Thicker/thinner fork oil then recommended

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I always used the recommended for oil or whatever was the equivalent, since fork oil weights aren’t exactly standardized.

However my 2000 Valkyrie has a very bad tendency do dive, even at pretty low speeds. It is abrupt and then settles once the suspension is compressed.

Reading forums people talk about changing springs with progressive ones, but to me this didn’t feel like a spring rate issue.

Found a guy on YouTube, Dave Moss Tuning and he talks quite a bit about fork oil weight and level and how it can be used to better tune the front suspension.

So I decided to give a heavier oil a try. Changed the fork oil over the weekend to Motul 15w. I put about 100 miles so far and I must say the improvement is definitely noticeable.
The dive is less pronounced and much better controlled. There is no abruptness to it anymore.
The suspension also feels more planted over rough surfaces.

I would say this was a cheap and easy improvement for sure.
 
The reason that a lot of motorcycles have excessive brake dive is that they use cheap damper rod forks, which have an exponential damping profile, which don't provide enough low-speed damping, but provide too much high-speed damping. Using thicker oil will improve the low-speed damping, but it makes the high-speed damping even more harsh, which may not allow enough suspension movement over harsh bumps. That said, you live in a warm climate, and a thicker grade of oil might be warranted for that reason alone.

The best solution is to upgrade the valving with cartridge emulators like those from Racetech, which have a more linear damping profile. They're typically only around 2-$300, but they require completely removing and disassembling the forks to install. They've made a big difference on the bikes I've ridden, but I'm not sure if the difference would be as great on a cruiser.
 
Yeah, cheap components for sure, not only that, but on this bike one fork is responsible for compression only, while the other handles compression and rebound. For such a heavy bike, it’s cost cutting at its finest.

But I’m happy with the result of just the thicker oil.
 
Glad it worked out for you. Similar experience (but opposite problem), I am a dirt rider and I used to have a 1992 WR250R, that bike was great at high speeds on fire roads or desert riding but was terrible in technical or rocky terrain (which was 95% of my riding). It was simply too harsh on compression and I would bounce all over the trails / boulders etc. I switched to Redline "Like Water" fork fluid (so, significantly lighter than the factory weight oil) and the tough terrain riding improved 10 fold. I never forgot this lesson and in subsequent bikes I paid close attention to the fork oil weight to tune my riding, sometimes I need to go up in weight... sometimes down. It makes a big difference.
 
You're right, fork and suspension fluids are not standardized. You have to look at the ISO grade and viscosity at 100C
to make an apples to apples comparison.
It's far cheaper to experiment with hydraulic fluids, atf and thf's than overpriced consumer products.
For example, an ISO 22 hyd fluid is a good place to start with dirt bikes.
A LV atf like Dex 6 is a half step thicker at 27. A D3M HV atf is an ISO 32, and a hyd fluid ISO 46 is the next thicker.
There's no need in getting into exotic synthetics, and all of the above can be blended for fine tuning.
The ISO grade of Motul 15W is anybody's guess if it's not printed on the label.
 
Damper rod forks can be quite sensitive to oil level. Raising the oil level decreases the air volume above the oil and this increases the air spring effect when the forks are compressed . You might be surprised how strong the air spring can be. A way of getting a feel for this is to remove the springs, replace the top caps and hand stroke the fork legs when you will feel substantial resistance as the forks get towards full travel.
 
advertised viscosities for fork fluids VARY in actual aka CENTISTOKE real viscosity, so buyer beware if changing brands looking for thicker or thinner fork fluids, search + learn!!
 
Found a guy on YouTube, Dave Moss Tuning and he talks quite a bit about fork oil weight and level and how it can be used to better tune the front suspension.

I know Dave Moss... not only do I trust his suspension advice but also test hopping my rare and exotic RC45 at race pace...

DaveMossMr.RC45.JPG
 
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