Air Pressure on 80's forks bleeding down

Air Pressure on 80's forks bleeding down

Postby Samelak » Sun Jan 02, 2011 5:46 am

My front forks are not holding air pressure. I just had them rebuit less than a year ago. Does it need it again so soon? Could it be the cold temperature ouside (low 50's)? I really don't want to rebuild them again so soon. If I have to pull them apart, are there aftermarket springs that work with the stock forks to improve handling? Right now I am getting a lot dive (within an inch and half of bottoming out on the lower triple tree).
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Re: Air Pressure on 80's forks bleeding down

Postby zxbob » Sun Jan 02, 2011 9:38 am

I see the exact same thing happening with the 82 forks on my 80. They get alot of dive under braking !

Im going to install new progressive springs (from Tims) and that will help when aided with the air.

When I installed my 82 forks I was going to put new springs in prior to putting them on the bike, but
I thought being a larger spring and the fact the 82s had more weight up front with the fairing it might
work out O.K. . . . . . but no, it needs the springs. I want to make it correct with little or no air if
possible. If the springs are'nt enough, i'll look into re-valving them as well.


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Re: Air Pressure on 80's forks bleeding down

Postby Rick Pope » Sun Jan 02, 2011 9:53 am

Just add oil. By reducing the air volumm, you can run zero pressure, so no leaks. When the fork compresses, the reduced air capacity will rapidly bring up the pressure, but only until the fork extends again. Just give it a try. everyone I know who has done this runs zero pressure.
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Re: Air Pressure on 80's forks bleeding down

Postby tevan » Sun Jan 02, 2011 11:06 am

How much more oil? I have a 80 doing the same thing..
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Re: Air Pressure on 80's forks bleeding down

Postby Don » Sun Jan 02, 2011 2:40 pm

Bob - Try a slightly longer spacer on the top of the stock spring and maybe a bit higher weight fork oil before you resort to buying new springs. The front-end is quite 'tunable' is you play with all the variables

My ProLink forks had extra oil in them and even with 5 or 6 pounds of air I didn't like all the nosedive under heavy braking . . . . it was the only negative I can think of with a ProLink fork conversion on a CBX. You don't really notice it with the stock early model CBX brakes but with upgraded brakes the nosedive is really pronounced if you use the brakes aggressively . . . . it seemed like the pipes were going to drag the ground everytime I grabbed a big handful. I really liked the 'cushy' feel of the ProLink forks when riding around, especially on crappy roads . . . . it was only under heavy braking that I found them lacking


After reading up on the TRAC system used on the 1100F's, it sounded like the perfect solution to me . . . . but the USA marketed F's didn't use clip-on bars so their fork tubes are a couple inches shorter than those on the CBX. When Dave McMunn clued me in to the fact that the Canadian marketed F's did use clip-ons and the fork tubes were longer, the hunt for a Canadian front end was on. Got it all installed on the X about 3 months and 1,000 miles ago and it's a whole new bike . . . . at least to me. I was pretty sure I'd like it . . . . the TRAC system on my ST has always been great and the ST's use it on only one forktube. Kinda like the 1100F rear shocks, the front end is completely adjustable with 3 levels of rebound dampening and 4 levels of TRAC anti-dive . . . . plus adjustable air if you need it - I've never felt the need to add air to mine. They also come with a nice built-in fork brace and the stock CBX fender fits them perfectly

Probably not 'the solution' for everybody, but I sure like the way it works for me

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Re: Air Pressure on 80's forks bleeding down

Postby Samelak » Sun Jan 02, 2011 5:00 pm

I appreciate the advice from everyone. I'll try the extra oil first. How much oil will it be total and what weight/type should I use? If this does not work, should I consider progessive springs? I also saw a thread on a gold valve racing conversion in the "best of" section. Any opinions on that? I cannot convert to 1100F forks because I am currently living overseas and it woul cost too much to ship. So I plan on keeping the stock 80's forks.
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Re: Air Pressure on 80's forks bleeding down

Postby Rick Pope » Sun Jan 02, 2011 6:45 pm

For me, 15w is good. For a more "normal size" rider, I'd start w/ 10w.

Try filling the fork legs to 7.5" from the top with the springs removed and the forks fully collapsed. I'm heavier than the average CBX rider, I run 6.5 inches from full.

I can tell you that on the Mountain 2 Mountain ride a few years ago, Dan Carr (naked '81) did this at the Marlinton stop as an evening tech session. He rode back roads to Maggie Valley over the next two days, and reported a huge improvement.

Don't be afraid to experiment. What's the worst that can happen? I've yet to hear of a blown seal from doing this, and if it rides too harsh, drain a few cc's back out.
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Re: Air Pressure on 80's forks bleeding down

Postby Don » Sun Jan 02, 2011 7:32 pm

7 weight oil would probably be considered 'normal' and 10 weight a little stiffer - 15 weight is awfully heavy for most folks

You're trying to find the balance between a nice, compliant ride over so so roads and yet not have a ton of nosedive under heavy braking - Much of the solution is dependant on how and where you ride. If you like pushing the bike near it's (or your) limits, then it's harder to find something that's soft enough for good handling on crummy roads and yet doesn't nosedive on you when you brake. If you weigh more, it's harder to sort out too but if you're say 175 pounds and don't ride it *hard* then going up one weight on the oil and adding a bit more of it will go a long way - Maybe adding 3/16ths to 1/4 on the spring spacer will help too . . . . you can easily make spacers out of PVC pipe and play around with the lengths

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Re: Air Pressure on 80's forks bleeding down

Postby Samelak » Mon Jan 03, 2011 5:56 am

I want to get this right. First, what kind of oil should I use? The manual says ATF and I don't believe ATF comes in "weights". Are you talking regular fork oil? Next, I guess I need to drain my forks empty while the bike sits on centerstand and let the front end collapse. Then remove the fork tops and springs. Refill to within 7.5 inches from the top. Reinstall springs and tops. Do I need compress the front a couple of times and recheck the level? Will I need to add air afterwards? Am I missing anything? I will probably start with 10W since I'm 6'2", 220lbs.
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Re: Air Pressure on 80's forks bleeding down

Postby zxbob » Mon Jan 03, 2011 8:50 am

FWIW: I have 11oz of 15W oil in my 82 forks and it too soft ~ thats why im thinking of changing springs.

