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 Post subject: ?Thoughts on fork oil
PostPosted: Thu Nov 26, 2015 3:28 pm 
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I'm considering swapping my factory fork oil for 20W.
After all, I'm a lot heavier than the average, Asian SR rider.

I can see where heavier fork oil would result in a harsher ride for someone who weighs a buck-fifty.
Being nearly twice that weight, any fork oil will flow faster through the orifices with a heavier rider than with a flyweight.

I'm thinking that heavier fork oil would be a good part of tuning the bike's suspension to my weight.

What do my esteemed brethren say?

TIA

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PostPosted: Thu Nov 26, 2015 4:14 pm 
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I am not a suspension expert, but a few thoughts to ponder...

It's the job of the spring to take the "hit" and the job of the oil to control the speed of the resulting motion (and the speed of the rebound, which is a different thing again).

It's Kinetic Energy that the fork has to absorb, which (1/2 mv^2) is linear with mass, but goes with the square of velocity so an extra few pounds of rider is far less significant than a few more mph.

On top of that, oil "weight" is actually not a good guide to its viscosity, which is measured in centistokes. If you change manufacturer and go up 5wt, you could still have the same viscosity and therefore no change in performance.

FWIW and IMO, if you're going to spend on the suspension then the progressive spring swap is the first thing to do.


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PostPosted: Thu Nov 26, 2015 5:13 pm 
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^^what he said (I think)^^
My seat of the pants understanding is that you get the spring rate right first and fine tune with the oil. If you have to go too far from factory it indicates the spring needs changing.
The worst case for me was a Koni shockset that could not rebound right because of too light weight a spring and too high a viscosity oil so they compressed after a short ride and stayed pretty much down until the bike sat for a while. I went to a higher weight spring and ATF and it was good to go again.

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PostPosted: Fri Nov 27, 2015 2:16 pm 
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I've already installed the Progressive fork springs. They are a big improvement over stock.
I'm using a 1" long preload spacer. I'm thinking about making a 1-1/4" long spacer.

What is the viscosity of Dexron ATF?
Is it OK to use as a fork oil?
ISTR Type F ATF is thicker than Dexron. It used to be common to use it in GM/Chrysler automatics that were worn.

I have Tractor Oil, Dexron ATF, and AW32 Hydraulic Oil at my disposal.

I have little money and less free time.

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PostPosted: Fri Nov 27, 2015 6:38 pm 
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Hi Jaymo, get a stack of washers and keep adding them till you you get the springs right, then decide on the oil, if you start using atf etc you will only be guessing, if you use fork oil and as long as you stick to one brand you have an actual reference point so can go thicker or thinner from there once you change the oil you might find you want to fine tune the spacers again, after you are happy then make some permanent spacers the same length as your old ones plus the washers you added, if you still aren't happy save up for some cartridge emulators.


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PostPosted: Fri Nov 27, 2015 9:46 pm 
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On 'ATF', which is not all the same as you say, Ive heard from 2.5 - 5 wt. I choose it back then because it was what we used in the then new MotoGuzzi front forks on the 850T, and on, models. It has high film strength, lots of antioxidents and is not prone to foaming when hot. The Konis had some thin viscosity oil when I disassembled them and 'seemed' no too dissimilar to the ATF when poured. It worked well for two sets of Koni's I reburbed.
As has been recommended, by using one brand of fork oil you will be able to utilize their rating in a systematic way. There does not seem to be a defined range for fork oils as there is for motor oils so you cant change brand to brand; one guys 5wt might well be anothers 10wt.

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PostPosted: Fri Nov 27, 2015 11:26 pm 
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I never tried atf cause I never knew what weight to buy, a lot of guys seem to use it though, I know what you mean about fork oil brands all differing, I changed brands (cause my local bike shop sells it) and used the 15 weight and it is way too thick, need to go to 10 weight or even thinner if I stick with this brand.


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PostPosted: Fri Nov 27, 2015 11:51 pm 
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We ran ATF in a Hewland transmission in a Formula Ford once on a very rainy weekend and everything in the transmission surface rusted, which made me wonder if it's not prone to attract moisture like an open container of brake fluid. Just my personal experience, I realize every automatic car and a lot of manuals run it with no problems.


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PostPosted: Sat Nov 28, 2015 2:50 am 
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Up to Dexron 2, it would often cause corrosion.
Dexron 2-1/2 and later were formulated to correct that problem.

I'm going to stick with fork oil.
I'll measure the OD of my springs and get some washers with the same OD or a hair smaller.

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PostPosted: Mon Dec 07, 2015 3:06 pm 
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fork oil comes in all sorts of weights, from at least 5 up past 20.
Thicker oil makes things more harsh and is not the way to stiffen up suspension.

I have progressive springs but need a bit more preload for sport riding.
Adding more oil makes the forks get more progressive as the air compression inside goes up and acts as a spring.
Many times the oil level is low in new bikes, not sure if the dealer adds it or the factory but my TU250 came well short of the amount the shop manual said it was supposed to have.


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