Author Topic: Front Suspension Tuning with Cartridge Emulators  (Read 9118 times)

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Offline stueveone

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Front Suspension Tuning with Cartridge Emulators
« on: March 27, 2011, 08:09:55 pm »
After months and months and countless tear downs, talking with Suspension Gurus, and borderline OCD front end tweaking, I am finally happy with the handling of my Cartridge Emulator equipped front forks. Here is the "final" addendum and tuning notes to my previous Emulator post http://forums.sohc4.net/index.php?topic=76521.0 including a side-by-side compare of Reacetech vs. MikesXS Emulators.
Hopefully, this will save you guys some of the grease time and clear up some of the suspension mystery when custom tuning your own suspensions. . .

For those of you with short attention spans or reading aversions, here are the cold hard numbers of my final setup:

Racetech Cartridge Emulators (Blue 40lbs/inch springs) 3 turns
Racetech Springs 1.00kg
15W Bel Ray Oil
180mm of Oil
1 Rebound hole welded shut
Rider Weight: 190lbs
Spacer length 66mm with 1 washer provided
20mm of preload
32mm of static sag


The basic problem of my front suspension post Emulators was an unwieldy front end rebound. Here is the eventual tuning solution that I’ve been working out these last months along with resources to general motorcycle front fork tuning.
If you don’t know the difference between compression damping and rebound damping, or preload vs. spring rate, check out the Suspension Tuning Guide at Sportrider.com and pour over the associated links first to better familiarize yourself with general suspension tuning.

http://www.sportrider.com/suspension_settings/suspension/146_1004_sport_rider_suspension_guide/index.html



On Emulators, brands, and tuning. . .
I originally tried the MikesXS emulators thinking, heck, I’ll save some bucks. Well, call it my perfectionist tendencies or never ending search for piece of mind, but I finally broke down and bought a pair of Racetech Emulators. In comparison, these units are very similar, albeit identical. There are at least two key differences which led me to use the Racetech brand vs. the cheaper MikesXS brand.
1. Racetech has 2 valve plate bleed holes, compared to 1 on the MikesXS version. An extra hole means the Racetech unit will navigate low speed compression damping far ie small bumps, road chatter, better than the MikesXS unit.
2. Racetech gives you 3 sets of springs with different spring rates depending on your set up.
Let’s be honest, the MikesXS Cartridge Emulators are knockoffs of the original design Racetech developed and tested over who knows how many prototypes. What was the R&D process of MikesXS besides blatantly ripping off the design? I’m guessing not much. I also couldn’t tell you what spring rate they are using in the valve spring or spring shim. Racetech, however, DOES provide you with spring rates and 3 choices. So, considering I am a guy who usually plays it safe, and got tired of taking my front end apart constantly, the Racetechs were the obvious choice. This is not to say someone won’t find the MikesXS units an improvement over the archaic damper rod style front end these bikes came with. But if you are a perfection psycho like me, again, the Racetechs are the better option.

Springs. . .
In my original build I went with cutting the Progressive springs I already had. Don’t do this as the progressive springs render it fairly impossible for the accurate tuneability of your forks with Emulaotrs installed. At the very least, find a set of original single rate stock springs and cut those. Although, this too is a poor option. The best option to get the best performance out of your front end with emulators, which is why you are doing all this in the first place right? is to get a pair of single rate springs made for Emulators.
The reason you can’t use normal length CB springs is because the emulators are about 20mm or so tall. And because the emulators are installed below the springs, you are adding 20mm of preload to the springs. Not a good thing. So that leaves two options, cutting original springs or getting shorter springs. Cutting springs makes them stiffer. So unless you want to figure out how much you can cut the springs in relation to spring rate in relation to your body weight, just buy shorter springs with known spring rate. I called Racetech for mine, who calculated both bike weight and rider weight (I weight 190lbs) before sending me out a pair of 1kg springs.
I believe spring rate and preload were the main contributers in my original problem of too much rebound or not enough rebound damping, however you want to state it. In aiming for 30-35mm of static sag, I had to preload the stock cut springs (shortened around 20mm) far more, than the stiffer RT 1kg springs. So with the RT springs, I am at what is considered the normal amount of preload for street application, around 20mm with a very firm spring. The end result is SWEET!
So, again, either way you need shorter springs with Emulators. If you choose to cut stock springs, figure out the spring rate you are shooting for, and cut accordingly. Or just save yourself the hassle and buy new springs.

