Spring Rates for NSR50 -> XR100 conversion
I am about to do the xr100 conversion on my NSR50. I am going to get fork springs and probably a rear shock. I considering rebuilding the stock shock but it was not a whole lot more to get a shock, provided I know my spring rate. For those of you who got the correct spring rate for your NSR50 or NSF100, what is your weight and what spring rates did you get?
If you are still looking for this info, I dove in to the NSR suspension a while back when I was racing them. Ended up making a spring rate calculator the NSR 50. Doubt there would be a huge difference to the XR conversion with a lot of extra weight.
You can access the calculator here: NSR50 Spring Rate Calculator
I ended up using front springs of .9 kg/mm and a rear spring of 1250 lbs/in. The spring were right on the money for what I needed. Had to play around with different oil weights to get the valving to work right. If you are interested, I could search through my notes on the oil. I was running a ratio of different weight of redline suspension fluid.
You can't by the fork springs pre-made. But you can custom springs. I had mine made by Cannon Racecraft out of Oklahoma City. They were real easy to work with. Just need to provide the spring dimensions and weight you are looking for and they can make them up for you. And the price was real reasonable, on par with pre-made springs. Dirt Bike Springs - Cannon Racecraft
I did a write up on my research. I'll see if I can find it and post it here.
Here is the write up. This was from a few years back not fresh in my memory. Take what you can from it.
During the time trials a few of us got in to a discussion on suspension and springs and some of the changes I have been making on my bike. I was going to try and write up everything I have learned. But it is just too much, so I am just going to discuss the shock, which seems to need the most help.
I have only been racing the NSR for about half a year, and one thing I have been surprised about is the lack of focus or information on proper suspension set-up. It is pretty obvious the bike is not set up for heavier riders. Once I set proper sag for it, the bike would slam to the top out position. Also the compression and rebound adjustments were useless. So I searched the net for keys on setting up proper suspension on the NSR and just came up some standard advice that keeps getting passed along.
“Put 20wt in the forks, crank preload up, and set all settings on the shock to full hard”
That is all the wisdom I could find on the net, summarized of course. So I decided to try and figure out how to make the stock shock workable.
The first thing I focused on trying to figure out is whether I have the proper springs, or how to determine the proper springs for my weight. I thought this would be simple, just go to the Racetech website and use their calculator. Well the NSR50 rear is not covered in their calculator. I tried a couple of other sources with no real insights on choosing the right spring.
So how do you know if you have the proper spring for your weight? Well for the big bikes, the first thing to do is set up proper rider sag, then measure the free sag. Comparing the rider sag to free sag will let you know if your spring is too light, too heavy, or just right.
Here are some guidelines getting the right spring from sag numbers taken from Peter Verdone Designs
Rider Sag - 30-35mm (25-30% of Full Travel)
Free Sag - 15-20mm (60-70% of Rider Sag)
Rider Sag - 20-30mm (race), 30-35mm (street) (25-30% of Full Travel)
Free Sag - 5-10mm (extremely light bikes use less) (15-25% of Rider Sag)
These are general guidelines. Well here is the kicker with the NSR, it has no free sag in the rear. From further research it turns out this is normal for the GP style bikes like the NSR. Also the GP style bikes should have 10-15mm of free sag at the front. So the NSR is a bit different from the larger production bikes.
Since there is no free sag, I got the idea of calculating the amount of force on the spring while topped out rest. With that value it should be possible to calculate the right spring rate. But Honda in there infinite wisdom supplies the string rate as KGF which cannot be converted to kg/mm or lbs/in without knowing the distance they used. So I would to find another way to determine the spring rate. Lucky for me Peter Venrdone had a nice little formula determining the spring rate.
(11,500,000 x (wire diameter) ^(4)) / (8 x (ID + wire diameter) ^(3) x active coils)
I found this formula on a few other sites, so it seems to be and industry standard. Or at least a physics standard, and so decided to trust it. With this, I busted out the caliper and took some measurements. My bike came with the standard red spring from HRC. I switched it out this the yellow HRC spring, on advice to stick with the available HRC springs. So I decided to measure them both out.
The Red spring had Wire Dia of .431in, ID of 2in, and 5.3 active coils. This works out to 651.46 lbs/in or 11.66 kg/mm.
