Jump to content

How does RaceTech calculate fork spring for YZ450FX ?

Recommended Posts

I don't get how RaceTech calculates the fork spring rates for the YZ450FX or any bike for that matter.

The stock YZ450FX fork spring is 0.46 kg/mm.  The stock YZ250FX fork spring is 0.45 kg/mm.

I find the stock YZ450FX fork springs way too soft.   Touch the front brake and fork dives about 4 inches with the stock clicker settings.  Steep downhills are scary.  Test reviews say similar things.  Should probably have a 0.48 for a stock spring with a std rider ?

I weigh 220 pounds in street clothes, maybe a bit less.  I like to run the fork clickers pretty far open for lots of compliance.

If I use 230 pounds ( I carry a heavy pack), 30-44 years old, Expert, the Racetech calculator tells me I should use a spring rate of 0.486 Kg/mm with 4mm spacers.

I'm running a 0.48 springs in my YZ250FX and I still find them soft and the 250FX weighs 11 pounds less than the 450FX.  I ran 0.48s in my 2012 WR450F lightened 25 pounds (weighed about 5 pounds less than the YZ450FX does) and they were too soft.

I'm tired of riding bikes with soft forks.  I think I should run 0.50 in the YZ250FX and 0.52 in the YZ450FX.  What say ye ?

Funny story... my WR450F forks with stock springs (0.46s) would bottom right out on big hits.  Doink.  Doink.  Funny that the YZ450FX (262 pounds) and the 2012 WR450F (273 pounds) come stock with the same fork springs when they supposedly have dramatically different capabilities.    I suspect the soft fork springs contribute to the high speed stability problems that some YZ450FX riders mention.  

 

  • Like 1

Share this post


Link to post
Share on other sites

Fork dive has 100% almost nothing  to do with spring rate....and NOTHING to do with bike weight.  It is the valving that controls the bikes motion. The springs just keep the bike and rider in the correct part of the stroke and steering geometry.

I never even rode my bike with the stock springs and valving. It went out to Enzo before I even started the bike.  It's going to be wrong from the factory, so why struggle ?

The stock valving is typically way off, even for the 'target' rider. Yamaha has a budget, and they are given X amount of forks to try by Kayaba, and they pick the one they hate the least. They are typically not involved in the actual valving design or implementation, just like all other brands (accept BETA, TM).  Acceptions abound.

The only information that matters in figuring out spring rate are SAG measurements, taken standing up with full riding gear on, and again sitting down in the sweet spot.

You average and skew the numbers  depending on the kind of riding you do. (Fast standing vs. Slow sitting)

You get the rear sag/spring rate correct, then install the comenserate front spring to match, and then have the valving re-done to match the springs, and your riding skill/speed.

If you are truly honest about your weight, your riding skills and your speed with the suspension tuner,  you can get the best suspenion you have ever experienced, typically the first time out.

 

If the springs are too light, you will put the fork valving in the bottom half of the travel, making it wallowy and harsh (too much sag)

If the springs are too heavy, you will put the fork valving in the top half of the travel, making it unresponsive and with reduced travel, as well has making the entire bike drop, instead of just the wheel, when you ride  through a dip (not enough sag)

Add to this the problems of geometry when the bike is no longer in control of itself, and you have a really un-fun bike to ride.

 

 

  • Like 2

Share this post


Link to post
Share on other sites
11 minutes ago, Krannie McKranface said:

Fork dive has 100% almost nothing  to do with spring rate....and NOTHING to do with bike weight.  It is the valving that controls the bikes motion. The springs just keep the bike and rider in the correct part of the stroke and steering geometry.

Fair enough on the valving controlling how fast the fork dives when you apply the brake, but if you hold the brake for more than a second or two the fork is going to reach a static point no matter what valving you have because of the bleeder holes.

On the other hand a certain braking G force transfers a certain weight to the front tire which is applied via the forks.  And the softer the fork springs are, the more fork travel is needed for a given force. 

The same thing happens on downhills.   For a given grade a certain amount of weight transfers to the front wheel.  The softer the fork springs are the more fork travel is needed for a given weight transfer.  

What are you running for springs, Krannie ?

I know that sag is supposed to determine spring rate.  It is commonly done on the rear shock but almost nobody talks about it for the fork springs.   Thanks for reminding me, I'll investigate.

What are you running for springs, Krannie ?

Share this post


Link to post
Share on other sites
1 hour ago, MidlifeCrisisGuy said:

Fair enough on the valving controlling how fast the fork dives when you apply the brake, but if you hold the brake for more than a second or two the fork is going to reach a static point no matter what valving you have because of the bleeder holes.

On the other hand a certain braking G force transfers a certain weight to the front tire which is applied via the forks.  And the softer the fork springs are, the more fork travel is needed for a given force. 

The same thing happens on downhills.   For a given grade a certain amount of weight transfers to the front wheel.   The softer the fork springs are the more fork travel is needed for a given weight transfer.  

