Yamaha TRX850 Enthusiasts Forum

Copied from the main TRX forum...

Spring Rate Calculation
Here is the formula to calculate your current spring rates.
(1010.781765 * d4) / (D3 * N) = ??Kg/mm
Where:
D = Mean diameter (Note: Measured from the middle of each wire)
d = Wire diameter
N = Number of active coils

NOTE: This formula only works on Linear Springs; Progressive springs have to be calculated in sections as they rise in Spring Rate. Progressive springs will varying Pitches/”Gaps” between the Active Coils, calculate the spring rate for each section of equal “Gaps”, usually 2 sections.

Setting Rear End Free & Loaded Sag;
1. Extend the suspension completely by getting the wheel off the ground. It helps to have a few friends around. Using the bikes sidestand it can usually be carefully rocked up to unload the suspension. Measure the distance from the axle vertically to some point on the chassis. Mark this reference point because you'll need to refer to it again. This measurement is A. If the measurement is not exactly vertical the sag numbers will be inaccurate.
2. Carefully lower the bike back down WITHOUT bouncing the suspension and hold it upright. Now re-measure as above. This is measurement RFS1. Lift the rear wheel off the ground using the above method and lower it back down as above and re-measure. This is measurement RFS2.
3. Rear Free Sag = A – [(RFS1 + RFS2) / 2].
4. Race Bikes 5-10mm, Road Bikes 15-20mm.
5. Sit on the bike in your normal riding position. Have a third person balance the bike from the front. You must take friction of the linkage into account. First, push down on the rear end about 25mm and let it extend very slowly. Where it stops, measure the distance between the axle and the mark on chassis again. It's important that you do not bounce or use the brakes! This measurement is RLS1.
6. Have your assistant lift up on the rear of the bike using the frame about 25mm and let it down very slowly. Where it stops, measure it. Remember, don't bounce! This measurement it RLS2.
7. The Loaded Sag is in the middle of these two measurements. In fact, if there was no drag in the linkage, RLS1 and RLS2 would be the same.
8. Rear Loaded Sag = A – [(RLS1 + RLS2) / 2].
9. Race Bikes 25-30mm, Road Bikes 30-35mm.
10. Adjust the preload. If you have too much sag you need more preload; if you have too little sag you need less preload.
11. If you can’t get within BOTH the Free Sag and Loaded Sag ranges you will need to change the spring for a harder one if outside the ranges or softer one if you are inside the ranges. There is no formula to calculate what strength spring you will need on the rear shock as it depends on the mounting position and linkage ratios.



Setting Front End Free & Loaded Sag;
1. Extend the fork completely by lifting it off the ground in the same way as with the rear. Measure from the dust seal to the bottom of the triple clamp or brake calliper holder on USD forks. This measurement is B.
2. Carefully lower the bike back down WITHOUT bouncing the forks and hold it upright. Now re-measure as above. This is measurement FFS1. Lift the front wheel off the ground using the above method and lower it back down as above and re-measure. This is measurement FFS2.
3. Front Free Sag = B – [(FFS1 + FFS2) / 2].
4. Race Bikes 20-25mm, Road Bikes 25-30mm.
5. Sit on the bike in your normal riding position. Get an assistant to balance the bike from the rear, then push down on the front end without using the brakes and let it extend very slowly. Measure the distance between the dust seal and the bottom of the triple clamp again or brake calliper holder. Do not bounce. This measurement is FLS1.
6. Lift up on the front end and let it drop very slowly. Where it stops, measure again. Don't bounce. This measurement is FLS2. Once again, FLS1 and FLS2 are different due to stiction or drag in the seals and bushings, which is particularly high for telescopic front ends.
7. Just as with the rear, halfway between FLS1 and FLS2 is where the sag will be.
8. Front Loaded Sag = B – [(FLS1 + FLS2) / 2].
9.Race Bikes 30-35mm, Road Bikes 35-40mm.
10. Adjust the preload. If you have too much sag you need more preload; if you have too little sag you need less preload.
11. If you can’t get within BOTH the Free Sag and Loaded Sag ranges you will need to change the springs for harder ones if outside the ranges or softer ones if you are inside the ranges.


This method of checking sag and taking stiction into account also allows you to check the drag/condition of the linkage and seals. It follows that the greater the difference between the measurements (pushing down and pulling up), the worse the stiction. A good linkage (rear sag) has less than 3mm difference, and a bad one has more than 10mm. Good forks have less than 15mm difference and bad forks has more than 25mm.

It's important to stress that there is no magic number for Free Sag or Loaded Sag. Your personal sag and front-to-rear sag bias will depend on chassis geometry, track or road conditions, tire selection and rider weight and riding preference. Using different sags on the front and rear will have huge effect on steering characteristics. More sag on the front or less sag on the rear will make the bike turn more slowly. Increasing sag will also decrease bottoming resistance, though spring rate has a bigger effect than sag.

Note the “Road Bike” ranges are aimed and Tourers and Cruiser Motorcycles. While the Lower number of the “Race Bike” range is aimed at Race Bikes, the rest of the range is aimed at Sports Bikes; once again these are only guidelines.



