How to increase the spring rate in your forks
Your stock front springs are likely to be about .7 kg/mm, a surprisingly common (insufficient) value for front springs on street bikes. You want your springs to be more like .85 kg/mm for street use, as much as 1 kg/mm for track use. The best solution is to buy some good progressively wound springs from HyperPro, Bartels, Racetech, or some such. If you don't want to spend $100 on springs, here's a $2 alternative.
This procedure involves some measuring and simple arithmetic. If you skip the measuring and arithmetic, you run the risk of having your forks bottom out due to spring binding instead of running out of travel, meaning you will lose fork travel.
| Spring Wire Diameter: | _______________________ | (a) | About 5mm |
| Number of coils: | _______________________ | (b) | About 40 |
| Spring Free Length: | _______________________ | (c) | About 450 mm |
| Spring Binding Length: | _______________________ | (d) | a * b |
| Available Spring Travel: | _______________________ | (e) | c - d |
| Travel per coil: | _______________________ | (f) | measure, or e / b |
| Fork Travel: | _______________________ | (g) | About 100mm (street) 150mm - 200mm (dual sport) 300mm (dirt) |
| Excess Spring Travel: | _______________________ | (h) | e - g |
| Excess Spring Coils: | _______________________ | (i) | h / f |
| Number of widely spaced coils: | _______________________ | (j) | About 40 |
| 10% of coils: | _______________________ | (k) | j * .1, About 4 |
| Length of (k) coils: | _______________________ | (l) | About 35mm |
| Length of stock spacers: | _______________________ | (m) | About 80mm |
| Length of new spacers: | _______________________ | (n) | l + m + 25 |
| Desired static sag: | _______________________ | (o) | g / 4 |
- Put the bike on its center stand and/or use an engine jack to get the front wheel in the air. ½" is fine.
- Remove the fork caps and remove your springs.
- This is a good time to do a front end alignment - see the instructions above.
- Using a micrometers, measure the thickness of your spring wire. This is likely to be about 5mm. Write this number down.
- Count the number of coils in your front springs. Write this number down.
- Measure the free length of your springs in millimeters. This is likely to be roughly 450mm.Write this number down.
- Calculate the spring binding length. See worksheet above.
- Calculate the available spring travel. See worksheet above.
- Calculate the travel per coil. If you have progressive wound springs, then measure the free space between coils in the part of the spring with the largest gaps. See worksheet above.
- Write down your fork travel. This will likely be in your owners manual, or somewhere on the Internet.
- Calculate the excess spring travel. See worksheet above.
- Calculate the excess spring coils. See worksheet above.
- If you have progressively wound springs, count the number of coils which are widely spaced and enter it in cell j.If you have straight wound springs, copy b to j.
- Calculate 10% of the widely spaced spring coils. See worksheet above.
- i is the most coils you can cut off. If k is larger than i, put the springs back in the forks. Buy some new springs.
- We'll plan on cutting 10% off of your springs. This will raise the spring rate by 10%. You can consider cutting as much as 20% off of your springs, but I don't recommend you do this all in one cut - cut 10%, put your forks back together, and see how things are. If you want, you can cut off up to 20% of your coils, so long as this number is less than i.
- If the arithmetic says it's ok, cut the springs. Spring wire is pretty tough, this can be done with a hack saw but it's a lot easier with a dremel tool and a cutting wheel. Use a bench grinder to grind a flat ramp on the last ½ coil, like the factory did.
- Measure the length of the spring you cut off. This is likely about 35mm, about 1 ½". Add an inch to this length, here we get 2 ½". If your forks already had spacers in them (likely), make new spacers which are 2 ½"longer than the stock spacers. Cut a couple pieces of PVC water pipe to this length. These are spacers we'll use to make up the length of the new spring. We're intentionally starting with spacers that are too long, so that we can measure sag carefully and cut them down to the correct length.
- Get a washer that's the same diameter as your fork springs. This is likely to be about 1 ¼". Place the fork springs in your forks, with the factory end down. Place the washers in your forks on top of the cut end of your fork springs.Place the PVC pipe that you cut into the forks on top of the washers.
- If you have pre-load adjusters on your fork caps, set them at the middle position. Put your fork caps back on the forks, and turn them down to finger tight. Most likely you're going to be removing these caps again in a couple of minutes.
- Measure the distance from the bottom of your lower triple clamp to the top of your fork leg seal. If this is inconvenient, pick some other place on the lower fork leg, like one of the bolts on a disk caliper. Write down this number.
- Take your bike off the center stand. Sit on the bike, grab the front brake, and work the forks up and down a bit.Get the forks to their resting place while holding up your weight.
- Measure the distance from the bottom of your lower triple clamp to your fork leg seal, or the disk caliper bolt, or whatever you chose. Subtract this distance from the unloaded distance you measured above. This is your static sag.
- Your static sag should be ¼ of your total fork travel. If your travel is 150mm, your static sag should be about 38mm. If your sag is less than this, calculate the difference. Remove the PVC spacers one at a time and cut them down this amount. If your sag is more than this, calculate the difference. Remove the PVC spacers and make new ones which are this amount longer.
- Replace your fork caps, this time torquing them down to the factory spec.
That's it, you have increased your fork spring rate by 10%. As long as the binding length calculation on your springs says you can make more cuts, you're allowed to make more cuts. Remember, this is like hair: you can't cut it longer.
Lowering your motorcycle
I get a surprising amount of email on this. If you want to ride a bike that's a bit tall for you, all is not lost. If you ride a Harley, your dealer will sell you a kit. Otherwise, here's what you can do:
- Get a lower seat. Having the seat narrower at the nose also makes a big different in reaching the ground. Some manufacturers make lower seats for their bikes, and you can just buy one. Or, get a custom seat made, for example by Rick Mayer (links in Seats). Or, take your seat to a local upholstery shop and tell then what you want - for about $50 they can reshape your foam and reattach the seat cover. Or DIY - instructions in Seats.
- Slide your forks up in the triple clamps. On most bikes, you can loosen the pinch bolts and slide the forks up about ¾".
- If you have a single shock in the rear, get different dog bone links. Generally speaking, if your rear shock has a linkage system, it will have a ratio of about 2.5 to one. If you get dog bone links that are 5mm to 10mm longer, your rear end will be lowered by about½" to 1". Any local machine shop can make you replacements for about $50 or so. Links to pre-made lowering links below.
- Get a shorter rear shock. Works Performance.com will work with you on a shock that will lower your bike by 1" to 2". Their shocks are excellent. About $700, depending on model, options.
- Get your forks shortened. Racetech.com generally can lower your forks by 1" to 2" and improve their function at the same time.About $300, depending on model, options. These guys do good work.
- If you're brave, you can do this yourself. The basic idea is to cut your fork springs by the amount you want your forks lowered. See the instructions above. Then, take your forks all apart. Under the damping rod you're likely to find a small spring - this is the top out spring. Add the portion of spring you cut off to this spring. This will make your forks work lower in their travel range. You can cut your fork springs by yet more and replace the second cut with a spacer to raise your spring rate. Not all forks have a top out spring, so get a service manual and study the fork diagram before you cut anything.
If you lower your suspension, try to lower the front forks and the rear shock by the same amount. Otherwise, the handling of the bike will change.
If you choose to lower your suspension, either by changing fork tube level, changing dog bone links, or getting your suspension modified, your bike will sit lower and have less ground clearance. This means your bike will start scraping more stuff in corners, so these are not such great ideas for track days. Also, your kick stand and center stand will now be too long. You can get them cut and welded at any decent machine shop for about $25.