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Old 03-01-2013, 02:07 AM   #31
PeterW
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Quote:
Originally Posted by craydds View Post
In previous posts inmates have mentioned mixing 10 wt. with 5 wt., (whatever) blah, blah, to get a 7.5 wt. oil. You mix sugar with water and you have sugar water. Molecularly, you still have sugar molecules and water molecules. If you mix 30 wt. with 10 wt. you do NOT have 20 wt. You only have an miscible molecular mixture of 10 and 30. Would you you "mix" 20 wt. motor oil wit 10 wt. to get 15 wt.? Run the correct wt. motor oil. Run the correct wt. fork oil. I am running 5 wt. in my forks. 7.5 or 10 wt. might be okay, hey, it's your bike. There was one of my local NM inmates said he was running 30 wt. motor oil in his forks, said he liked it... hmm. To each his own. I recommend sticking with the stock recommendations - nothing wrong with listening to the suspension gurus who have more experience than me.

p.s. I am a chemistry major. I have a degree in chemistry. That means I am qualified to dig ditches with a DIPLOMA in my back pocket.

Not correct, mixing fork oil does work. It's a mix of molecules of different length anyway. Professional suspension tuners will often start with 'too heavy' oil then thin it with something like 2.5 until it's just right.

ATF is worth trying anyway, it's a fraction of the cost of fork oil, viscostity is better controlled - so if it works once, it'll work next time as well - and you can get it anywhere. It also maintains it's viscosity better than most 'real' suspension oils. i.e it lasts longer.

p.s. also a chem degree, also one in engineering - on this one I'll lean on the engineering side, whatever works if it saves $$$

Pete
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Old 03-01-2013, 03:24 AM   #32
Johnny Locks
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Quote:
Originally Posted by craydds View Post
In previous posts inmates have mentioned mixing 10 wt. with 5 wt., (whatever) blah, blah, to get a 7.5 wt. oil. You mix sugar with water and you have sugar water. Molecularly, you still have sugar molecules and water molecules. If you mix 30 wt. with 10 wt. you do NOT have 20 wt. You only have an miscible molecular mixture of 10 and 30. Would you you "mix" 20 wt. motor oil wit 10 wt. to get 15 wt.? Run the correct wt. motor oil. Run the correct wt. fork oil. I am running 5 wt. in my forks. 7.5 or 10 wt. might be okay, hey, it's your bike. There was one of my local NM inmates said he was running 30 wt. motor oil in his forks, said he liked it... hmm. To each his own. I recommend sticking with the stock recommendations - nothing wrong with listening to the suspension gurus who have more experience than me.

p.s. I am a chemistry major. I have a degree in chemistry. That means I am qualified to dig ditches with a DIPLOMA in my back pocket.
I'm going to have to go ahead and respectfully disagree. I don't have any credentials, and I don't know a thing about chemistry. Don't need to for this, it's pretty simple. Oil mixes with oil. If you mix some 5w with 10w you get a slightly thinner oil that will flow through the dampers a little faster. For damping purposes it's pretty much that simple.

I have heard it discussed (and I believe based on my own experience) that there is a pretty big difference between brands of of the same weight fork oil in terms of how they affect damping. To call something a 10w for lubricating purposes it just has to be in the ballpark of 10w, but fork dampers are A LOT more sensitive to viscosity differences. Different brands of the same weight fork oil can be quite different in how they affect damping.
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Old 03-01-2013, 04:52 AM   #33
craydds
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Originally Posted by Johnny Locks View Post
Oil mixes with oil. If you mix some 5w with 10w you get a slightly thinner oil that will flow through the dampers a little faster.
Johnny Locks and PeterW - and all who are interested - you are both correct, but think about what I really said. I believe I stated that if you mix 10 with 30 you do not have 20 wt. oil, you simply have a 10/30 MIXTURE. I did not say that oils do not mix. I did not say that it would not work. I only recommended to use the "correct wt." oil and not use a mixture (seems odd to me). If one chooses to mix different wt. fork oils, it is not a crime. We use some mixtures purposefully, like 2-stroke oil/gasoline. Do you mix 30 wt. engine oil with 10 wt. to get a "slightly thinner oil"? I'll bet that you do NOT. You may be advocating mixing fork oils based upon recommendations of a master suspension tuner. I'd be interested in reading more on this.
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Old 03-01-2013, 05:10 AM   #34
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Originally Posted by PeterW View Post
Not correct, mixing fork oil does work. It's a mix of molecules of different length anyway. Professional suspension tuners will often start with 'too heavy' oil then thin it with something like 2.5 until it's just right.

