I'm actually nervous of letting them down completely! I had already asked Mojo and now they have replied:

"Hi Anthony,
That is a normal thing to happen when running lower pressures and you have no need to worry. 5mm shouldn't make a huge difference to the angles of your bike, it might be worth checking your sag to make sure you have it set up correctly. We have a video here of how to do this - http://mojo.co.uk/sagsetup.html.
Cheers
Mojo Suspension Hoodoo Ltd."

Still seems rum to me, but to be fair I believe they have air negatives in more recent forks, so presumably you can tune them better. Also I have another Float RL80 from around 2003, and that hasn't shrunk.

Your list of Ms sounds quite expensive! At my weight, SIDs are quite good forks, although you do feel the advantage from the 30mm stanchions of the Marathon I bought for my RM and the 32s on the Fox.


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PostPosted: Thu Aug 23, 2012 1:24 pm 
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Anthony wrote:
Your list of Ms sounds quite expensive! At my weight, SIDs are quite good forks, although you do feel the advantage from the 30mm stanchions of the Marathon I bought for my RM and the 32s on the Fox.


No, not that expensive as non of them were bought new. Funny thing is that I'm still using one example of each make - Marzocchi Marathons on the Kilauea I have in Greece, Magura Phaons on one of my wife's bikes, Maverick SC32's on one of my Singular Hummingbirds and Manitou Minute 29's on my Trek Top Fuel 69er.

Of all of them, both to ride and to work, on I rate the Mavericks the best of the lot (although mine have a few changes from standard spec.), the Maguras and the Marzocchis about equal and the Manitous the worst, as they're not as small bump sensitive as I'd like, the problem being not enough bushing/stanchion clearance resulting in too much stiction.
I'll sort them though, I've always enjoyed working on suspension.....


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