Fork Oil Adjustment

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Garry Coates
Posts: 10
Joined: Tue Aug 11, 2020 1:57 am

Fork Oil Adjustment

Post by Garry Coates »

Hi All,
I am new to this forum but have followed it for awhile. I have my 1995 Honda ST1100 ABS/TCS fitted with re-valved VTR1000 fork cartridges and WP springs since 2015. I am reasonably happy with the result as it got rid of the hybrid damper rod/ cartridge set up of the OEM. The bike feels better under brakes and hard cornering. The only issue being the compression dampening is a bit too stiff in comparison to rebound across the entire range. I have not been able to adjust this out. Now with 53,000 km up I am looking to get a re-valve but Covid-19 restrictions here in Melbourne have stopped me. Therefore I am thinking of doing an oil refresh only possibly to reduce it from 5wt to 4wt and see if I will give me a better adjustment compromise. The rebound adjuster impacts compression as well (70/30?). Probably also raise the air gap from 170 mm (OEM) to 190 mm as have plenty of spare fork travel.

I would interested in what others have done with oil weight changes to improve the adjustability. I am already aware of the Ablett modifications.

Regards
Garry
1995 Honda ST1100 ABS/TCS
1999 Honda XR600R
2013 Husqvarna TXC310R
tony.mon
Posts: 15943
Joined: Wed Jul 25, 2007 10:46 pm
Location: Norf Kent

Re: Fork Oil Adjustment

Post by tony.mon »

I'd make those changes one at a time, starting with increasing the air gap in 10mm increments.
If you increase the air gap and also change oil weight you won't know which change has been effective; worse, one could cancel the other out.
But changing oil weight by 1 won't be noticeable, move in 2.5 weight steps initially. 5 is already very thin, so that shouldn't need to be adjusted thinner, more likely a bit thicker, after the air gap has been increased.
It's not falling off, it's an upgrade opportunity.
Garry Coates
Posts: 10
Joined: Tue Aug 11, 2020 1:57 am

Re: Fork Oil Adjustment

Post by Garry Coates »

Hi Tony,
thanks for the reply. In response
1. Increasing the fork air gap only impacts the last third of the travel (softer). I am only using 115mm of the 150mm travel so hoping to increase travel and soften the last bit. SAG is at 30mm without additional preload.
2. The cartridge valves are tuned for 5wt oil. Changing from 5wt to 4wt is 20% not dissimilar from going from 10wt to 8wt. It would soften dampening over the entire range. 4.5 wt (10%) maybe a better first step.

Therefore I am pretty sure both changes are in the right direction. The question is how the oil weight change impacts the rebound adjustment which changes compression as well.

Regards
Garry
1995 Honda ST1100 ABS/TCS
1999 Honda XR600R
2013 Husqvarna TXC310R
MK_WF
Posts: 130
Joined: Mon Aug 31, 2015 6:02 pm
Location: Wolfenbuettel/Germany
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Re: Fork Oil Adjustment

Post by MK_WF »

You should consider that oil of brand A does not fully compare to a "same" oil of brand B.

There's heaps of viscosity measurements around the net that prove a #5 oil can vary more than 20% between brands.
Here's an example from down under : https://transmoto.com.au/comparative-oil-weights-table/

So my first shot would be to use a brand that delivers constant specifications over a long term like White Power or Oehlins.
If damping needs to be adjusted afterwards, you can at least rely on the numbers and it's not more or less pure luck to grab the "right" bottle.
Bye
Martin
Garry Coates
Posts: 10
Joined: Tue Aug 11, 2020 1:57 am

Re: Fork Oil Adjustment

Post by Garry Coates »

Hi Martin,
thanks for your reply. I have tracked down the oil used previously (Maxima 5wt, 16.2 Cst)) so I can keep the kinematic viscosity consistent or about 10% lower.

Regards
Garry
1995 Honda ST1100 ABS/TCS
1999 Honda XR600R
2013 Husqvarna TXC310R
Garry Coates
Posts: 10
Joined: Tue Aug 11, 2020 1:57 am

Re: Fork Oil Adjustment

Post by Garry Coates »

Thanks Guys for your feedback,
it has been surprising to me that the general consensus (elsewhere) is that oil viscosity is not a significant factor. There is a hell of a lot of oil being marketed with different weights (cSt).

In conclusion I will be staying with the 5wt Maxima oil and just increasing the air gap to soften last bit of travel. My ST1100:

1. Weighs 323kg wet plus a 196cm, 115kg rider plus gear and luggage gives a total of 450kgs

2. Has 50% higher springs at both ends because OEM rates barely cover sag of bike only. Now Rider sag is 30mm at both ends.

3. Only uses 115mm of the 150mm fork travel available. Fork dive is fine with this setup. The bike just squats with both brakes applied. Rear brake is very important on a touring bike as the back never leaves the ground.

4. Going from 170mm to 190mm airgap is just within the range for that model. Hopefully use another 5-10 mm of travel. Besides putting extra oil in is easier than taking it out.

I have gathered some useful information to discuss with the tuner next time which is probably the XR600R refresh post covid

Regards
1995 Honda ST1100 ABS/TCS
1999 Honda XR600R
2013 Husqvarna TXC310R
MK_WF
Posts: 130
Joined: Mon Aug 31, 2015 6:02 pm
Location: Wolfenbuettel/Germany
Contact:

Re: Fork Oil Adjustment

Post by MK_WF »

I didn't catch that we're talking about HGV suspensions :-)

Seriously: There's one thing more than spring stiffness and air chamber.
If you have a +50% spring rate and adjusted the "right" sag, but still not use the full suspension travel, then your stiffness was too high.
The oil level can only influence this a small range.
Bye
Martin
Garry Coates
Posts: 10
Joined: Tue Aug 11, 2020 1:57 am

Re: Fork Oil Adjustment

Post by Garry Coates »

Hi Martin thanks for your reply. The front sag at 30mm with no additional preload is on the stiff side for a touring bike. The WP springs have been on the ST since the first service when new. They fixed chronic weight transfer on corner entry which gave the feeling the bike would run wide mid corner.
I now can trail brake as deeply as I need to in emergencies.

Remember changing the air gap will probably get me to 120+mm travel which is pretty much what any road bike gets now. Saving a little for unusual loads or conditions etc is fine.

The real elephant in the room is the VTR front travel. 41 mm conventional showa forks have been around since the 80s and they gave atleast 118-120mm of travel. The CBR600F3 has identical forks to the VTR and gets 118mm. F2 was around 130mm.

The base ST1100 also has the same forks with identical 633mm length tubes and gets 150mm. Even with stock VTR cartidge/damper setup I get 150mm if I want it.

The VTR is running with long forks for only 109 mm of travel which you can improve just using ST1100 ABS/TCS springs and no spacers.

Nobody has raised this.

Regards
Garry
1995 Honda ST1100 ABS/TCS
1999 Honda XR600R
2013 Husqvarna TXC310R
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freeridenick
Posts: 1574
Joined: Fri May 23, 2014 1:30 pm
Location: Derbyshire

Re: Fork Oil Adjustment

Post by freeridenick »

Are you saying that the VTR forks could have more travel and this would be an improvement? If so how would that be achieved? The stock setup only has 120mm of stroke so more would have to come from raising the front end which would have to be matched with raising the rear to maintain handling.
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