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Selection Of Rear Shock Spring & Fork Springs


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I'm looking to learn an empirical/scientific based methodology to determine the correct shock and fork spring(s) for my bike. I am intentionally not going to mention my bike or my weight - just that my riding style can best be described as aggressive twisty riding.

 

I have consulted with all of the suspension "gurus" - Race Tech, Traxxion, Jim Lindemann, Phil Douglas, etc. I have had shock spring rate recommendations anywhere from 950 #/in to 1400 #/in for the same bike and parameters. My shock is a Penske double adjustable by the way. I run that on all my bikes.

 

Surely there has to be some sort of semi-scientific method to determine the optimum spring rate. After all, the springs are the building blocks of a suspension system. Most all of the articles seem to be focused on compression and rebound damping and do little to address whether you have the correct spring to begin with.

 

So far all I have come up with is Andrew Trevitt's "Sportbike Suspension Tuning".

 

I'm not looking for a particular spring rate recommendation (hence why I don't mention which bike or my weight) - I'm looking for a way to determine the parameters for calculating the correct spring rate for both the shock and the fork springs.

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Hi Duke.

 

Can I call you Duke?

 

The baseline I was given many years ago by an international superbike racer (with several national championships to his credit) was that the best spring for a baseline is one that takes zero or as little pre-load as necessary to achieve the desired static sag (0.75-1.0 in or whatever). In fact, he said that he set his bikes up with "negative" pre-load so the bike could come up off the spring under heavy braking and allow the rear wheel to stay in contact with the pavement better. I guess if the spring was short enough and you dialed the pre-load ring all the way out so there was a gap between the spring and adjuster ring when you lifted the rear of the bike, that would do it.

 

Anyway, I also hear about WSB or MotoGP riders changing shock springs from track to track, so I guess there can be some subjective feel about how one likes to tune a suspension, ie. if you prefer .75" or more sag, or perhaps a softer spring for certain conditions, then different springs would be needed under different conditions or circumstances.

 

My personal feeling is that typically, the less you have to pre-load a spring the better as that should theoretically give you the most potential range of motion and keep you furthest away from the binding point of the spring, ie. operating in the area that the spring was intended to operate in. That said, this doesn't take into account the subtleties of different rates that might be important depending what type of track you are riding and what the conditions are. Personally, I set it and forget it. I have enough to worry about tuning the carb(s) on a two stroke. I make suspension adjustments for rain or a bumpy track, or between a slower tight twisty track vs a fast sweeping track maybe, but, I don't change springs.

 

At the end of the day, unless you are Rossi or racing AMA nationals, you probably don't need to worry too much about it. Though, I do find it interesting that you got such a wide range of answers from people. However, remember that Traxxion Dynamics are the Penske people here, so, my gut instinct would probably be to lean toward what they tell you about their shock. If you were running Ohlins, I would listen to Ohlins.

 

Finally, the bottom line is that I don't think there is any single answer for everybody. If you want to know what is good for you, buy several springs and spend days testing each of them. However, if you aren't already running competitive lap times with what Max tells you, it probably isn't worth your time and money. I'd say concentrate on your riding. A good rider can ride around most anything well. And then you will have the necessary feel to know what you prefer. I know that probably isn't the answer you wanted to hear, but, like so many things, any choice you make is a compromise or a trade off somwhere. I'd start with what Traxxion Dynamics tells you. But consider what other experts have to say too.

 

Hope that helps.

 

racer

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Hi I'm new to this board but i've got to agree wholeheartedly with Racer for you to stick with the recommendations given by the people who distribute your shock.At the minute I'm guessing you're on a Honda and you're on the large side judging by the spring rates you mentioned but it is still a big variation.The answer to your question about the correct spring rate doesn't exist in as much as there isn't enough information in that statement to get a definative.There are calculations that can be used to get repeatable results but in all honesty they require a preformed plan regarding both static and rider sag figures which are all part of how the tuner sees his personal solution to make the best job.The fact that these initial ideas are formed based on what he knows about the geometry and weight distribution of the machine he is dealing with along his preffered(and fiercely guarded)damping curves makes it impossible to say one guy is more right than another.Personally I find the subject fascinating and the more you learn the more you feel like you are juggling an ever increasing amount of balls to increase one aspect without affecting others and would say if you are really interested in learning then do as racer says and throw a few springs at it and formulate some ideas of your own based on what you find.My own endevours have brought some brilliant results as well as some scary moments and more dissapointments than I care to think about but it's all good for me

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Thanks for the inputs guys. Trevitt devotes a pretty large amount of ink to figuring out the spring selection process.

