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One of the most common fork problems I see is bound up forks due to improper installation of the front wheel. It’s very easy to avoid this problem by taking these easy steps...

1.) Install both forks and set
fork height using calipers or a ruler. It is important that fork height be exactly the same for both sides. Tighten pinch bolts to the proper specs (Top: 18 Ft-Lbs | Lower: 15 Ft-Lbs- check with your owners manual).

2.) Install the axle and tighten the axle nut to proper spec. If the axle turns while tightening, tighten one right side axle pinch bolt to hold axle in place.

2a.) Never use a metal hammer to install the axle. It can mushroom the end of the axle making it almost impossible to adjust. If that happens just take the axle out and use a bench grinder to remove the lip. May also need to take a round file to the hole of the axle holder.

3.) Tighten both left side axle pinch bolts to spec (17 Ft-Lbs).

4.) With the left side axle pinch bolts and axle nut tightened, loosen the right side axle pinch bolts. Take a small flathead screwdriver and carefully tap it into the slot between the two right side axle pinch bolts. This will enlarge the axle hole. You will then be able to grab the bottom of the right side
fork tube and push it in and pull it out freely. The fork will settle naturally into position on the axle without binding
.

5.) Remove the screwdriver and tighten the right side axle pinch bolts to spec (17 Ft-Lbs).



If you're wondering if your forks are currently bound up, put your bike on a stand and start this procedure at Step 4.

Bound forks are a leading cause of premature bushing wear, air build-up, and fork harshness. All of which can be avoided by following these easy steps.  




The following was posted by Terry Hay of Shock Treatment Australia on DRN back in 2003. Its a great piece and I'm sure Terry would not mind it being posted again here.


Fork seals - Why do they leak?
 

The fork seal has become the scourge of the modern motocrosser. Leaking at the most in-opertune time and creating the highest level of incovenience. But what causes them to leak. I have been reading many of the posts on this forum and it would appear that fork seals create the greatest amount of havoc for the greatest number of riders. Fork seals, contrary to popular opinion, rarely blow. Mostly they leak because they have a foriegn object lodged between the seal lip and the fork leg, holding the seal open and allowing oil to leak out. Usually this is dirt or mud, sometimes grass and so on. As modern forks grow in diameter to increase rigidity we also see an increase in seal drag due to the increased suface area. This additional drag creates unwanted harshness due to additional stiction or - static friction. In order to reduce the unwanted stiction the modern seals have less tension against the actual fork leg. Now when your wheel hits a bump we see the impact work in two directions. One tries to compress the fork and absorb the hit, the other tries to break the fork in half and push the front wheel back into the motor. This secondary force causes the forks to flex, usually at the lower triple clamp area. As the forks flex the seals are stressed and any dirt on the fork tube may get past the primary seal lip, lodging uderneath the seal and creating a path for oil to escape. Another from of stress to the fork tube is caused by the brake rotor. As you apply the brake there is a rotational toque applied to the wheel, opposing it's natural momentum. This force also creates stress to the fork leg which can affect the seals and create leaks. Hence the reason why most leaks occur on the brake side first. Obviously the dirt or foriegn matter needs to be removed from the seal to ensure correct performance. This is best done by removing the seal. Whilst the seal itself may not actually be damaged, I would still recommend replacing it with a new one. Only use genuine seals as aftermarket ones seem to leak shortly after installation. To the idiot that first come up with the suggestion that you dislodge the dirt by shoving something between the seal and the fork leg, I would like to see your balls in a vice and I will happily do the cranking. Ninety nine percent of the time you will simply shove that dirt further into the fork where it can do some real damage. Someone is making a tidy profit out of seal-savers. You can actually produce a more effective seal saver for almost nothing. Cut a small strip of foam appr. 10mm x 10mm x 160mm long. Apply a small amount of fork oil and massage in, similar to an air filter. Remove the dust seal and place the foam in the cavity between the dust seal and oil seal. Replace the dust seal and ride. Remove and clean the foam every time you do your air filter. Re-oil and replace. If any dirt gets past the dust seal it will be trapped in the foam before it gets to the oil seal. The oil on the foam lubricates the leg and further reduces stiction. Sometimes fork seals can leak because the seal lip burns due to excessive friction. If bikes are left for a long time or are washed with aggressive detergents the chrome tubes can dry out. When you next ride the seals are subjected to increased friction and a hardening of the seal lip occurs. This hardening reduces the seals flexibility and further promotes leaks. If you haven't ridden for a while or often use harsh detergents, apply a little lube to the fork leg to lubricate the seals before you ride. Good Luck!
Regards
Terry Hay

 

Jm Racing Illinois makes u fast lolBack to the Basics Article


Problem/Solution PDF.


Fork Installation PDF




Proper Setup for the 2013 KX450F KYB PSF Fork - More Motocross Videos



Race Sag

 

Setting the proper race sag is the most important adjustment you can make. Without the sag properly set a proper balance cannot be obtained. Race sag refers to the amount of rear wheel travel used by your bike at rest with rider on board. As a general rule of thumb, the race sag should be about one-third of the maximum travel. Ride height is changed by adjusting the rear suspension spring pre-load.

