Hey All

I am on a '23 RGST. I am considering changing the stock fork oil for a heavy/performance fork oil. I understand the denser viscosity slows the rebound of the fork. This decrease in "sag" translates to better responsiveness and an increase in "effective torque". I also understand that the front end is not as soft and perhaps not as comfortable.

My question for the forum here is if there are other considerations I am not aware of? How does the heavy fork oil affect the slow speed dynamics of the bike? Or, perhaps the performance of the bike under heavy load while touring with luggage or a passenger?

Thanks!

 

Sag and rebound are not the same thing.

 

A change in ride height, which is affected by your change in sag, will affect the rake angle. This will affect how your bike reacts to swerving and weaving. If your front end is taller than stock, the front end will feel light, and the bike will be more prone to wobble at higher speeds.

If your luggage is located at or behind the rear axle, you'll continue to increase the height of the front end, effectively lightening it more.

Sag/ride height can be adjusted with the amount of fork oil, but your rebound and compression will change as well.

P.S. I thought there was some fancy forks on the RGST?

 

@Brandon Smith most of what you said I believe to be true. I don’t think sag is changed by fork oil, viscocity or level. Those would change dampening and rebound. Please correct me if I’m wrong. Mechanics control sag measured with weight on the bike. Oil controls rebound and dampening based on viscocity and flowing through ports/valves.

 

I always change my fork oil to SE Performance Oil. It has worked great on multiple Dynas and Touring bikes. I think it improves handling and brake dive with very little fuss.

 

Rebound and dampening speeds are manly dusted by fork oil weight if not adjusters.

Ride height sag is adjusted by spring tension or spacers, or in the case of air shocks, air pressure.

So yes, thicker fork oil in the front forks will make them stiffer, and if you need to reduce the amount of rider height sage in the front forks, then can double or tripple up on the washer 2, or just change out front fork springs to something stiffer.


As for rear shock, yes, could pull them apart to rebuild them with thicker fluid, and nitro recharge using Racetech spnV0512 add on charging ports, but just better off going with shock that are adjustable the same both sides. Hence with only one side begin adjustable, least just say not the best way to begin with.

 

Abitheavier oil will change, well just fresh fluids will make a difference but increased viscosity will change the nose dive. On my 07 I experimented with different weights until I put progressive cartridges. On my 14 tri glide I went with heavier fluids and it helped, on my current 21 tri glide I went with Ricor Intiminators and 10wt fluid. I like it a lot. The feel is different but compliant and smoother

 

FWIW.
Suspension set up has "personal" desires/ rider feeling.
It may take multiple attempts / disassembly to get it right.

Best advice I can give: start with start and set your sag.
(Oil weight will make no difference to sag "tuning"/ set up.)
Typically one can make different length PCV pipe spaces (low cost) to attain a proper sag.

Heavier fork oil indeed slows rebound and compression.
Not so sure about how to answer the passenger or luggage question.
I guess I would say that front suspension feel and setup choice could have a major difference when traveling solo or 2-up, so one single set up may not fit all riding desires.

Therefore:
Once sag is set, knowing that fork oils are sold in weights at 2.5 weight intervals, you could start experimenting with weights (but stay with the same brand so weights are stable).
Without research, I believe the stock HD oil is 10 weight and Screaming Eagle is 20 weight.
Oil level (air gap) can also "stiffen" a fork, but do raise the oil more than a 1/4 inch.
There is no harm with your original plan to just change to the heavier weight oil.
PS I also find a fork oil level measurement tool (@$30.) is well worth the cost to purchase.

Fork springs are a good step also, especially if you are telling us the forks feel too soft.

With all that said, have fun.

 

Here is a report on how front suspension works, and a comparison of different options.

Front Suspension Report #1

Front Suspension Report #2.(info on sag, damping, fork oils & more)

For most, the Intiminators give the best quality ride for the $$$ spent.

 

I can tell you this for your consideration:

I have a 2011 FLHTK ( Ultra Limited ) and over the years as I gained weight I experimented with changing the oil in the forks and the air shocks. At first I left the forks alone with the stock oil and replaced the air shocks with 10W Maximua fork oil. when I did this the research I conducted led me to believe that these bikes came from Harley with 7w oil in the rear air shocks ( * some of this information comes from the Gold Wing forum because the rear air shocks on these bikes come from the same company that makes them for the Gold Wing ) the bike did handle better with the new oil in the rear air shocks but I was still not happy with it because I still had to up the air pressure to 45# to keep from bottoming out and the brake dive on the front end was more than I liked . Now I do not know for sure if it felt better because of the weight difference in the oil, or because it may not of had the correct amount of oil in them to begin with because I did not measure the oil I took out of them, but it was an improvement.

After a couple of years like this, I finally hit 400 lbs and was not happy with the ride of the bike again. I decided to use the HD SE Heavy fork oil in the front forks and the Maximua 15W oil in the rear air shocks. This time I measured what came out of the front forks and one fork was 2oz less that the other. The rear air shocks had the same amount in them that I put in them with the 10W. ( 12 oz witch also comes from the Gold Wing forum )

I filled the front forks with the HD SE heavy fork oil to the HD recommended amount, and the rear air shocks with 12 oz of Maximua 15W oil and the bike is much more manageable for me now. I use to run 40 psi in the tires and 45 psi in the air shocks ( that are 50 psi max ) but now I can run 38 psi in the tires and the air shocks . This allows me a bit more cushion room before bottoming out.

Some say that the Harley Davidson Screaming Eagle HEAVY fork oil is also 15W but I am not 100% sure on that, I only know the overall feeling of the bike seems a little stiffer but it is much more comfortable for me with my weight . I do not know if you can expect the same results on your '23 RGST because I do not know how the suspensions compare to my '11 FLHTK , I only mention my experience for you consideration. Good luck and keep us posted .