Converted front suspension to run 2.5W fork oil

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Converted front suspension to run 2.5W fork oil

Postby Freddy » 03 Jun 2019, 13:17

While there are a number of factors that make a modern cartridge fork superior to the stock damper rod suspension used on a Zephyr, one of the reasons is the use of very light fork oils. So to match that technical feature I have just finished modifying and tuning the front forks on my 750 to run the lightest fork oil I can buy. In my neck of the woods that's 2.5W.

MASSIVE improvement in stability and handling (because I've now got an effective spot-on rebound damping), and great compliance over big sharp bumps because the very light oil can move quickly . My only regret is not doing it years ago. For those wondering why light oils make a big difference.....

Changes in fork oil viscosity have a major effort on suspension performance. The lighter a fork oil the less its viscosity changes in response to ambient temperature changes. That is a BIG problem in Winter where I live with heavy weight traditional fork oils where the temperature in a day may change by up to 30C, let alone big temperature variations across the seasons. To get effective rebound damping on a standard fork you have to theoretically run 20+W fork oil which will rarely be right (because of the radical viscosity changes). And I say 'theoretically' because you can't safely run 20W fork oil or above in a standard fork because it is way to thick to pass through the compression side of the damper rod when a big hit is encountered. The suspension will just lock up.

Heavy fork oils are 'sticky'. The lighter an oil the more easily the fork parts move, and a significantly greater volume of oil can be moved in a given time in response to a big 'hit'. Both give rise to a far more 'plush' suspension over big sharp bumps.

The modifications necessary to run such a light oil are not difficult, but would take a bit to explain. Happy to explain what needs to be done if anyone is interested.
Freddy
 
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Re: Converted front suspension to run 2.5W fork oil

Postby 750R » 05 Jun 2019, 21:16

Freddy wrote:The modifications necessary to run such a light oil are not difficult, but would take a bit to explain. Happy to explain what needs to be done if anyone is interested


despite the language difficulties - definitly YES!

MY 1100 with Stock Fork doesnt give me the suspenion I want - maybe your experiences/modifications will help? The Cartridge in the other 1100 makes Life better - but why spending much money if there is a cheaper way to try ?

Greeting from Bavaria
Daniel
750R
 
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Joined: 14 Sep 2017, 18:42

Re: Converted front suspension to run 2.5W fork oil

Postby Freddy » 06 Jun 2019, 07:09

750R wrote:
Freddy wrote:The modifications necessary to run such a light oil are not difficult, but would take a bit to explain. Happy to explain what needs to be done if anyone is interested


despite the language difficulties - definitly YES!

MY 1100 with Stock Fork doesnt give me the suspenion I want - maybe your experiences/modifications will help? The Cartridge in the other 1100 makes Life better - but why spending much money if there is a cheaper way to try ?

Greeting from Bavaria
Daniel


Hi Daniel,
I started typing an explanation and reasoning for going in a certain direction but it quickly became long and complex. So I'll just list the key steps as to what I think gives a really good front suspension on a 750 for the minimum possible cost. Another advantage is no suspension 'experts' need to be engaged (and the cost is not the reason for wanting to have nothing to do with suspension 'experts').

Fit correct weight linear springs
Fit a fork brace
Fit cartridge emulators
Use two part metal epoxy to seal up one of the two rebound holes in the damper rod.
Tune the rebound using 2.5W or 5W fork oil (or an appropriate mix)
Solder up one of the two slow speed bleed holes in the emulator valve. They come supplied with 2 slow speed bleed holes on the expectation someone will be using around a 15W fork oil. Only one is necessary for thin 2.5w or 5W fork oil.
Use the blue emulator springs set to 2 turns.
Start with fork oil height 130mm and fine tune from there.

As having correct weight springs is going to be necessary in every option, their cost can be disregarded in making comparisons. Same deal for fork oil. Unless you go with UDF's a fork brace is a virtual must in all conventional fork options, so its cost can also be disregarded. The only actual cost for comparison purposes is the cost of a set of a Race Tech Cartridge Emulators (don't even think about any other brand/design) verses the cost of retrofitting upgraded ZRX1100 internals.
Freddy
 
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Joined: 06 Sep 2017, 11:06
Location: Sydney Australia

Re: Converted front suspension to run 2.5W fork oil

Postby Freddy » 06 Jun 2019, 12:53

An additional comment to help explain a bit more.

The idea of completely blocking off the rebound damping holes is not mine. It seems to have originated out of the the SV650 racing community. They run light oil as it keeps a more consistent viscosity and hence rebound, and is less prone to aeration. I also believe it has a significant advance in being able to pass much faster through the emulator valve on a big hit.

The SV650 crowd totally block off both rebound holes, and rely solely on leakage past the seal on the damper rod head, and check valve in the bottom (yes there is one in the bottom of an inner fork tube) to control rebound. Aftermarket damper rods sold to this crew (promoted as having tighter tolerances) do not contain rebound holes at all.

