Front fork oil replacement - How much?

[Pigeon]

Wondering if I can pick your brains again.

I'd like to replace the 10w oil in the front forks with 15w to try and slow down the dive a little (I'm a fat git on a small bike). To all intents and purposes my bike is a 2010 YBR125

My question is, how do I know the correct amount of oil is in without taking the legs out and apart?

The Jianshe manual says 156ml But apparently, there is usually some oil left in the system and you should "measure the air volume"
I've googled this and read a few posts, but don't get how I measure it and what I'm measuring.

I think I understand I need to:
1. Unscrew drain bolts from bottom of fork legs and drain oil into 2 trays.
2. Pump forks up and down to make sure its all out.
3. Measure the volume of oil removed from each leg.
4. Slacken bolt at top of fork leg
5. Add drain bolt back into bottom of leg
6. Raise the bike up so the front wheel is off the ground and the forks are fully extended.
7. Finish undoing the cap bolt at top of fork leg, using a cloth and downward pressure to avoid it firing into my face.
8. Poor in the new oil, totalling (156 - volume extracted)ml per leg.
9. Confirm the correct amount by measuring the air volume.
10. Button up.
11. Pump the forks lightly.

I have some spare syringes and pipe from when I replaced the brake fluid. And I've seen some people use these to get the correct amount of oil in each fork leg.
By measuring a specific length of tubing.
But do you know how they knew what to measure?

I figure with this snow, if I can't ride my bike, I'll do my best to brake it.....eehhh, I mean get it in on song for spring and the start of a glorious and long English summer.....maybe even as much as two weeks sunshine.

I'm sure if anyone is going to know the tricks and dark arts to this, its the BCF crew

[P.]

For a YBR, I'd just drain all the oil out, fill it up with 150ml in each leg and see how you get on.

[Bikeless]

I was going to do this on my CBR400 as the forks were white,spoke to Maxton and they said don't bother,it will make little difference,needs to be resprung and damping shims replaced.

[Kickstart]

Hi

The air gap is the important bit. And that is measured with the forks compressed and no springs in them (normally).

The amount of the air above the oil (ie, the air gap) acts as an extra spring. Bigger the air gap the softer it acts as.

Have you got a figure for the air gap?

All the best

Keith
____________________
Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Islander]

[Pigeon]

Ahh, ok, its measured when they are compressed and spring out.

I've been unable to find the air gap in the manual.

I've seen other posts on the YBR refer to 154ml oil and 166mm air gap.
My manual says 156ml oil.

If I start with 155mm air gap, is that too drastic a change?
Presumably once its all back together, I should be able to tell if its wrong and take more out etc

Thanks again.

[Islander]

If the manual specifies a quantity of oil then go with that.

[Kickstart]

Hi

I have no idea how close your bike is to a YBR, but wouldn't take much difference to change the air gap (different fork springs would quite possibly result in a different standard air gap).

Also be careful about using a thicker fork oil (ie, 15W instead of 10W). The oil weight controls the damping, so the speed the forks dive (and return). But will have stuff all effect on how far the forks dive. Probably better to fiddle with the air gap and / or try a spacer above the fork springs and / or try different springs.

It is easier to add oil than remove it.

All the best

Keith
____________________
Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Pigeon]

[Kickstart]

[Robby]

Fork oil does get worn out over time - possibly not over two years, but the original oil could have been shite.

I would recommend refilling with 10W and using some spacers.

[Bikeless]

All spacers will do is preload the forks so you have less sag,won't make the spring any stiffer.

[barrkel]

[weasley]

But the only thing compressing the springs is the weight of the bike and rider, which isn't changing. The springs will still compress ("sag") by the same amount but the level they settle at will be higher because of the shims.

This is an age-old misconception due to the misnomer "pre-load". You are not pre-loading the spring (and thus increasing its tension), you are adjusting the level it settles to when pre-loaded with the weight of the bike and rider.

The only time a shim (or pre-load adjuster) will actually have the springs "pre-loaded" will be when the forks (or rear shock) are fully extended, but this is not a situation the bike ever really encounters (wheelies and hump-back bridges excepted).
____________________
Yamaha XJ600 | Yamaha YZF600R Thundercat | KTM 990 SMT | BMW F900XR TE

[barrkel]

[YBR Ric]

"A slower dive and rebound is what I'm looking for initially."
You need to fit a heavier weight spring matched to thicker fork oil.