What are you running for springs, Krannie ?

I know that sag is supposed to determine spring rate.  It is commonly done on the rear shock but almost nobody talks about it for the fork springs.   Thanks for reminding me, I'll investigate.

What are you running for springs, Krannie ?

Bleeder holes?

:smashpc:

Share this post


Link to post
Share on other sites

Spring rate has everything to do with brake dive and bike weight.  You can't resist brake dive with JUST compression damping because it's still a G-load.  If you're braking down a hill, that's a quasi-static load on the front end.

If your spring allows 33% sag, then your system will take 3 G's to bottom out.  20% sag will give you a 5 G bottom out.  This is all neglecting the effects of oil height or reservoir charge on the rear shock.

I don't know what the hell Yamaha was thinking putting springs that light in a 450 with a bunch of gear on it made for big dudes.  Stupid move if you ask me, especially since they kept the same rear springrate as the motocrosser.  The only thing I see that could justify that is them running the 50T sprocket in the rear which shortens the swingarm and takes weight off the front end.

 

At 220, you need .50's or more.  I'm 190 and I'll be running .49's or .50's.

  • Like 2

Share this post


Link to post
Share on other sites

Hmmm I have a few sets of forks.  I have a set with .51s valved by Factory connection for MX.  They are really plush initially then stiffen up.  I also have a set with .48s valved for supermoto.  I have Ridden supermoto with the Factory Connection with the heavier springs and fork dive is way way worse than the set with lighter springs that are valved different.  Just an observation.  I will say I am about your weight (230) B level rider in the desert and I tend to like a softer fork and usually run .50s in my YZ which I think is a bit lighter than the FX.

  • Like 1

Share this post


Link to post
Share on other sites

What's the preload on the set with the .48? Supermoto setups typically have more coil preload.

Share this post


Link to post
Share on other sites

 

12 hours ago, Krannie McKranface said:

Bleeder holes?

:smashpc:

Krannie: If you look at the diagram below, you'll see that the low speed compression adjuster is just a orifice that is partially closed by the adjuster.  It is always open.  Oil can flow through that passage at any time and does. 

When you apply the brakes this passage allows the oil to flow and reduce or eliminate any damping force that the fork may be adding.  In a second or two the fork is adding no force whatsoever to keeping the bike up.

Even worse, the rebound adjuster also allows oil flow.

 

3-32.jpg

33 minutes ago, GHILL28 said:

What's the preload on the set with the .48? Supermoto setups typically have more coil preload.

I'll measure it today.

Edited by MidlifeCrisisGuy

Share this post


Link to post
Share on other sites
28 minutes ago, MidlifeCrisisGuy said:

 

Krannie: If you look at the diagram below, you'll see that the low speed compression adjuster is just a orifice that is partially closed by the adjuster.  It is always open.  Oil can flow through that passage at any time and does. 

What you apply the brakes this passage allows the oil to flow and reduce or eliminate any damping force that the fork may be adding.  In a second or two the fork is adding no force whatsoever to keeping the bike up.

Even worse, the rebound adjuster also allows oil flow.

 

3-32.jpg

I'll measure it today.

Your assessment is incorrect. There is always damping, at all times, unless there is no oil.

Share this post


Link to post
Share on other sites
2 minutes ago, Krannie McKranface said:

Your assessment is incorrect. There is always damping, at all times, unless there is no oil.

There is only damping force when the fork is in motion.  When it stops moving the damping force is zero.   The damping force is caused by oil moving to the other side of a piston.   When the piston stops moving the oil isn't trying to move to the other side and the force is zero.

Edited by MidlifeCrisisGuy

Share this post


Link to post
Share on other sites
15 minutes ago, MidlifeCrisisGuy said:

There is only damping force when the fork is in motion.  When it stops moving the damping force is zero.   The damping force is caused by oil moving to the other side of a piston.   When the piston stops moving the oil isn't trying to move to the other side and the force is zero.

Whatever.

  You just want to be argumentative and disscussive.

I choose not to be.

Share this post


Link to post
Share on other sites

The shim stack makes a hugh difference in ride and what the bike does. The 07-11 WR450 didn't come with a real shim stack. They come with a spring instead of the shim stack. When the spring sacks out there is pretty much nothing that fork springs will do to keep the bike up. It sacks through the travel to the harsh mid valve instantly. 

When a real shim stack is installed it is a new bike. So I agree with Krannie. Also I wonder if Racetech's spring calculator is calculated using their shim stack. Which means a stiffer Racetech shim stack than stock may take a lighter spring. 

Share this post


Link to post
Share on other sites

Guys, have you ever rebuilt a shock ?   What does the shock shaft do before you pressurize the reservoir ?  Does it push out at you ?  It just sits there.   It generates no force.

Share this post


Link to post
Share on other sites
What's the preload on the set with the .48? Supermoto setups typically have more coil preload.


