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 Post subject: PRELOAD-Air in forks
PostPosted: Fri Dec 07, 2007 10:28 am 
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Joined: Sun Jul 01, 2007 12:00 am
Posts: 133
Location: Queenstown ,Tasmania
Whats the general consensus on preload using air in front forks? Is it of any value? or better going to higher viscosity fork oil? Can anyone provide feed back/knowledge on this issue relevant to Paso/907? Must be kind of hard to measure low air pressures without losing air, I would imagine. ..........Thanks Marty


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 Post subject: Re: PRELOAD-Air in forks
PostPosted: Fri Dec 07, 2007 11:34 am 
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paso grand pooh-bah
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Location: southern Germany
model: 750 Paso
year: 1987
don`t do that, you may blow your seals. The valves are only for letting out air, not for raising pressure in the fork.

G.


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 Post subject: Re: PRELOAD-Air in forks
PostPosted: Fri Dec 07, 2007 10:14 pm 
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Location: Queenstown ,Tasmania
OK, thanks for the tip, regards Marty


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 Post subject: Re: PRELOAD-Air in forks
PostPosted: Thu Mar 05, 2009 5:26 am 
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Joined: Sun Feb 22, 2009 4:55 am
Posts: 46
Location: Camas, Washington USA
model: 907 I.E.
year: 1992
Valves are for only letting out air!! What air? How does it get air in? Give me some details on this fork "adjustment" on my '92 907IE. Does not make sense to me, a two week new owner of the bike.

Thanks


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 Post subject: Re: PRELOAD-Air in forks
PostPosted: Thu Mar 05, 2009 9:01 am 
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paso grand pooh-bah
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Location: southern Germany
model: 750 Paso
year: 1987
that`s mainly after a fork rebuild or fork oil fluid change


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 Post subject: Re: PRELOAD-Air in forks
PostPosted: Thu Mar 05, 2009 11:41 am 
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Joined: Fri Mar 21, 2008 8:57 pm
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Location: Arlington, Massachusetts USA
model: 907 I.E.
year: 1992
see also this thread on fork oil and air pressure:
viewtopic.php?f=3&t=2845&p=21207&hilit=fork+oil#p21207

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 Post subject: Re: PRELOAD-Air in forks
PostPosted: Fri Mar 06, 2009 5:57 pm 
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Joined: Sun Jul 13, 2008 5:54 pm
Posts: 14
Location: Bisbee, AZ
model: 907 I.E.
year: 1993
This air pressure topic is of increasing interest. I'm familiar with air pressure forks, having run a variety of Marzocchi, Manitou and Rock Shox on bicycles. These forks come in all configurations, including just air, air and coil, and coil. All have fluid dampening in addition to the air or coil. Air pressure by itself is not nearly as linear in response as is air and coil or coil alone. All of these forks provide a adjustable dampening and rebound, effectiveness highly dependent on the quality of the unit.
My assumption--as a guess--would be that the air valves at the top of the M1R forks were placed there with the intent of adding air preload. There seems to be no point in releasing pressure. After the forks are assembled and weight placed on them, pressure shouldn't be much more than normal atmospheric load. This pressure would increase with fork compression, as is part of fork design. Releasing pressure would have to take place under compression and the resulting negative chamber pressure would seriously affect rebound dampening let alone causing seal lip stiction.
I've never experimented with the preload on the M1Rs, though it's obviously easy enough to do. A simple 160psi shock pump sells for about $25 or so. The feed end is a standard Schrader fitting that screws onto the fork valve and has a release fitting designed to maintain set pressure when the pump is removed. I've considered playing with the preload on the Ducati just to see what it would do. I don't see why adding pressure, maybe start with 20 pounds or so, would blow seals. After all, the seals are designed to deal with significant cavity pressure when the forks compress when braking.
Adding preload--as referred to in the other thread--would decrease static fork sag and should add improved control over fork dive. It would also contribute to stiction, though I don't know if this would be a significant factor. Has anyone on the forum done any experimentation with adding air preload to the old Marzocchi's? If so, what pressure was used? Were there deleterious effects, such as blowing the seals all to hell?
Most of this is purely intellectual questioning. I don't take the 907 out for hard rides and have no intention of testing the limitis of its handling. It's a show bike for me, rather the same as my Norton. I don't like the idea of putting bikes down--I'm old enough and have been riding long enough to know how painful a fall can be--but I'm still curious about those valves. Maybe someday...

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'93 Ducati 907ie
'75 Norton 850 Commando
'98 BMW K1200RS
'85 VW Vanagon (to haul parts for the bikes...)


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 Post subject: Re: PRELOAD-Air in forks
PostPosted: Sat Mar 07, 2009 12:40 am 
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Joined: Thu Apr 03, 2008 4:38 pm
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Location: Boston, MA
model: 907 I.E.
year: 1993
Several points:

-The pressure rise in a set of forks between full extension and full compression is on the order of 0.6 bar/10 PSI - not as much as you'd think.

- One of the key reasons to -not- raise fork pressure is stiction: the seals will hang up much more s pressure increases.

- The pressure in the forks will naturally increase over time because the oil evaporates/outgases. This is significant on a hot day. Release it with the fork legs fully extended.

- you can adjust preload using fork pressure, but you're better off doing it with spacers. They're not that hard to put in.

- you can adjust anti-dive by adjusting fork oil level. In fact, that's all setting the fork oil level really does. Raising the pressure affects preload, but doesn't affect dive much at all.