ATF is worth trying anyway, it's a fraction of the cost of fork oil, viscostity is better controlled - so if it works once, it'll work next time as well - and you can get it anywhere. It also maintains it's viscosity better than most 'real' suspension oils. i.e it lasts longer.

p.s. also a chem degree, also one in engineering - on this one I'll lean on the engineering side, whatever works if it saves $$$

Pete
Pete, I think the old BMW fork oil was Aero Shell or something like that. I have heard that it was a "hydraulic fluid" and was actually an ATF. I used that fork fluid back in the day, but have long since used "real" fork oils like Bel Ray, etc.
For us average Joe's who are NOT "professional suspension tuners" - we want to change our fork oils annually and be done with the chore. Who among us is going to mix oils, test it on the track, go back to the paddock, thin the mixture a bit more, and VIOLA, the perfect mixture? I don't know any racers who do this, I certainly do not. If you do, I am NOT saying it is WRONG - I say... GO FOR IT.
p.s. My son is a double major - mechanic and materials engineer - way smarter than his old man. Bet I can still beat him on the race track. ha!
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Old 03-01-2013, 05:17 AM   #35
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oh, and craydds,
are you trying to say that those of us who have mixed different rate fluids have come up with some sort of special dual rate fork oil?
I have mixed (currently 5 and 10 mix in my R90 due to not being able to buy 7.5W at the time) and have been happy enough with the results, so I'd be interested to actually know what is going on physically in there..
I state that if one mixes 5 with 10, then you simply have a 5/10 MIXTURE - you do not have 7.5 wt oil. If your 5/10 mixture works for you then all is well.
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Old 03-01-2013, 05:54 AM   #36
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suspension set-up

Quote:
Originally Posted by PeterW View Post
Professional suspension tuners...
I am not a "perfesional" suspension tuner, but I have set up suspensions on my dirt bikes and sport bikes for many years (decades, actually). I have a good book on the matter (somewhere in storage now). Today, my brother from San Diego is here, he IS a racer. I can still hang with him! We are going to fine tune my suspension on my R90S - kind of hard to set the laden sag by yourself. Going to do some adjustments if necessary then my bike will be AWESOME. I have brand new everything - tires, fork springs, oil, ToasterTan, rear shocks, forks aligned "perfectly" by the Randy Glass method (supershaft, are you listening?), etc. - stay tuned for a ride report.
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Old 03-01-2013, 06:03 AM   #37
Beemerboff
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Ontic, just think of your spring as air in the top of your fork.

At zero pre compression it doesnt take much effort to compress it.

Pre compress it to 50 PSI and it will be a lot harder.
It is still the same air, but only pre compressed.

Your spring works exactly the same way.

Easiest way to check the difference preload makes is to find a bumpy corner, find a bike with preload adjusters on the forks, and watch the difference in the fork movement as you change the the preload, from close up.

You can pick a quarter of a turn harder of the preload adjuster.

In the days before telemetry this was how race bike suspension was set - when you get close to the spot the difference between a little to hard and to soft is hard to pick from the saddle.

Most suspensions work best with some preload, and Ohins and most others supply their shocks with 18mm, with the instruction to change it no more than 2mm either way, then change the spring.

So it would seem that 4mm change in preload covers the gap in their spring range, which is 25 lbs/in in the old money or around 0.05 in the current whatsits.

If you can find a bike where the suspension is adjustable on the move you will learn more about suspension in half an hour than you will ever learn by reading set up guides.

And you will find that most of what you read is extremly hard to replicate on the road - it is mainly bullshit interspersed with stuff and nonsense.

Remember that we are dealing with the dynamics of a body in motion, most suspension guides don't consider anything other than static weight and loads, and what little the put in , like gyroscopic precession is simply stuff and nonsense anyway.
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Old 03-01-2013, 12:11 PM   #38
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Originally Posted by ontic View Post
Yes, what you are saying here 'throws' me if that is how you would like to put it. It seems contradictory to me. Preload, to me, changes the position of the shock/fork in its travel, not its rate of travel, as you seem to be saying. If it did, and this seems to be what you are saying, changing preload would be the equivalent of changing the springs rate.
Please explain more.



You seem to be responding as if I've said there is something wrong with preloading a spring? The point I am emphasising, and it is a blindingly simple one, is to get the right rate spring first, and then set the sag by preload, and the right rate spring is determined by the weight of the rider, the set-up of the bike (big tank, etc) and of course personal preference, etc, etc.
My whole point is that selecting from two options of 'regular' or 'heavy' springs is kinda like shopping for underwear in XS or XL sizes only. That is why I mention custom winding springs, to get a given rate, not to eliminate preloading the spring.
I also find it difficult to imagine any correct weight spring (custom wound or not) that gave the correct sag and required NO preloading...
no spacers required for preloading maybe, but no actual preloading? I doubt it.
FWIW, my custom wound springs have about 3 inches of spacer, for preloading, on top.