Same preload.

Share this post


Link to post
Share on other sites

Midlife,

I won't pretend to know much about how my suspension works, but I'll share my experience for what it's worth. I've got the '17 FX and while not new to Yamaha, this is my first backward cylinder bike. I weigh just under 200 pounds, and 220 in gear. I'm a 50 year old novice, maybe an intermediate. My intended purpose for the bike was about 60% moto and 40% desert. I never rode it with the stock front springs. My first spring advice said to up the fork springs to .51 and leave the rear stock. Most recommendations I saw said to set the sag at 103. My first rides took place in the desert and I struggled to turn the thing, it kept wanting to stand up in the middle of corners, and the front end felt like it was deflecting off of every rock. Skipping the steps along the way, I finally settled on the .51 springs in the front, I went with a 6.1 rear spring, and went the other way with sag...settling with 100 race and 32 static. I tried to find a balance between turning and straight line stability with fork height and went with 3mm up and tightened up the steering stem bolt a little too tight. I didn't like the stock tires so I've got the Dunlop MX3S on front (which seems to be universally appreciated) and I'm trying the Michelin Starcross Medium on the rear. My current settings are 17 compression 16 rebound front, 14 compression/17 rebound and 1.5 HS on the rear. I'd describe it as a little stiff initially, especially on a single track type of trail, but high speed stuff and big hits it gives me a lot of confidence and I seem to hit these things with little fear. On the moto track I'm happy with it as well. I'm running stock gearing and the MXA map designed for the YZ that's mostly zeros on fuel and +2/+3 on ignition.

I'm sure a revalve would be great, but in the mean time this will do for me. I try to convince myself that the quirky things about the bike are an opportunity to ride better; if it's too stiff then ride faster. If third gear is too tall for that corner on the track then take a wider, faster line or just hit the corner faster. If it's too snappy on the throttle then work on your throttle control. I'm still learning...

  • Like 1

Share this post


Link to post
Share on other sites

Are you running the stock size tank and do you wear water/tools? Those rates could be just a hair on the stiff side if you're not. I ran those at that same weight with the bigger tank and pack on my 14 450F and it was almost too stiff but still worked well.

Share this post


Link to post
Share on other sites
1 hour ago, MidlifeCrisisGuy said:

There is only damping force when the fork is in motion.  When it stops moving the damping force is zero.   The damping force is caused by oil moving to the other side of a piston.   When the piston stops moving the oil isn't trying to move to the other side and the force is zero.

 

In other news, water is wet and if your aunt had balls she'd be your uncle.

Springs determine sag.  In a twin chamber fork you have the main spring and the ICS assembly holding the fork up.  Between these two you have X amount of sag with you on the bike.  A general purpose off road fork might be set up with almost 3" of sag.  A hardcore MX fork would maybe be half that.  As soon as you hit the brakes the springs will allow it to dive several inches regardless of rate.  A stiffer rate will dive a little less but it'll still dive.

As you are keenly aware, valving is speed sensitive.  Bleed orifices have the greatest effect at very low speeds.  You can overwhelm the bleed circuit by standing on the pegs and jumping up and down.  As soon as the bleed is overwhelmed the cartridge will start to build pressure and generate damping.

You might try running the compression clicker in to see if that has the desired effect.  If you run the clickers too far open for compliance you will be allowing too much bleed during low speed compression resulting in dive for little benefit in terms of compliance which has more to do with base and mid valving than bleed. 

  • Like 2

Share this post


Link to post
Share on other sites
2 hours ago, stevethe said:

The shim stack makes a hugh difference in ride and what the bike does. The 07-11 WR450 didn't come with a real shim stack. They come with a spring instead of the shim stack. When the spring sacks out there is pretty much nothing that fork springs will do to keep the bike up. It sacks through the travel to the harsh mid valve instantly. 

When a real shim stack is installed it is a new bike. So I agree with Krannie. Also I wonder if Racetech's spring calculator is calculated using their shim stack. Which means a stiffer Racetech shim stack than stock may take a lighter spring. 

What?  Are you trying to say it has a check plate instead of a shimstack?

20 minutes ago, turbo dan said:

As soon as you hit the brakes the springs will allow it to dive several inches regardless of rate.  A stiffer rate will dive a little less but it'll still dive.

No, in fact that is directly related to rate.  If you throw 100N of force into the front end of a bike with a 5.0 N/mm spring versus one with a 4.5 N/mm, guess which one is going to ride lower.  One might take a little longer to get there depending on the damping, but one with the 45 N/mm spring will settle out at a lower ride height.

 

 

I just looked at the RaceTech recommendations for the 450FX.  For the same inputs, it's giving fork springs that are way softer than the equivalent MX model but keeping the same rear spring, even for MX applications.  I'd ignore those recommendations based on my experience already with this chassis.

  • Like 2

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:


×