How do I lower the front and rear end of a yz450f


19 replies to this topic
  • cr8ter

Posted April 21, 2013 - 05:54 PM

#1

I'm new to the dirt bike thing and I just recently picked up an 06 450. It's a lil tall for me, I have ridden my buddies but i was never comfortable because i can't reach the ground when i'm on it. I like to be close to flat footed on it so i need to lower it about 2 inches. I did some research and a lowering link is an option but it only drops the rear 1.25 inches. Is there another way to lower it more? Also I couldn't find anything on lowering the front, atleast without disassembling the forks. Is there an easier way to lower the fronts? Some help please, thanks.

  • ekulb14

Posted April 21, 2013 - 08:30 PM

#2

only thing you can do for free is shave your seat.
moving the forks up in the tress can get you a little bit lower (will mess with geometry)

if you have 200 bucks you can get a lowering link that will take it down (after you have set the sag maybe 2 2.5 in..)
if you have more cash go get your front springs shorted and suspension re done.

Edited by ekulb14, April 21, 2013 - 08:30 PM.


  • 2grimjim

Posted April 21, 2013 - 08:40 PM

#3

The forks can be disassembled and a spacer placed on the damper rod between the inner cartridge and top out spring. This will require a shortened main spring. The spring will need to be shortened the same amount as the spacer. Race Tech sells the spacers (1 or 2 inch) and they will shorten the fork springs for $50.

The Shock can be shortened the same way. The shock will need to be disassembled and the shock shaft taken apart. A spacer is placed between the shock piston and seal head. The thickness of the spacer needs to be determined before taking the shock apart. This can by removing the shock, taking the spring off, and reinstalling the shock without the spring. Support the weight of the bike with the rear suspension fully extended and have someone take a tape measure to a mark on the rear fender directly above the axle. Let the bike down to the height you want to lower it to (equal to the amount you will lower the front end), and use a fine tipped magic marker to mark the shock shaft even with the seal head. Remove the shock and measure the distance from the seal head to the mark on the shaft and this will be the thickness of the spacer that needs to be placed inside. Race Tech sells these too. You don't need to change the rear spring. There is plenty of adjustment for the spring.

You can lower your bike for pretty cheap this way, under $100, but you will have to do some work. It's not hard, just a little time involved.

Edited by 2grimjim, April 21, 2013 - 08:42 PM.


  • cr8ter

Posted April 21, 2013 - 08:56 PM

#4

Thanks for the replies guys. I will look into all of that and try to familiarize myself with working on the suspension. I know the engine very well but never messed with the suspension components.

  • 2grimjim

Posted April 21, 2013 - 09:16 PM

#5

Thanks for the replies guys. I will look into all of that and try to familiarize myself with working on the suspension. I know the engine very well but never messed with the suspension components.


If you can tear an engine apart and put it back together, you can work on your suspension. Just takes a little determination and commitment. Any questions you have can be answered here.

  • grayracer513

Posted April 22, 2013 - 06:42 AM

#6

Note that there is no top out spring in an SSS fork as used on the YZF.

Also, shortening a spring will increase its actual spring rate significantly, so if you don't want that, you need to start with a lighter set and shorten them instead. A much better method is moving the spring seats farther up the cartridge, or using alternate seats.

  • 2grimjim

Posted April 22, 2013 - 12:07 PM

#7

Also, shortening a spring will increase its actual spring rate significantly, so if you don't want that, you need to start with a lighter set and shorten them instead. A much better method is moving the spring seats farther up the cartridge, or using alternate seats.


Shortening the spring doesn't affect the rate but shortening the suspension will usually require a higher spring rate. most places that shorten springs simply cut of the end, anneal and flatten the end and retemper the last 1-2". Heating is done with an induction oven (similar to what's used for shrink-fit cnc tool holders and wrist pin heating fixtures for press-fit wrist pins).

Sending the springs out is quick and cheap and doesn't require any (semi) permanent mods to the fork that couldn't be un-done during the course of a normal service.