Rebound damping tuning. . .
After speaking with a few race tuners and suspension techs, and working under the assumption that my bikes rebound evils were due to an under-sprung condition, I decided to play it safe and re-drill 1 rebound hole (I had previously welded both on each damping rod). Welding both holes closed is still probably fine, especially considering how bad your damping rod seal, inner fork tube wear, and rebound check valve wear is. However, with new fork tubes, new damping rod seal, and 15w oil, my suspension rebound is about right where it needs to be with 1 hole welded closed. It has been asked, but if you weld both holes closed, where does the oil go?

Check out diagram 1 here: http://www.sportrider.com/tech/146_9502_tech/photo_01.html

On the compression stroke, oil is pumped up thru the rebound check valve built in to the fork tubes. Then, on the rebound stroke, the check valve’s job is to close the rebound chamber so that the pressurized oil has no where to escape but the rebound holes. Problem is, and as I found in my tests, the damper rod OD is not exactly a tight fit within the ID of the check valve. Meaning, some check valve oil blow-by is inevitable.  This, to answer the question, is where the oil naturally goes if there are rebound holes present or not. So again, depending on the sealing of your rebound check valve, you may be fine welding both holes and using 15w or welding 1 and using 20w. It kind of depends on your set up. For the moment, I have settled on 15w with fairly good rebound results. Though I might try 20 or 25 just for testing.

Oil height. . .
Oil height, or oil level, or air chamber are all the same measurements. Basically, you want to use as much suspension travel as is allowed before either spring binding or bottoming out occurs. Fork oil does basically 2 things: 1. It provides the necessary damping characteristics of your suspension and 2. It saves your fork internals from bottoming out by providing a kind of cushioning effect. Because your fork is air tight (unless it’s leaking!) the oil level, or more importantly, the amount of air above the fork oil will compress when you hit a bump. Too much oil/not enough air space, and your suspension will not allow full travel. Not enough oil/too much air space, and your fork internals will have nothing to stop them from mechanically bottoming out when you hit a harsh bump.
You want to measure fork oil level with the forks completely compressed and with springs and emulators removed. After much testing, I found that the forks work the best in the ballpark of 175-190mm oil level. This is confirmed by RaceTech’s guide, http://old.racetech.com/evalving/menu/searchdirtvs.asp which given my weight, recommends 180mm of oil.

That is about it. After all the work, months of tuning and head scratching, my final thoughts on the matter are: unless you want to spend the time tinkering, tuning, and repeatedly tearing apart your front end, spend the money and get the kit from Racetech. That is not to say the MikeXS route won’t work. And there are other springs makers that can make you springs. But at the end of the day, I like to ride. If I wanted to work on my bike all the time I would have gotten a Triumph or Harley!
I am super stoked on my final suspension set up. Well, with my front forks anyways. Sigh. With a set of Ikons on the way and swing arm sent out to Hondaman, my rear suspension awaits. . .


Other reading etc.
http://www.racetech.com/page.aspx?id=30&menuid=93#19
http://www.vmaxoutlaw.com/tech/cartridge_emulators.htm
http://www.feelthetrack.com/downloads/Suspension41.pdf
« Last Edit: March 28, 2011, 11:09:29 pm by stueveone »

Offline Steve_K

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Re: Front Suspension Tuning with Cartridge Emulators
« Reply #1 on: March 28, 2011, 05:21:51 pm »
I have had simlar results with RT products on Suzukis and I love the way the forks work.  Makes you want to do the same with the rear shocks.
Nice write up
Steve
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76 CB 550, 73CB750, 86 GSX-R750, 16 Slingshot
Old rides:305 Honda, CL350, 74 CB550
 05 SV1000S, 88 CBR600,92 VFR, 88 Hawk GT, 96 Ducati 900SS, 98 Kaw ZX6R, SV650

Offline bwaller

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Re: Front Suspension Tuning with Cartridge Emulators
« Reply #2 on: March 28, 2011, 07:03:55 pm »
Very thorough & informative stueveone. Like you I started with MikeXS emulators but have RT's on the way. A while back asked Mikes what weight emulator spring came on their units and never got a reply. I bought RT's 40lb/in emulator springs to compensate for not meeting the fork spring ID, valve plate OD suggestion for clearance. Part of the reason for finally buying RT valves.

