Hi forum members,

I experience my front fork springs are to soft in my MC21 SP. After track days I can see that the forks have been fully compressed. The springs measure to 295mm length which is just at the service limit.

My weight is just under 90 kg including gear and the bike is ~10 kg lighter than stock.

I have tried to find the spring rate for the original springs, but without luck. Has enyone measured the spring rate of the original springs for a MC21 SP bike?

If I were about to replace my springs, has anyone themselves changed springs and what was your spring rate? What was your total weight that was used for calculating the spring rate?

The spring rates I can choose between are 10 N/mm, 9,5 N/mm and 9 N/mm.

Appreciate any help in the matter.
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MC21, GP-Replica (Daijiro Kato, Telefonica Movistar Team 2001)

http://www.nsr-world.com/tuning/250-tuning/suspension/

HRC fork springs for the MC21 are part# 51402-NH3-761 rated at 0.6-0.85 kgf/mm & 51403-NH3-761 rated at 0.7-0.9 kgf/mm, and for the MC28 are part# 51401-NKD-970 rated at 0.95 kgf/mm. The fork springs are NOT interchangeable between MC21 and MC28.

I would use that as a starting point and say that street settings are marginally softer than that.

That is a place to start, but more info is needed.

0.7-0.9kgf/mm progressive spring translates to what spring rate on a linear spring? What weight on rider and bike are these springs suppose to match?

The springs I am thinking of getting are not progressive, but linear.
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MC21, GP-Replica (Daijiro Kato, Telefonica Movistar Team 2001)

Yesterday I did some measurements on the springs, and these are the results.

Both springs had a free length of 295mm (just at the service limit for SP springs according to service manual), when loading them with 10 kg they extended to 312mm, and when loading them with 20 kg they extended to 331mm.

Weight/displacement
10kg/17mm => 0.59 kgf/mm
20kg/35mm => 0.57 kgf/mm

If I would have measured smaller weights I would probably get a lower spring rate, but since I am not going to use progressive springs and comfort is not the issue on the race track I focused on the heavier weights. Also it is more difficult to measure displacement for small weights.

I round of the result and use the spring rate 0.6 kgf/mm.

Afterwards I did some measurements on the static/free sag and ride height.

No preload, sag = 33,0 mm
Medium preload setting (dont remember exactrly how much), sag = 24,1 mm
Full preload (using tyga fork caps = 25mm preload), sag = 14,3 mm

While using full preload I got a ride height of 36,9 mm, and then I wasnt wearing any equipment (adds a few kg to ride weight and mm to ride hight.)
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MC21, GP-Replica (Daijiro Kato, Telefonica Movistar Team 2001)

A lot less hassle and a lot lot cheaper though possibly not as good in the long run... fresh 12.5w oil (made by mixing 10 and 15 grade togeather) and play with the amounts a little bit.
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Please do not PM me technical questions, if you can't find it on the Forum start a thread

Bodoe wrote:Yesterday I did some measurements on the springs, and these are the results.

Wow... thanks for the info!
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2t-racelab.com

Thought I should update the thread with some new info.

Bought myself some new springs and fitted them yesterday. I went for Öhlins springs (of course) with a spring rate of 9N/mm. These springs have exactly the same diameter as original and are 299mm long.

http://ohlins.com/Our-products/Motorcycle/Products/Hypersport/08713-90/

I started checking the spring rate on the new Öhlins springs, mostley to validate my method of measuring the rate on the original springs.

Weight/displacement
10kg / 11 mm => 0.91 kgf/mm (8.9 N/mm)
20kg / 23 mm => 0.87 kgf/mm (8.5 N/mm)

Given my measuring method using a ruler I think the results shows my previous measurings are reasonable.

After installing the springs I started looking at static sag and preload settings.

No preload:
Free sag - 15.9 mm
Ride height - 31.3 mm

3 "rings" preload (should be the same on both the tyga preload adjusters and original):
Free sag - 11.6 mm
Ride height - 26.5 mm

6 "rings" preload:
Free sag - 6.4 mm
Ride height - 20.7 mm

I ever felt the need to measure the sag with full preload setting.

The sag measurements were done by using cable ties on the fork leg and measuring the position after both compressing the fork and after rebound, then using the mean value of those two.

Next step will be to look at the rear suspension, raise/lower the front/rear, oil level in the forks, rebound dampening and least compression dampening settings.

I will take the bike to a track next weekend, and perhaps let you all know how the new setup felt after that. Until then I have some more work do to.
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MC21, GP-Replica (Daijiro Kato, Telefonica Movistar Team 2001)

To show the difference between the worn original springs (1st and 3rd from the left) and the Öhlins springs (2nd and 4th from the left) I uploaded a picture.

Does anyone know why the Honda manual shows a linear spring for the original SP forks while the R forks have progressive springs? My forks should be SP forks, but they had progressive springs.

According to the manual the free length of the spring on the SP is 301.2mm and 291.5 mm for the R model. It is strange then that my original springs are progressive like the R-springs but longer than the R-springs. Could they have been replaced, but why use such low spring rate?

Anyone have any ideas?
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MC21, GP-Replica (Daijiro Kato, Telefonica Movistar Team 2001)