  • grayracer513

Posted April 22, 2013 - 01:50 PM

#8

Shortening the spring doesn't affect the rate


It absolutely does. Specifically, it raises the rate. It does this by reducing the number of active coils. The formula for rate in a coil spring is:

Dw^4*80400/(8*Dc^3*N)
Where:
Dw= wire diameter
Dc= the center to center diameter of the coils
N= number of active coils

Measurements made in millimeters
Gives the answer in N/mm
Divide by 9.81 to get the answer in kg/mm

Try it yourself. Remember that a coil spring is nothing more than a torsion bar that is more conveniently packaged. The coil wire is twisted under a load, not bent, and shortening a torsion bar, it should be obvious, will stiffen it.

  • 2grimjim

Posted April 22, 2013 - 07:57 PM

#9

It absolutely does. Specifically, it raises the rate. It does this by reducing the number of active coils. The formula for rate in a coil spring is:

Dw^4*80400/(8*Dc^3*N)
Where:
Dw= wire diameter
Dc= the center to center diameter of the coils
N= number of active coils

Measurements made in millimeters
Gives the answer in N/mm
Divide by 9.81 to get the answer in kg/mm

Try it yourself. Remember that a coil spring is nothing more than a torsion bar that is more conveniently packaged. The coil wire is twisted under a load, not bent, and shortening a torsion bar, it should be obvious, will stiffen it.


For the the amount that the spring is shortened it isn't going to stiffen it up enough. The reduced amount of available travel is usually going to require a rate that is somewhat higher than the formula shows. I had a long conversation with one of the Race Tech employes the first time I did this. I did this on my wifes '07 WR250. She weighs 130 lbs and the stock front spring was a .44kg/mm. her weight required something like a .38 with the stock fork length, but with the forks shortened 2 inches, I was advised to use a .42kg/mm (before cutting). The effective spring rate works out to something close to the stock .44 but the available travel has been reduced to 10 inches. It's worked great for her. She hasn't had any complaints about bottoming or harshness. For most riders that are contemplating shortening the suspension, unless the riders weight is way out of range for the stock fork spring, just shortening the stock spring works fine. Chances are you aren't going to have someone that is a AA rider wanting to drop their bike 2 inches.

  • cr8ter

Posted April 22, 2013 - 08:04 PM

#10

Now i wounder if i need to redo the suspension because the guy i bought it from has it setup for a 220 lb rider and i am 185

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  • 2grimjim

Posted April 22, 2013 - 08:35 PM

#11

Now i wounder if i need to redo the suspension because the guy i bought it from has it setup for a 220 lb rider and i am 185


If the previous owner re-sprung the bike for him, that would go a long way in explaining the stiff ride. The stock springs work pretty good for a 185 lb rider but springs for a 220 lb rider are going to knock your teeth loose. Do you know what type of riding the previous rider used the bike for? If he used it for woods or off-road riding, all is not lost. The valving may still work if it was changed and you may be able to get away with only swapping the springs.

  • cr8ter

Posted April 22, 2013 - 08:53 PM

#12

If the previous owner re-sprung the bike for him, that would go a long way in explaining the stiff ride. The stock springs work pretty good for a 185 lb rider but springs for a 220 lb rider are going to knock your teeth loose. Do you know what type of riding the previous rider used the bike for? If he used it for woods or off-road riding, all is not lost. The valving may still work if it was changed and you may be able to get away with only swapping the springs.


He rode it on the tracks. He is in his late 50s and is the president of a racing club for older people

  • grayracer513

Posted April 23, 2013 - 07:18 AM

#13

For the the amount that the spring is shortened it isn't going to stiffen it up enough.


A spring I have on the shelf for an '04 YZ250F has a 5.2mm wire diameter, a center-to-center coil diameter of 41mm, and 25.5 active coils. It's rated at .42 kg/mm, and calculates to .4262. for the sake of this example, we'll call it a .43. Shortening this spring 2" will remove 3.25 active coils. Recalculating with this change yields a new rate of .4830 kg/mm. That's close to a 12% jump in rate. I don't see that as insignificant, somehow.