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Re: Air Pressure on 80's forks bleeding down

Postby Rick Pope » Mon Jan 03, 2011 9:56 am

ATF runs between 3 and 7 wt. That's why we now use real fork oil, as it's a known value.

I've experimented with up to 20wt. It was pretty harsh, in a stock '79. I would think 10w would be a good place to start for an average sized rider.

Drain the fork legs, if the fluid coming out looks nasty, flush w/ some sort of solvent to wash out crud. Remove caps and springs, fill each side to about 7.5" from the top w/ the fluid of your choice. Install springs and caps. Go for a ride. First, notice whether the overall ride is better. If still to soft, use a little heavier grade oil. If ride is good, but it dives too much under braking, add a little oil to each side. If you add too much oil, it will feel like the front end becomes solid too quickly.

You might go to a farm supply store and get a large animal syringe, and a piece of tubing the correct size to slip over the end. That way you can stick the tubing into the fork leg to your desired depth, and suck out oil. Easier than trying to drain just a little out the bottom.
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Re: Air Pressure on 80's forks bleeding down

Postby Don » Mon Jan 03, 2011 10:27 am

Samelak wrote:I guess I need to drain my forks empty while the bike sits on centerstand and let the front end collapse. Then remove the fork tops and springs. Refill to within 7.5 inches from the top
It would probably be easier and safer to remove the wheel, so you can take the fork tubes off the bike. You'll need to be able to easily compress the forks all the way to change the oil. Likely, you'll need to flush them out with some mineral spirits to clean out the crud and it's helpful if you can invert them to drain

When you add the oil, the springs are removed and the forks need to be compressed all the way, so you can get an accurate measurement with a ruler to get equal amounts of oil in each fork

Reinstall springs and tops. Do I need compress the front a couple of times and recheck the level? Will I need to add air afterwards? Am I missing anything? I will probably start with 10W since I'm 6'2", 220lbs.
It all depends. What Rick is explaining to you is that if you overfill the forks with oil, the remaining air in the fork will compress pretty quickly when the forks compress under braking and that makes them stiffer . . . . not stiff enough? Add more oil (I think Dave McMunn fills them to 6 inches from the top) . . . . or go to heavier fork oil and add more oil. Too soft when not compressed? Add a longer spring spacer on top of the springs

If you play with all the variables, you can probably get everything the way you like it without adding any air

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Re: Air Pressure on 80's forks bleeding down

Postby daves79x » Mon Jan 03, 2011 5:23 pm

Guess the only thing I'll add is that you can safely add oil to 6 inches from the top of the tube with springs out and forks collapsed. This has been written about here more times than just about anything I can think of.

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Re: Air Pressure on 80's forks bleeding down

Postby Samelak » Tue Jan 04, 2011 4:40 pm

Well, I was hoping to do this without pulling the forks out. Since I have to remove the forks, would it be worthwhile to upgrade to progressive springs and then add 10w fork oil to the 7.5 inch mark? The springs aren't really expensive and if its saves me the trouble of having to remove/replace the forks several times to get the adjustment right. I'm not an aggressive rider, but I do prefer a bit stiffer ride.

Thanks for everyones' input,
Lee
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Re: Air Pressure on 80's forks bleeding down

Postby NobleHops » Wed Jan 05, 2011 12:02 am

Samelak wrote:Well, I was hoping to do this without pulling the forks out. Since I have to remove the forks, would it be worthwhile to upgrade to progressive springs and then add 10w fork oil to the 7.5 inch mark? The springs aren't really expensive and if its saves me the trouble of having to remove/replace the forks several times to get the adjustment right. I'm not an aggressive rider, but I do prefer a bit stiffer ride.

Thanks for everyones' input,
Lee


Springs are about sag, and affect ride quality mostly when they are too hard or too soft. Spring rates should not be used to 'firm up' suspension, that's a function of damping. Progressive fork springs add a variable to suspension action that actually makes them a little harder to tune. Damping is a function of oil volume (and the corresponding air 'spring volume'), oil weight, and any fluid or mechanical dampers eg primitive damper rods like we have, or cartridges like the TRAC devices Don was recently mentioning, or cartridge emulators like the RaceTeck Gold Valves, or Ricor Intimators.

IMO you will chase your tail with suspension settings until you get the bike properly sprung so that when you are on it you are 'sitting' in the optimum location within the suspension's range of travel, roughly 1/3 of the way down from the top of the travel, for a rule of thumb. Once the spring rates are correct and sag is properly set, then tackle damping with volume and weight of oil, or better yet, with mechanical dampers like GoldValves or Intimators.

Further complicating matters is that so called 'rated' fork oils vary widely in actual measured weight between manufacturers. Both of the cartridge emulator manufacturers recommend 5 weight oil and to tune further with the mechanical damper, with a static fluid height of ~100mm, as measured with the fork compressed and the spring removed. That's the only repeatable way, measuring fluid volume is a crap shoot. But that's for use with emulators. If I was not running cartridge emulators (I am), then varying oil weight would be my next avenue personally, using a height of 100mm from the top of the fork oil as a baseline measurement.

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