No matter how many shims you add, once you're on the the bike and the forks start to compress the preload is
removed from the equation leaving you wholly reliant upon a pair of underweight springs.


Measure your spring length and diameter, work out the compression rate and find something from another bike
off of the K-Tech site with a heavier spring rate closer to your needs. You may only have to increase the spring
rate by 1.5N to achieve what you're after.
Adjust the sag, air gap and fork oil weight only after the springs have been installed.

[Pigeon]

Well, the bike is all back together and working exactly as it did before. Not because I've yet again used your knowledge to to try and learn a thing or two..... but as usual, I don't have the right tool!

I was expecting a bolt with a head on it. Instead I got these and zoomed in this

Measuring the internal diameter from straight edge to straight edge is 15mm

Looks like I need a 15mm Allen key, or maybe 14mm.

I'm not sure I'd be able to get enough leverage on an allen key (bearing in mind I'm trying to avoid wheel, mudguard, fork removal if possible).

I've seen T shaped tools, a bit like this
But I need an outy not an inny

Can you convert my idiot speak into correct terms please?

Has anyone else seen this fitting on front forks before?

[weasley]

Google "15mm hex key" (or whatever size it is you need). There are not lots to choose from but you should find something.
____________________
Yamaha XJ600 | Yamaha YZF600R Thundercat | KTM 990 SMT | BMW F900XR TE

[Pigeon]

Looks like a 14mm Hex Plug

BCF with the answer again

[Pigeon]

Well it's a week on and the fork oil has been replaced.....mostly.
The 14mm drain plug key arrived. Helps when you have the right tools

Turns out, the forks on this bike do not have an oil drain screw. So the manual says the caliper, speedo, wheel, mudguard and forks have to be removed to change the oil.

Being a lazy sod, I thought I'd try cutting corners and it seems to have worked.
I put togther:
1* 60ml syringe with 150mm length of 5mm tubing
1* 60ml syringe with 1500mm length of 5mm tubing

Left everything on the bike, simply undid the drain plug bolts on top of the forks, removed the springs. Springs measured 340mm, spot on.
Shoved the long pipe down the fork legs, used the syringe to extract the old oil.

Managed to remove about 130ml of dark brown thick oil (supposed to be 10w) from each leg. Leaving 25ml (assuming it was correct to begin with) in the leg and dripping from the removed springs.

Used the smaller syringe to squirt 150ml SAE 15w Silkolene Fork Oil into each leg, pumped the legs several times. Then just sucked back again on the same syringe because the pipe length meant a 150mm air gap (manual says 166mm).

Springs in, bolts back on, job done. Nice and quick

Now, it's not as good as taking forks off and tipping them upside down for 30mins.
And measuring the air gap is not 100% because the pipe can bend (although its pretty stiff stuff, but I used a ruler too) and the forks were at a slight angle still. But they were pretty straight when compressed and the nose on the deck (using weights in top box to lessen the chance of bike falling forwards off centre stand).
But I'm happy with the job and would do it this way again.

And the result. Well. I pumped the forks for a bit before starting to get a bit of a comparison. And now, with the 15w shiny new blue fluid compared to the supposedly 10w 18mth old gravy. I can't tell any difference
Forks go up and down smoothly, but it's no slower at all.

I did replace the front brake pads too. Stuck on some EBC's. So the "English" bike now has a keyring made in England and front brake pads made in England.

Had a quick google for 340mm springs, to see about getting stiffer ones. But so far, not had any joy. Might just leave it.....but the urge to tinker

EDIT: Just re-read YBR Ric's post and K-tech website. Didn't measure diameter of spring, doh!

[Bikeless]

Ideally it needs re shimming and stiffer springs,springs you can get from,KAIS or Maxton for example. I have used both of these companies and they know their stuff.

[Pigeon]

Think I spoke too soon. Might hold off spending money on parts that aren't actually broken / need replacing.

On the road today and its fantastic. Felt like the bike was floating over the bumps, I could feel them there, but they were smoothed out.

In total, I've just added a notch of preload to the rear shocks (from pos2 to pos3 out of 5). And replaced the 18mth 10w front fork oil with 15w. But the result is noticeable, bike feels planted everywhere.

Feels brand new again, really pleased, thanks all!