You started this by stating that "Shortening the spring doesn't affect the rate". Now that it's been demonstrated that it does, you're attempting to equivocate by saying that "it isn't going to stiffen it up enough". Stuff like that does not help one's credibility.

  • YamaLink

Posted April 23, 2013 - 07:49 AM

#14

Alluding to what Grayracer stated, often times a revalve is required for spring shortening because the bike often rides a bit more harsh.

  • FinchFan194

Posted April 23, 2013 - 09:33 AM

#15

A spring I have on the shelf for an '04 YZ250F has a 5.2mm wire diameter, a center-to-center coil diameter of 41mm, and 25.5 active coils. It's rated at .42 kg/mm, and calculates to .4262. for the sake of this example, we'll call it a .43. Shortening this spring 2" will remove 3.25 active coils. Recalculating with this change yields a new rate of .4830 kg/mm. That's close to a 12% jump in rate. I don't see that as insignificant, somehow.

You started this by stating that "Shortening the spring doesn't affect the rate". Now that it's been demonstrated that it does, you're attempting to equivocate by saying that "it isn't going to stiffen it up enough". Stuff like that does not help one's credibility.

Mind=blown.

  • 2grimjim

Posted April 23, 2013 - 09:52 AM

#16

A spring I have on the shelf for an '04 YZ250F has a 5.2mm wire diameter, a center-to-center coil diameter of 41mm, and 25.5 active coils. It's rated at .42 kg/mm, and calculates to .4262. for the sake of this example, we'll call it a .43. Shortening this spring 2" will remove 3.25 active coils. Recalculating with this change yields a new rate of .4830 kg/mm. That's close to a 12% jump in rate. I don't see that as insignificant, somehow.

You started this by stating that "Shortening the spring doesn't affect the rate". Now that it's been demonstrated that it does, you're attempting to equivocate by saying that "it isn't going to stiffen it up enough". Stuff like that does not help one's credibility.


So...if we are going to pick each other apart, here we go....
http://i1118.photobu...opOutSpring.jpg

This is a top out spring in the bottom of an SSS fork I just disassembled. You stated that "SSS forks don't have top out springs". Im sorry, but they in fact, do have a top out spring. And so do the AOSS forks. But you would never know this if you haven't actually torn one down entirely. You have to emove the seal retainer at the bottom of the cartridge to get at it, but as I said, you would most likely never know this if you haven't actually been inside a set of these forks.

  • 2grimjim

Posted April 23, 2013 - 10:19 AM

#17

A spring I have on the shelf for an '04 YZ250F has a 5.2mm wire diameter, a center-to-center coil diameter of 41mm, and 25.5 active coils. It's rated at .42 kg/mm, and calculates to .4262. for the sake of this example, we'll call it a .43. Shortening this spring 2" will remove 3.25 active coils. Recalculating with this change yields a new rate of .4830 kg/mm. That's close to a 12% jump in rate. I don't see that as insignificant, somehow.

You started this by stating that "Shortening the spring doesn't affect the rate". Now that it's been demonstrated that it does, you're attempting to equivocate by saying that "it isn't going to stiffen it up enough". Stuff like that does not help one's credibility.


The rates I used were recomendations from Race Tech. I'm not going to argue with them. I would think that Paul Thede knows what he's doing.

I'm not here to boast of my credibility, I'm just offering the op one possible solution to his problem. I demontrate my credibility every day to the customer machines I work on at my shop. And guess what? I have a lot of happy customers. There machines come to me to get fixed, and when they leave, that's what they get. And they don't have to bring there machines back for the same repair twice (or three, or four times). I also on occation prove to the OEM's that their machines have product faults that their engineering departments cant explain.

I spent many years in manufacturing and a lot of time working with engineers. A bunch of them didn't have the common sense that god gave a lump of dried poop. They had all kinds of reasons why something would or would not work but in reality they were unable put their knowledge learned from their engineering degree to practical use. I don't claim to be an engineer and if I did, I certainly wouldn't be making posts hear to gain "credibility".