 

Spring Preload & Race Sag Adjustment

The following adjustment procedure establishes the correct starting point for any suspension tuning - the proper rear spring preload adjustment for your specific needs. Your bike should be at normal racing weight, including fuel and engine oil. You should wear all your normal protective gear. To calculate the proper adjustment, it's necessary to measure between two fixed points - from the fender down to the axle, for three different situations:


1.     Unloaded (without rider): Bike on a stand with rear suspension fully extended


2.     Loaded - with rider: Bike on the ground


3.     Loaded - without rider: Bike on the ground

     

Calculating the Race Sag

Support your bike on a stand with the rear wheel off the ground. Measure the distance from a fixed point on the rear fender to the center of the axle. Write this measurement down. Remove the stand, with a couple of helpers’ available, sit in a neutral position (center of hip over the foot pegs) on the seat. Ask one helper to steady your bike perfectly upright so you can put both feet on the pegs. Bounce your weight on the seat a couple times to help the suspension overcome any sticking action and settle to a good reference point. Ask the other helper to measure the "loaded - with rider" distance.

 

To calculate the race sag dimension, subtract the "loaded - with rider" dimension from the "unloaded" dimension.

 

         Unloaded - 671mm (26.4")

         Loaded w/rider - 568 mm (22.4")

         Race Sag = 103 mm (4.0")

 

Adjust spring preload as necessary to obtain the desired handling results:


·         Decreasing the race sag dimension (i.e. 98mm, 3.9") improves turning ability for tight terrain at the cost of slightly reduced straight line stability.


·         Increasing the race sag dimension (i.e. 108mm, 4.3") may improve stability on faster terrain with less turns, but will reduce turning performance slightly


·         The ideal race sag (ride height) will vary with bike model and rider preference (100mm-105mm is a good starting point). Individual preference may produce race sag from 95mm-115mm. It is important to know your ideal race sag measurement before changing spring preload. Different abilities, riding styles, and measuring techniques will vary the ideal race sag among individual riders.

 

         Unloaded - 671mm (26.4")

         Loaded w/o rider - 631mm (24.8")

         Free Sag = 40mm (1.5")


Free Sag

Free sag indicates the distance the rear suspension should sag from the weight of the sprung portion of your bike. To calculate the free sag, subtract the "loaded - without rider" dimension from the "unloaded" dimension. Do this with your bike set at the standard race sag.

 

With the spring preload set to obtain the proper race sag, the rear suspension should sag 25mm to 45 mm. If the rear of your bike sags less than 25mm from its own weight, the spring is too soft for your weight. Too much pre load is needed to obtain the proper race sag adjustment.

If the rear of your bike sags more than 45mm from its own weight, the spring is too stiff for your weight.

 

Spring Rate

If you are lighter or heavier than the average rider and cannot set the proper ride height without altering the correct spring preload, consider an aftermarket spring.

 

A spring that is too soft for your weight forces you to add excessive spring preload to get the right race sag and, as a result, the rear end is raised. This can cause the rear wheel to unload too much in the air and top out as travel rebounds. The rear end may top out from light braking, or kick sideways over lips and square-edged terrain.

 

Because of the great absorption quality of the shock bumper rubber, it may be difficult for you to notice when your bike's suspension is bottoming out. Some riders may think the damping or perhaps the leverage ratio is too harsh and in reality, the problem is most likely insufficient spring preload or a spring that is too soft. Either situation prevents utilizing the full potential of the suspension.

 

Keep in mind that a properly adjusted suspension system may bottom slightly every few minutes at full speed. Adjusting the suspension to avoid this occasional bottoming may cost more in overall suspension performance than it is worth.

 

A spring that is too firm for your weight will not allow the rear tire to hook up under acceleration and it will pass more bumps on to you.

This is a basic guide line. Contact us a more detailed analysis and advisement.

 

 Balance front end and rear end static ride height

 

- If the rear end squats under acceleration along with too much front-end lift, or the bike doesn't want to turn sharp enough, tighten your preload by adjusting your rear sag -5mm (example: 100mm adjust to 95mm).  (Note: Raising the rear end helps turning)

 

- If the front end rides low, turns too sharp, and/or tends to Head Shake at high speed, try a combination of lowering the front forks in the triple clamps and loosen your rear sag to +5mm (example: 100mm adjust to 105mm).  (Note: Raising the front and lowering the rear helps stability)

 

Adjust Rebound damping front/rear. (Adjust 1or 2 clicks at a time)

 

- If either front or rear tends to kick up (rebound) faster than the other does after landing from a large jump, the rebound is not balanced front to rear.  You can slow down the end that kicks up or speed up the end that is hanging down.

 

- Adjusting the rebound screw "in" (clockwise) creates more damping, causing the suspension to return slower to its original ride height. If the front end bounces up after landing from a jump, turn the slotted screw at the bottom of the forks "in" 1 click at a time to slow their return. If the rear end kicks up after landings or kicks up on high-speed straights, turn the slotted screw at the bottom of the shock "in" one click at a time to slow the rear wheel return.

 

- Remember; too slow a rebound setting causes "packing" because the suspension does not have time to return to its original ride height before you hit the next bump. This causes the second bump to feel stiff and causes the rider to tire quickly.  (Note: that being soft or stiff on compression will cause kicking as well.)

 

Adjust compression damping front & rear

 

- If "bottoming" is accurse at either end, the compression adjuster should be turned "in" (clockwise) to stiffen the compression stroke. The front fork compression adjuster is the Slotted Screw at the top of the fork. The rear shock compression adjuster is the Slotted Screw and the Hex Nut in the shock reservoir

 

- If either end is stiffer than the other, turn the adjuster “out" (counter clockwise) at the stiff end. This will soften the stiff end making it more compatible with the other “softer” end.

 

Note:    On hard pack tracks, set your rebound at both ends "slightly faster” to follow the terrain.

On deep sand tracks, set your rebound “slower”, especially at the rear, to prevent bouncing out of the rollers. 

JM Racing Illinois 342 W Embarras Street Sainte Marie, Il 62459 (618)-455-3152 mborgic@jm-racingmb.com


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