I'm using 750F2 forks that are excellent, no wear marks, like new. Following RT's suggestions I bought their .85kg springs but I'm afraid they may be a little light. This for a 325 lb CB550 race bike. I just seem to have a bit too much of an issue with stiction when measuring for laden sag. Does the heavier spring help overcome this for you?

I had welded one rebound hole closed but on the advice of a RT tech I redrilled it. Now even with good damper seals I think I may weld them both closed and if need be open the emulator bleed holes slightly larger if need be. I have way too much undamped rebound.

Anyway well done, you should help answer a lot of peoples questions.

Offline stueveone

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Re: Front Suspension Tuning with Cartridge Emulators
« Reply #3 on: March 28, 2011, 08:02:30 pm »
"I just seem to have a bit too much of an issue with stiction when measuring for laden sag. Does the heavier spring help overcome this for you?"

How many mm are we talking? I have read that anywhere between 5-15mm of stiction (difference between L1 and L2 measurements) is considered normal. I'm not sure how much these bikes came with. I am right around 10mm myself. Did you lube your seals with grease before installing them?

What are your other stats? Static sag? Free sag? Preload? As I said, I am at around 32mm of static sag. However, I have read multiple times that your sag should reflect about 30% of fork travel. Since cb750's have around 140mm or so, that would mean static sag of 42mm!!! Hmm...

To be honest, I was pretty disappointed in the Tech Support at Race Tech. After about 10 calls and 2 voice messages, I finally got a hold of their tech, who then told me I needed to use 130mm of fork oil?!!! So, as far as them knowing to drill rebound holes or not, at this point, I'm not confident in their recommendations.
Heavier oil weight and trying to figure out how to make that rebound check valve seal better are the only things that are going to help add damping to the rebound.


stueve

Offline stueveone

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Re: Front Suspension Tuning with Cartridge Emulators
« Reply #4 on: March 28, 2011, 11:07:25 pm »
Interesting video from "The Man" Dave Moss advocating 20+ weight oil for emulator installed damping rod forks.
http://www.ustream.tv/recorded/5690943

Offline bwaller

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Re: Front Suspension Tuning with Cartridge Emulators
« Reply #5 on: March 29, 2011, 04:23:30 am »
The .85kg RT springs are 23.5x410mm. I don't expect anyone to have CB specific fork springs, but these are very small diameter. Because of the need for a bellypan/ground clearance and not raising the front too high, I'm limiting travel to 115mm. Stiction is anywhere from 6-14mm, one test to the next, was playing with different preload but quit when I figured it would be necessary to re-weld both rebound holes So the front suspension is on hold while other parts of the job get done.

Oil weight will help too, I have 15W right now as well. I'll weld them first.
 


Offline tweakin

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Re: Front Suspension Tuning with Cartridge Emulators
« Reply #6 on: March 29, 2011, 05:25:19 am »
Great write up!  Saving this one for reference.

Offline Kong

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Re: Front Suspension Tuning with Cartridge Emulators
« Reply #7 on: March 29, 2011, 07:18:22 am »
You discussion of the emulators is very interesting.  In the end I will follow you lead, and thank you for taking the time to figure all this out and report it - this is mighty useful information and very much appreciated.  None the less, these things sort of stick out in my mind.  About the emulators, I don't know what the advantage is of having several springs available on a device that offers infinite preload adjustment and I don't know if one hole in the emulator is better or worse than two holes.

I think what interests me most is the continuing question about welding holes shut in the damper.  If I got it right you first welded both of yours shut and then redrilled one of them, is that correct?  If it is what size did you drill the replacement hole and at what location.  That aside, on the question of closing them off at all,  as I understand it Race Tech is silent on doing this or doesn't recommend it, instead suggesting that rebound is a function of oil weight.  Well, if that's what they say and they are the ones who did the years and years of testing, then why doesn't anyone believe them?

See what I'm saying?  Why is it that we rely on their years and years of testing on the compression stroke (3 springs available and 2 relief holes), but think that its irrelevant on rebound (closing rebound holes) issues?  Once again, I'm gonna do what you did, but the question lingers none the less.