 

To give you a little back ground (and being completely honest here), I have bought 3 Penske shocks from Traxxion for three different motorcycles I have owned. There was not a single one of them correct as they were shipped to me. The first exprience was a Penske shock I purchased from them for an SV650. It came with no half inch to 10 mm reduction collars and the manual told me how to set up the shock if I was running it on an Indy car at Pocono, Daytona, etc. It being my first Penske purchase and hearing so many rave reviews of them, I figured that was the way it was and to make do with it. Making a long story short, I wound up drilling out the shock bosses on my SV650 to accept a half inch bolt. It wasn't until later that I learned of the existence of the shock reduction bushings.

 

The second shock I purchased from them was for my VFR800. It was shipped to me as a shock for a VFR750. A half inch too long. Had to send it back to be shortened. 1400# spring.

 

My third Penske/Traxxion purchase was for my ST1300. The shock wouldn't even fit the bike let alone the spring was 300 # too soft. I went onto some ST1300 websites and learned that I had one of two choices: either grind some material from frame for the shock to fit or send it back and have them install a long top on it (apparently there is a standard top and a longer top for a Penske). By this point to say I'm burned out on Traxxion is an understatement. They are 3 for 3 with me on screw ups. That's after spending a couple of thousand dollars with them. Not exactly confidence inspiring to say the least. Therefore, I'm very leary of their recommendations.

 

Out of desperation, I called Penske themselves. They told me to ship the shock for the VFR800 and the ST1300 to them and they would straighten the entire situation out. Took them about a week and they called and told me my shocks were ready. I asked what the damage was ($$$). $17.95 for shipping. No charge for the work they did at all including parts. They were more concerned with a satisfied customer. So I can't say enough good about Shane Sweigert and Cole at Penske Shocks in PA. Those guys are the best and do take care of you.

 

I know I digress here, but that is why I don't trust Traxxion's recommendations but still think Penske shocks are the best there is and hence why I am trying to learn more of how to setup my bike than just having somebody tell me to run this spring because they said so.

 

So far I have come up with two parameters that seem to apply to the correct spring rate:

 

1. Select a spring that requires the lest amount of pre-load to achieve the desired amount of static and dynamic sag.

2. Select a spring that is at least twice as long as the amount of shock travel to prevent spring binding (this applies to the shock spring).

 

Luckily I have found a source for shock springs at very reasonable prices so I can do some experimenting: Hoerr Racing Products (www.hrpworld.com). Hoerr is a race car suspension dealer that sells Hyperco springs. A Hyperco spring is a Hyperco spring is a Hyperco spring. It doesn't matter if it's going on a motorcycle, an Indy car, a NASCAR, etc. Most motorcycle suspension shops sell Hyperco springs for $99.95. HRP sells them for $66.95. That's a third less.

 

Penske shocks take a 2.25" I.D. x 6" long spring. HRP are great people to work with. I have spoke with them many times and they tell me the motorcycle world is just beginning to learn of them.

 

The first point of my "journey" here is to get some good accurate measurements and go from there. Exactly what does my bike weigh front and rear, exactly how much rear end travel do I have, etc. Not relying on numbers from the shop manual or a sales brochure.

 

RDR

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Hey Red,

 

Sorry to hear about experience with Traxxion Dynamics. Assuming you clearly communicated your needs (and it sounds like you did and do communicate well), then that looks really bad for them. And, obviously, going three for three, you have given them the benefit of the doubt to be sure and paid a high price for it. Get me once shame on you, get me twice, etc.

 

I don't like to speak ill in public of a business or persons but unfortunately, yours is not the first story I have heard along these lines re: TD lately. I used to race with the guy (10-15 yrs ago) who started the company, no idea how involved he is anymore, but, I know he had the best intentions when he quit racing to go into business. In any case, Im glad to hear that Penske took care of you, though I don't blame you for not returning to TD.

 

As for your testing prgram, it sounds like you have a good handle on a good beginning. I have heard good things about Tony Foale as a suspension/chassis guru. He has books and gives seminars. My specialty is more motors than chassis, but, I will try to find some time to do some research to add as, obviously, I have a lot to learn and could beneift as well.

 

Good luck,

 

racer

 

PS - Technically, the shock will (should) define your range of travel, ie. you will hit the bump stops before binding the spring (unless PL is wound way down on a soft spring or something). I was using the spring binding to physically locate where I was re: my point about operating closer to the top of the spring.

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Thanks for the reply racer.

 

Taking the spring off and on a Penske couldn't be any easier. What I plan to do is jack the bike up, take a measurement, then remove the shock spring, and using either a screw type jack or bottle jack, begin compressing the swingarm until the shock body just contacts the rubber bump stop. The difference will be the amount of shock travel (or more accurately, rear wheel travel). While I'm under there I'll also place a small tie wrap on the shock shaft to see how much travel I'm using.