  • grayracer513

Posted April 23, 2013 - 10:35 AM

#18

Whether the rate needs to be modified to adjust for lowering the bike depends on a number of variables too large to accommodate any general rule. It is true, however, that the rate will need to be raised "some" in most cases. Here's why. Under a condition when the fork would be expected to bottom, the spring will need to be compressed to a point at which it applies some specific amount of force in resistance in order to support the suspension. A suspension unit that has had its travel shortened 2" will require a spring that will produce this force with 2" less compression than before the shortening, and that means that the spring rate, which is the amount of resistance the spring creates for each increment of compression, will need to increase if it is to have the same preload and the same force at the bottom. This could be calculated simply by taking the original travel and dividing it by the new travel, then using that to multiply the original spring rate. That would give you that same force at bottom, but that mat still not be the rate you actually end up wanting.

I stand corrected regarding the topping buffer springs. Since they are held captive in the seal head until it's disassembled (unlike the open bath fork), they aren't an everyday item unless you replace the cartridge seals on every job, and I don't. That's a really bad picture of one, BTW.

Interesting that while you place great significance on the need for a higher spring rate when lowering the front that you make no such recommendation for the rear, instead saying that there is "plenty of adjustment" available. But preload adjustment doesn't change the rate.

This requirement may change the way the fork valving has to be handled, or not, depending on the use the fork is to be put to, and the question of spring rate needs to be evaluated case by case accurately. Note that the preload will also need changing, since it is usually set up as a fraction of the total travel.

None of this, of course changes what I said in the first place, which was that shortening the springs increases their rate.

  • 2grimjim

Posted April 23, 2013 - 12:20 PM

#19

My point was that most folks that are going to consider lowering the machine aren't going to notice if the spring rate is off unless by some significant ammount. That is unless there is a gross mismatch between the riders weight and springs to begin with. UsusllyI'll discuss this with whoever I'm doing the work for. Only once did I recommend replacing the springs with shortened springs that were a different rate and that was for a rider that weighed 300 pounds. I understand the mechanics of springs. I didn't see the need to complicate the post with a physics lesson for such a simple request. Like I said before, I've been following the advice of a company that manufactures springs and that's what I have gone with. I've since lowered a handfull of bikes this way and to manage costs for the customer, I have just sent whatever springs were in the bike to begin with out to be shortened. Not exactly the 'engineers aproved method', but no one has complained about it.

As far as the pic of the top-out spring, I have another set of these forks completely stripped down. When I get to them, I can post detailed photos if you like. Oh, btw gray, apology accepted.

  • grayracer513

Posted April 24, 2013 - 07:03 AM

#20

I know what the spring looks like and where it is, but no one is going to encounter one during, as you say, a typical fork service.

IMO, as to lowering the fork, it is best done by cutting the lock ring retainer band off of the stop ring for the spring seat, removing the ring, and cutting a new groove for it farther up the cartridge tube. The machinist who does this can then make a new ring keeper ( a solid ring with a step on the ID to be slid over the wire ring from the bottom) for it. That work should be maybe $50 in most small shops and requires nothing more exotic than a lathe. Once done, it will allow the use of off-the-shelf springs to suit the riders needs, and the job can be reversed simply by putting the ring back in its original spot and removing the upstroke limiter sleeve from the damper rod. Simple for the same money as a cut spring, and you don't have to throw the spring away when you raise it back up to resell or whatever.

To an earlier comment regarding damping rates, they usually have to be increased. The fork has to resist compression a little more quickly when shortened simply because there is less travel to work with, and if it didn't, it would bottom sooner than it should and tend to run too deep in the stroke.

There are two kinds of people who want a bike lower; short recreational riders, who, as Jim says, won't always be riding fast or hard enough to notice less than optimal suspension, and those who are serious flat trackers, or SM or road racers, who often can and will.





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