The Yellow spring had Wire Dia of .442in, ID of 2in, and 5.1 active coils. This works out to 738.74 lbs/in or 11.66 kg/mm.
If we look in the NSR50 manual they have a set length that the springs are supposed to be installed at. The red spring is to be installed at 102mm, while the yellow is to be installed at 103mm. With a full spring length of 107mm, this gives a preload of 5 and 4 mm respectively. You multiply this by the spring rate each spring and you end up with a load in the mid 50 kg range for topped out force. (58kg red, 52kg yellow)
To give perspective, with the yellow spring installed, my installed length for proper rider sag was 93mm. This equates to 15mm of preload and 198kg of force when topped out. Almost 4 time the amount of the HRC engineers intended. My weight is at 180, if you are wondering. This made me curious at what rider weight the stock spring was designed for. Doing a little reverse calculation, the bike is set for a rider and gear weight of 76lbs at 30mm of rider sag. I got that beat by over 100lbs. Clearly the bike wasn’t meant for me.
So now that we have the amount of force when topped out, we just need the ratio of travel between the shock and the axel. After a little work I was able to determine that the shock had full travel of 34mm compare to the swingarm travel of 100mm. This gives us a nice ratio of 34:100 to work with.
Now we just need one more thing to figure out the right spring, the sprung weight of the bike. Weighing the rear of the bike gives us the total rear bike weight. Then weigh the wheel assembly gives us the majority of the un-sprung weight. Subtract the two and we end up with a sprung weight of 61.5lbs.
Now we need to convert the sprung weight and rider wait to the amount of force on the shock. As a general rule 2/3 of rider weight is used for the rear. This leads us to the calculation:
((Rider Weight*(2/3))+61.5)*(100/34) = Force to shock
This is the force we use to calculate the sag. This is set up for lbs. Multiply this by .453592 to convert to Kg. Now we are set to calculate the spring rate that we need. But one thing to remember is we need to subtract the top out load force from the force to the shock we just calculated.
(Force to Shock – Topped Out Force)/ Desired Rider Rag = Spring Rate
Sweet, we now have the desired spring rate. But what amount of preload load do we need to install it at? We have the topped out force calculated. We can use that to determine how much preload in mm we need to achieve this desired force. This is a simple calculation of the force divide by the spring rate.
Topped Out Force/Spring Rate = Preload
When you go to install your new spring, just measure its free length before installed, subtract the preload value just calculated, and you will get your needed installed length to achieve the stock topped out force and desired rider sag.
I have tested this out and applied it my bike with success. The first time I did the calculations, the results were a significant jump in the spring rate. So much so, that I didn’t really believe it. So I ended up purchasing a spring a couple hundred pounds less than what I came up with. While the it did end up being a significant improvement, I should have trusted the math and got the right spring. The calculations told me I needed a 1250lb/in spring for a desired sag of 25mm. I went with an 1050ld/in spring due to my doubting mind. The ride was actually plush compared to my yellow spring, even thought it was over 50% stiffer.
The next item to address was the valving. Unfortunately the stock shock cannot be revalved. Well the rebound and low speed compression can be. I am still working on what can be done with the high speed compression. So what can be done to adjust the valving to make it work. Well just the forks, we can try different weight oils.
Since the valving cannot be changed, my thought was to get the rebound correct. The reason for this is from my experience, if the rebound is too slow, a bike will have a tendency to push though turns(run wide). If the rebound is too fast, your front end becomes a pogo stick and you have to fight it more. So I was going to shoot for an oil weight that allowed the rebound to be set to the desired amount. With this the compression damping will also change. I my mind this can be ridden around, and we should have some adjustment in the shock if we get it right.
The thing about oil weights between manufactures is they are not the same. You need to look at their rate viscosity. This gives you a number you can compare between brands and help you select an oil that is actually heavier or lighter than the one being used. I changed the oil in my shock over to Redline 10wt Red. This had a viscosity about double of what the stock shock oil was. This ended up giving noticeable change in the rebound adjustment.
I am still experimenting and trying to dial the suspension in. But maybe this info could be of use to you. If you have questions, observations, or opinions, I would love to hear them, as I am still working on figuring this all out. Or if you need help figuring yours out, maybe I can lend a hand.
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