Side notes: I was surprised by your spring rate.  You weight about as much more than I as my bike weighs more than yours (meaning our total weight on the tires is about the same, with me/my-ride being just a little heavier) and when I run my numbers it comes up with a street spring-rate recommendation of .78 kg/mm and for racing purpose .82, leading me to lean toward a spring rate of about 0.80 kg/mm.  That's quite a difference from the 1.00 kg/mm you came up with.   Then I was confused by this.  You note in your specifications that you are using 20mm of preload but later in the discussion you tell us that the spring has to be cut short by about 20mm to compensate for the height of the emulator. Maybe I'm confused, are you saying that you cut 20mm out of your spring (or ordered 20mm shorter springs) and then ended up putting a 20mm plug back in the top to give you preload?
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Offline stueveone

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Re: Front Suspension Tuning with Cartridge Emulators
« Reply #8 on: March 29, 2011, 07:17:20 pm »
"About the emulators, I don't know what the advantage is of having several springs available on a device that offers infinite preload adjustment"

Yes, but preload is different than spring rate right?

"I don't know if one hole in the emulator is better or worse than two holes."

It can be. I'm not sure if I can tell the difference. But it says right in the instructions that they even have a 4 hole plate where more low speed damping is needed.

"as I understand it Race Tech is silent on doing this or doesn't recommend it"

All RT says about rebound holes straight from the instructions is: "Do not add or enlarge the rebound holes and leave their edges sharp if any exist". So it doesn't recommend drilling new holes. It doesn't say anything about welding. But it also says "if any exist" meaning there are obviously bikes out there without any holes from the factory.
I have heard other tuners of cb's and other makes welding holes shut however. So, yeah, a bit of a grey area. Although, welding 1 hole makes sense to me.

It's not that I don't believe RT's advice on how the design of their part functions. It's that I'm questioning their experience with tuning a 1977 CB750! The reason is, their part affects compression damping ONLY. Rebound damping is another circuit all together and has nothing to do with the Emulator. So, again. I'm wondering just how much time they have spent with a CB. Again, their tech told me 130mm of oil and their website says 190mm? Just a bit wonky is all.

The spring rate was recommended by the sales person at Race Tech, not via my personal calculations.






Offline stueveone

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Re: Front Suspension Tuning with Cartridge Emulators
« Reply #9 on: April 02, 2011, 09:54:46 am »
Some more tuning thoughts spurred by a PM sent by Bwaller.

I was planning on wrenching on my forks this weekend but Bikebandit sat on my 30w fork oil order! 
Anyways. the RT and Mikes emulator bodies are identical as far as I could tell. I will measure them once that oil arrives. But I would bet money that they are the same. I see no reason why you couldn't re-drill that plate for another hole. RT has up to a 4 hole model BTW.

Checking your preload. More preload or a heavier springs means faster rebound and the need for more damping. It seems CB750's have a lot more usable travel that most bikes. So the 30-35mm static sag benchmark might not be accurate. Though I've also heard 35-45mm for street is appropriate. This starts making sense when you consider static sag in terms of percentage of travel. 141mm of fork travel at the recommended 30% of sag equals 42mm.

stueve

Offline bwaller

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Re: Front Suspension Tuning with Cartridge Emulators
« Reply #10 on: April 02, 2011, 10:55:16 am »
I was curious about the shoulder width of the valve plate between the RT & Mikes emulators. Sounds as if they're identical. RaceTech wants 4mm clearance between the fork spring ID and the valve plate OD, yet clearance on the parts supplied is 1.8mm...... that's why I went with 40lb/in emulator springs & will run less tension to compensate for half the clearance.

One thing I was told was if it's too harsh on the sharp bumps, to open the emulator bleed hole one #size drill, which makes sense.

Offline Steve_K

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Re: Front Suspension Tuning with Cartridge Emulators
« Reply #11 on: April 02, 2011, 03:43:50 pm »
RT says that rebound is controlled by fork oil weight.  Welding holes closed would let you use lighter wt oil.  Smaller holes or less would increase rebound. More range for tuning with oil.
Steve
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76 CB 550, 73CB750, 86 GSX-R750, 16 Slingshot
Old rides:305 Honda, CL350, 74 CB550
 05 SV1000S, 88 CBR600,92 VFR, 88 Hawk GT, 96 Ducati 900SS, 98 Kaw ZX6R, SV650

Offline MRieck

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Re: Front Suspension Tuning with Cartridge Emulators
« Reply #12 on: August 11, 2014, 06:56:29 am »
I am bumping this post as I am installing emulators and found the info worthwhile (especially the oil level as race Tech doesn't provide a level). I think the original posters spring rate is stiff but if it works for him that is OK by me.
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