The first time I tried blocking off both holes, using 10W oil to see what happen. The forks wouldn't budge at all. So I dumped that out and stuck in the lightest oil I could buy, 2.5W. At least they now rebounded, but still way too slowly. So it was a case of pull the forks apart again, and progressively drill out one of the blocked up rebound holes till I got the rebound speed I wanted. That hole size is now 3/32 inch.

While I didn't measure the original rebound hole size, my estimation is that my current 3/32 inch hole is one drill size smaller than the original hole. So my advice is just block up one hole, and leave the one slightly larger original hole than I currently have, and this will allow you the option of a slightly faster rebound speed than I have with 2.5W oil, or it can be slowed down if required with a heavier grade, say 5W or an appropriate mix.
Freddy
 
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Location: Sydney Australia

Re: Converted front suspension to run 2.5W fork oil

Postby 750R » 06 Jun 2019, 17:11

where can i find these Cartridge emulators? you maybe have some pictures?

Greetings
750R
 
Posts: 41
Joined: 14 Sep 2017, 18:42

Re: Converted front suspension to run 2.5W fork oil

Postby Freddy » 06 Jun 2019, 20:50

750R wrote:where can i find these Cartridge emulators? you maybe have some pictures?

Greetings


Everything you ever wanted to know about emulators ...... http://www.racetech.com/page/title/Emulators-How%20They%20Work

The main principals are that the compression bleed holes in the damper rod are drilled out to be so big which totally eliminate them. The compression damping is then controlled totally by the emulator. This frees rebound damping to be fully controlled by the weight of fork oil used i.e. heavier oils, slower rebound.

The addition step beyond the installation instructions is to also eliminate or reduce the size of the rebound holes in the damper rod so that very light oils will provide the level of required rebound instead of having to use 20+W fork oil (with all its associated problems).

IMO that is the weakness of the cartridge emulators, relying on heavy weight fork oils to try and establish correct rebound damping. After springs, rebound damping is the next most important system setup. It's virtually impossible to get consistently right with heavy weight fork oils whose viscosity swings widely even just throughout the day, let alone as the oil ages, seasons change, etc, etc. Because people struggle to get rebound damping right they try and compensate with endless adjustments to the compression damping in the emulators. They essentially fail because it isn't a compression damping problem, its a rebound damping problem. And making the changes to use light weight oils goes a long way to removing the problem.

Image

P.S. and another advantage of very light fork oil is that it is so clean to work with. Heavy oil clings to everything and drips everywhere.
Freddy
 
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Location: Sydney Australia

Re: Converted front suspension to run 2.5W fork oil

Postby Freddy » 08 Jun 2019, 03:12

Another significant advantage of using very thin fork oil I hadn't previously mentioned, is the greater ease of tuning the damping.

To explain why it is so much easier to tune light oil in a suspension that primarily relies on 'bleed holes', imagine trying to finely regulate the flow of water out of a bucket by drilling a certain size very small hole in it, verses trying to do the same thing with an appropriate size hole if the same bucket was filled with a thick liquid like honey.
Freddy
 
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Location: Sydney Australia

Re: Converted front suspension to run 2.5W fork oil

Postby 750R » 10 Jun 2019, 11:54

i have been looking for the Emulators for the 43mm Fork of the 1100 - but havent found anything. Have you may be ssen some?

Greetings
750R
 
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Re: Converted front suspension to run 2.5W fork oil

Postby Freddy » 10 Jun 2019, 16:59

750R wrote:i have been looking for the Emulators for the 43mm Fork of the 1100 - but havent found anything. Have you may be ssen some?

Greetings


The ZR1100 isn't listed on Race Tech's product search by make/model page, so I'd contact Race Tech directly at the following ...... http://www.racetech.com/page/id/18.

Their Excel product down load for cartridge emulators http://www.racetech.com/page.aspx?id=100#RACE_TECH_PRODUCT_LISTS shows two emulator part numbers that would seemingly fit a 43 mm fork. They are part numbers FEGV S4301, and FEGV S4303. There is a very tiny dimension difference in the 'Step D' measurement between the two part numbers.

My guess is the S4301 is likely to fit. They suits a ZX11 fork of the same era and are widely available on E-Bay.

P.S. A word of warning, be careful using Race Tech's spring calculator if you looking to replace the springs. It is well documented for recommending springs that are on the excessive heavy side. After a bit of trial and error I use 0.90Kg/mm springs in my 750 (I'm around 95Kg in gear). The spring calculator would have me running something like 1.00 springs which would be way too hard on bad roads.

I've found the Sonic Springs calculator gives the correct answer in my case ..... http://sonicsprings.com/catalog/calculate_spring_rate.php#calculate
Freddy
 
Posts: 479
Joined: 06 Sep 2017, 11:06
Location: Sydney Australia

Re: Converted front suspension to run 2.5W fork oil

Postby 750R » 11 Jun 2019, 19:14

Thanx foor the information - i'll try to contact them! will get funny wth my really bad English :lol:
W'll let you know what the answer was.
The spring for the 1100 can be a bit harder - 50kg more weight in the motorcycle (an don't ask for the driver ;) )

Greetings
750R
 
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