Setting Rebound on the Rear End
Find a relatively fast, sweeping corner. Reduce (Turn clicker out) the rebound damping until the rear end begins to stand up quickly after the corner apex. This will cause the rear to feel hinged or feel loose. Finally, increase (Turn clicker in) the rebound damping until the sensation goes away. Rebound damping is responsible for the traction of the rear suspension. As you gain traction you will often lose stability so it’s very important to find the right balance point.


Setting Compression on the Rear End
Find some hilly sweeping turn sections, a large roll and a steep street entrance. The shock should compress on these sections but it should not be a slamming sensation. Add compression to fight the bottoming load. (Turn clicker in.) But avoid going to far as the suspension's ability to react to small variations of surface, pebbles, or road cracks will be sacrificed in the trade. Remember the adjusters have a primary effect on the low speed, so even a large change in setting may only affect loading resistance slightly. Remember bottoming your suspension is not necessarily a bad thing. If you don't use your travel you're not getting maximum plushness from your suspension. Run your suspension as stiff as your comfortable with so stability will be maximized.



Setting Rebound on the Front End
The rebound damping is responsible for the stability and the cornering characteristics of the motorcycle.
1. Find a short sweeper. When the forks compress for the turn, the speed at which the forks return is the energy that pushes your front wheel into the ground. If the forks rebound too quickly, the energy will be used up and the bike will drift wide, or wash. If the rebound is too slow, the bike will tuck under and turn too soon to the inside.
2. With the bike turning well, the wheel should return to full length relatively fast or head shake is likely to occur.


Setting Compression on the Front End
1. The forks should react to all surface variations. If the forks seem harsh on small bumps, holes, rocks (Turn clicker out). If they are relatively smooth, (Turn clicker in.) until they do feel harsh and then turn back.
2. Now find the rolling part of the road again. The forks should bottom over the worst g-load.

tried above last night on my forks and it works out my static sag is in the race range 23 mm but my loaded sag ends up in the 42mm range

cheesie wrote:tried above last night on my forks and it works out my static sag is in the race range 23 mm but my loaded sag ends up in the 42mm range

Springs may be a tad soft. Try adjusting the preload to acheive a 35-40mm loaded sag. If the static sag reduces eg:<23mm, then your springs are definately on the soft side.

front end sorted now thanks Waz, the rear needs a stiffer spring but this should be sorted soon with the new shock if the spring is right on it

the front now feels a little harsh should I lower the oil level a bit ( bigger air gap) or try 5w oil instead of 10

What air gap are you running? Viscosity won't stop the harshness.

130mm

I found 130 a bit harsh. 140 is more comfortable.

Increasing the air gap will only really affect the middle to end of the suspension travel on compression. Lighter oil will soften the damping on slow speed (undulations etc) but will not affect the high speed (sharp edge bumps) and will soften the rebound damping (the TRX's weak point).......this equates to damper rod forks, cartridge are another proposition.
If you are compensating for soft springs by increasing the preload to acheive the correct sag, this will give the feeling of "feeling every little stone you run over", it won't really affect the rest of the suspension range but the initial feeling may give you the "harsh" conclusion.
Preloading the springs gives resistance to initial load, once the load overtakes the preload, the springs operate normally. ie: at their spring rate.
The only way to get the standard TRX forks to work properly IMHO is springs to suit your weight and riding requirements plus emulators.
If I were you, working with standard equipment, I'd forget about free sag and work on 40-45mm loaded sag for normal riding and adjust the preload to achieve 35-40mm for fast smooth road riding and forget about air gap and oil weight. Then save for a set of springs and emulators or Thundercat replacements (you would still need to get springs and more than likely a re- shim as well) or R1 USD Conversion (springs and re-shim possible also)

I have fitted stronger springs and they are better than standard, my loaded sag is at 40mm my static is at 23mm 5 1/2 rings showing, I phoned maxton today and they quoted me £380 plus vat for to rework the trx forks and £270 for to rework a set of thundercat forks with the thundercat forks you keep your damping adjustment while loose it on the trx forks, I am going to look at a set of t/cat forks hopfully this weekend £100 if they are ok I will go for them, I will try a bigger air gap in the mean time, what tyre presures do you run.

you know I could just ride the thing but making somthing work better is half the fun

what were maxton doing to the trx forks for that cost out of interest?

I am not up on all the jargon he talked but the damping rod will be scrapped, you will no longer be able to adjust the damping and a cartridge type kit fitted, different shims, new springs to suit rider weight and what type of use think that was it he said it was easier to work on the t/cat ones plkus you keep damping adjustment but you will basically end up with the same end result, I hope to get the t/cat ones so I can keep my bike on the road while they are being worked on,

Ah.......you have already changed the springs. Your static vs loaded sag is in the ballpark (ideally 25mm static for 40mm loaded). I would say the difference in cost between the 2 is the added cost of Emulators compared to pennies for shims for the T/Cat forks.With Emulators you can still adjust the damping by using the preload screw for compression and oil weight for rebound however this is a "remove forks/springs, drop out emulator and adjust and replace oil" vs adjusting externally with the T/Cats.

Is it a proper cartridge kit or, as Waz says, emulators?

I think for the trx forks its emulaters, all future work on the bike will have to stop as the firm I work for is being bought over so everything will be on hold, its a rumour but alot of things make sense the way the boss has been carrying on