So yes, I am a little thrown by how you describe all this, it seems blatantly contradictory to my theoretical understanding of the relation between spring rate and preload- but I am more than open to learn where my mistakes have been made.
Preload does not change the rate of the spring. It changes the amount of weight it takes to move the forks through its travel. The rate is the same. It just takes more weight at the same rate. Changing the rate of the spring takes a different spring but you can have different springs that take different amounts of weight to move at the same rate as a lighter spring. Something that, to a degree, preload alone can do.

I thought you were getting at preload being undesirable. My bad.

I have experience with the OEM regular and heavy springs. They aren't that big of a jump in "sizes". I think they are the same rate springs. I have different weight shock springs that are the same rate.
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Old 03-01-2013, 01:49 PM   #39
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Mixing Fork Oil

Besides issues of chemistry there are a couple of issues with discussing mixing 10 weight oil with 5 weight.

Most important in my mind is that unless your fork oil actually runs at 100 Deg C (and you'd have to be a flat out racer in a hot climate if it did) then it's all a bit meaningless.

Viscosity in cSt at 40 C makes more sense to a slow rider like me in a cold climate so I find it's best to judge oil at this temperature. Different brands will have a different viscosity index so even if they were exactly the same at 100 C they would be different at 40 C which is why you need to know the figure at the working temperature. If you think your oil runs at 50 C then it's not hard to calculate the viscosity at this or any other temperature.

Lastly even if you find out the actual viscosity at 40 C and and mix say
40 cSt oil with 20 cSt oil you don't get 30 cSt. It will come out approx. 6% thinner than you expect. If you really want half way between two oils mixing 60% of the heavier oil with 40% of the lighter will get you closer.

That's the chemistry bit and it's nothing to do with brand variation or the accuracy of labelling, it's something to do with the Refutas equation.

Someone earlier in the thread posted a link to the widman calculator. That whole site is a fantastic resource about oil in general.


http://widman.biz/English/Calculators/Mixtures.html
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Old 03-02-2013, 12:59 AM   #40
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Supershaft , what do you consider the difference is between the "WEIGHT" and the "RATE' of a spring, and what units do you measure them in?

And aint you just saying that changing the preload wont actually change the spring , it will just make it act like you had changed it.

Which is just what most folks find when the change the preload, hardly needs saying.
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Old 03-02-2013, 03:57 AM   #41
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I just found that 5W was too thin and 10W too thick, but mixing the 2 together gave me an oil whose weight was fine.......Don't sweat the numbers
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Old 03-02-2013, 01:32 PM   #42
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Originally Posted by Beemerboff View Post
Supershaft , what do you consider the difference is between the "WEIGHT" and the "RATE' of a spring, and what units do you measure them in?

And aint you just saying that changing the preload wont actually change the spring , it will just make it act like you had changed it.

Which is just what most folks find when the change the preload, hardly needs saying.
Weight is how much it takes to move and rate is how much it takes to move an inch, for instance.

What needs saying depends on who you are talking to. I have found that when it comes to preloading forks (for some strange reason not so much with shocks), you often have to start way back at the beginning.
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Old 03-02-2013, 03:42 PM   #43
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For those in the know, does this look to be the OEM fork spring for a /5? This is what's in my bike. I don't know if it's original, or a replacement by the previous owner.

It has a white paint mark on it. Not sure if this indicates top/bottom/if it even matters.

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Old 03-03-2013, 12:26 PM   #44
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Spring

I know nothing about \5's but later BMW springs were dual rate with the closer coils at both ends like that one. For this type of spring which way up doesn't matter.

Some aftermarket springs are dual rate but with the close coils only at one end in which case the close coils go to the top.

You might be able to identify your springs here http://www.bmwscotter.org/topics/sus.../john-chay.htm

If you want to identify the rate of your spring it's possible to calculate it by accurately measuring the wire diameter, number of coils and coil diameter. The formula is:

Rate =(wire Dia to power 4 * 1470000)/(coil dia to power 3*no of coils)

For the higher spring rate subtract the number of closely wound coils and run the calc again.

This is not an easy calc to get right unless you know roughly what the answer is. I have it on an Excel spread sheet if you need it.

Looking carefully at the picture and running the calc backwards, if you have something like 74 coils with 10 - 12 close wound at each end and the wire diameter is 4.0mm (this dimension is critical) I'd say there is a good chance that's an early soft \5 spring with 18.48\26.88 spring rate.


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Old 03-03-2013, 07:40 PM   #45
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The only way that the coil will have a dual rate is if the closer wound coils actually become coilbound, in the range that it runs when fitted and working on the bike.

And if you actually count them, you will be lucky if any more than half a coil each end becomes coilbound - all the gaps close by the around the same percentage, and very few seem to want to go to zero. I dont know why.

In theory it should be possible to make a progressive spring, but the technology simply does not exist to make the gaps accurately enough - remember that if the manufacturers sites are to believed be they are heat treated twice , scragged and powdercoated after they are coiled.
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