-There once was a penguin that took a vacation in Arizona. It was hot, and his car broke down somewhere near Bisbee. He brought it to a garage in town, and asked the mechanic where he could go to cool off while he was waiting for a diagnosis - after all he was a penguin, and not used to the hot weather. The mechanic recommended an ice cream shop aroud the corner. The penguin had never had ice cream before, and he loved it. Unfortunately, being a penguin, he had no hands, so he made a bit of a mess of himself while he gobbled down the delicious treat. When he was done, he headed over to the garage. The mechanic was just walking away from his car when he arrived.

"Well, it looks like you blew a seal!" he said.

The penguin stuttered: "No, honest - it's just ice cream!"



Forgive, me, I couldn't resist.

Sam


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 Post subject: Re: PRELOAD-Air in forks
PostPosted: Sat Mar 07, 2009 2:11 am 
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Joined: Sun Jul 13, 2008 5:54 pm
Posts: 14
Location: Bisbee, AZ
model: 907 I.E.
year: 1993
Jeez, I thought the penguin had seafood for lunch.
"Looks like you blew a seal," the mechanic said.
"Nah, it's tartar sauce."
Yeaaaah! Bisbee is renowned for its gay pride weekend, supposedly one of the five best in the country. I don't know, somehow I've managed to miss the affair each year. Nothing personal but...I feel out of place. I'm probably the only white-haired, bearded lesbian in town and I refuse to shave.

Back to forks, folks. I understand the effects of adding spacers and varying fluid levels/viscosity to modify dive and and rebound. The advantage of variable air pressure is the ease with which the pressure can be changed, thus quickly and simply altering ride characteristics. Last time I changed fluid and switched fork springs, the job took me a couple of hours. Thirty seconds with a pump...
Of course the "modern" scoots--the new BMW K series for example--has electronic suspension control. I've not tried the system but I understand it's quite effective. If anyone would like to loan me a K1300R for a weekend (and maybe a nice Ducati 1098S the weekend after) so I can get a taste for the real upgraded stuff, I'd accept the offer.

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'93 Ducati 907ie
'75 Norton 850 Commando
'98 BMW K1200RS
'85 VW Vanagon (to haul parts for the bikes...)


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 Post subject: Re: PRELOAD-Air in forks
PostPosted: Sat Mar 07, 2009 3:05 am 
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Joined: Thu Apr 03, 2008 4:38 pm
Posts: 149
Location: Boston, MA
model: 907 I.E.
year: 1993
I don't think there's anything wrong with using fork pressure as a way to try out a new preload setting, or perhaps to make up for carrying a full load for a trip or something. But it's a poor substitute for changing preload properly using spacers.

This, of course, is only my opinion.


Sam


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 Post subject: Re: PRELOAD-Air in forks
PostPosted: Sat Mar 07, 2009 9:45 am 
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paso grand pooh-bah
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Joined: Sun Jul 18, 2004 12:00 am
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Location: southern Germany
model: 750 Paso
year: 1987
I personally know someone who managed to blow his seals twice. It didn`t take long after applying air pressure that the forks started to leak fork fluid quite badly, but I have to admit I don`t know how much air pressure he applied. This may be the key factor.

G.


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 Post subject: Re: PRELOAD-Air in forks
PostPosted: Sat Mar 07, 2009 1:42 pm 
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paso grand pooh-bah
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Those valves are ONLY to take air OUT!!!! I would say, that its stupid to put air pressure inside M1R forks... :mrgreen:

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 Post subject: Re: PRELOAD-Air in forks
PostPosted: Sat Mar 07, 2009 4:14 pm 
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Location: Arlington, Massachusetts USA
model: 907 I.E.
year: 1992
I looked through the original 907 Owner's Manual again, and I can't find anything in there about airing up the forks. Just instructions on how to adjust settings. See page 31.

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 Post subject: Re: PRELOAD-Air in forks
PostPosted: Sat Mar 07, 2009 5:44 pm 
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paso grand pooh-bah
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"you" cant find such stupid advices from 750, or 906 owners manuals also at all.... :thumbup:

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KTM 990 Adventure -08 metal dark grey
Paso 750 -89 red/metal grey
Paso 907IE -91 red/metal grey
Paso 907IE -91 red
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ST2 -01 red/metal grey
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 Post subject: Re: PRELOAD-Air in forks
PostPosted: Sat Mar 07, 2009 6:16 pm 
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Joined: Sun Jul 13, 2008 5:54 pm
Posts: 14
Location: Bisbee, AZ
model: 907 I.E.
year: 1993
I stand corrected on two items. No air in the fork; the valves are "release" valves, not pressurization valves. Doesn't make a lot of sense to me but the older I get, the more I find out how little I know. If I want to change things in the suspension, I might try the stuff on modiflying M1R forks at http://www.moto-one.com.au/performance/851workinpro5.html, a site that has some information I'd not encountered before. Anyway, no air in the M1R forks. I'll keep the little fork pump safely tucked in my bicycle kit and I'll sit in the corner with my dunce hat on until teacher says I can play with others instead of playing with myself.

Again, all of this stuff about modifying the suspension is an intellectual matter. I have no intention of taking my old Ducati out on the track or even doing high speed through the twisties. However, I'm fascinated by just how good an old lump like the 907 could be if one put the effort into modernizing it. (Same can be said for the Norton. I watched Reg Pridmore ride Norts back in the '70's when I lived in Santa Barbara, many years later I attended his CLASS training. I'm still amazed at how well these old machines can make their way around a track.)

Second correction. My girlfriend tells me I should never refer to myself as a lesbian trapped in a man's body. She says it's rude and will alienate people. So, I won't tell anyone that I'm a lesbian trapped in a man's body. Not even Linsay Lohan will know.

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'93 Ducati 907ie
'75 Norton 850 Commando
'98 BMW K1200RS
'85 VW Vanagon (to haul parts for the bikes...)


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