 

I'll do something similar with the fork. We all pretty well run a tie wrap on one of the fork legs to see how much travel we're using but how many of us have put a mark (probably a colored piece of electrical tape) to designate the point when the fork bottoms out? Forks don't travel until the bottom triple clamp hits the top of the slider on convential forks or the axle holder hits the fork tube on USD forks. They bottom out quite a few millimeters before then. How few of us mark the bottoming out point to see just how much of the actual amount of fork travel we're using?

 

My racing days are long past me. Retired from competition years ago. Our choice in shocks back then was either S&W or Koni's. But I still like to ride hard and enjoy wrenching on my bikes when I'm not riding them. I'm an engineer by trade so I guess tinkering and improving things (and learning exactly how they work) is in my blood.

 

Please don't interpret my previous post as a slam on TD. There are thousands of guys out there who swear by them. I'm in the small percentage who swear at them.

 

Penske is a great company to deal with. I don't think people realize you can deal directly with them. They have a whole staff under Shane Sweigert who handle nothing but motorcycles. When you deal directly with Penske, you don't have to wait on them to "order" a shock or a part. They just whip one out. Penske has also starting sending Shane to races to offer racer support.

 

Thanks again for your input.

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  • 4 weeks later...
Forks don't travel until the bottom triple clamp hits the top of the slider on convential forks ..... They bottom out quite a few millimeters before then. How few of us mark the bottoming out point to see just how much of the actual amount of fork travel we're using?

 

 

So how is the bottom point calculated? I have asked this and have only gotten a range. I am close to being convinced that it's time to disassemble my forks (a whole other story). If I have to do it in order to find the bottom then it's +1.

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Forks don't travel until the bottom triple clamp hits the top of the slider on convential forks ..... They bottom out quite a few millimeters before then. How few of us mark the bottoming out point to see just how much of the actual amount of fork travel we're using?

 

 

So how is the bottom point calculated? I have asked this and have only gotten a range. I am close to being convinced that it's time to disassemble my forks (a whole other story). If I have to do it in order to find the bottom then it's +1.

 

It isn't calculated. It's measured. It bottoms where it bottoms.

 

 

Here's a decent diagram of a typical type of standard fork tube from "my day": http://www.racetech.com/articles/EmulatorFitting.htm

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Duke,

 

Sorry to hear that you had issues with Traxxion. I will not defend them in your situation, but will say that they have only provided the best of service to me every time that I have used them. Max and the rest of the team are great guys.

 

Back to your question. I have always been curious as how spring rates are chosen also. Usually as you stated they only ask for the bike model, your weight, and the application (road, track, race, etc). I am sure they are using years of empirical data to determine the spring rates that would work best. Also how does one account for fuel level? The difference in the weight of the bike with a full tank and a 1/4 tank of fuel could be around 15lbs alone. So I am guessing that the appropriate spring with the proper damping valves would work over a fairly wide range, well a tleast 15lbs.

 

Shane

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  • 1 month later...

Thought I would give an update to search for the proper spring(s) for my scooter.

 

I purchased a copy of Andrew Trevitt's "Sportbike Suspension Tuning". He goes into a fair amount of detail about empirical methods for determing the correct spring rate(s). I have followed his advice to a "T" and couldn't be happier with the way my bike is handling now.

 

I'm to the point now where I feel like I'm very close to the most ideal spring(s) for my bike. Luckily, as I mentioned earlier, I have found a much more reasonable source of shock springs for Penske shocks than the motorcycle suspension speciality shops. I've built up a nice inventory of springs (both front and rear) and after you've done it 4 or 5 times I've gotten to where I change either the shock spring or the fork springs in about 15 minutes.

 

My next stop in my odyssey is to take a couple of days off from work, get me a place in the mountains where I like to ride (which luckily has a garage), and spend a couple of days testing the difference between a 1250 # spring and a 1300 # spring. Same with the forks - try 1.2's, then try 1.1's., etc.

 

Keith himself writes an excellent prologue in the book he authored with Andy Ibbott about spring rates. Between the Trevitt book and the Ibbott/Code book, it's really got me in the ball park 10 times faster than posting on the internet.

 

I'm an engineer by trade and I'm excited about finally coming up with an empirical method for determining correct spring rates. I want to go beyond just a recommendation from someone on the internet. I guess I'm one of the unique guys who enjoys wrenching on his bike just about as much as riding it.

 

Sometimes it pays to go back to school (so to speak) and hit the books. I think all too often in today's world we let other people do our thinking for us instead of learning and figuring things out for ourselves.

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Hear, hear.

 

Thanks for the update, Red. And the book.

 

Keep 'em coming.

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