Front Spring Rates
Spring is coming, so a number are doing off season explorations and sourcing re front springing.
The attached is worthy of a chapter in a book.
It is also a perfect example of why simple charts and on line calculators are not the definitive source of what you "need" in terms of spring rates.
A great guide ? Absolutely !
Get you in the zone ? Yes.
All encompassing selection process ? Absolutely not !
A quick review in terms of 919s.
20-30 mm of preload is way too much for a properly selected spring rate for high performance or track oriented duty.
Such preloads are for soft springs and gentle road riding intended to not approach bottom out or top out.
No 919 front end that has any give will ever have as little as 25-33 of Rider Sag.
919S are not 50/50 weight split bikes, as there is more rear axle weight bias.
BUT 919s are super weight transfer bikes under front braking, especially with upright riders using higher bars.
AND 919s are terrible for stiction. (so stiffer springs make initial movements even harsher)
So, what does one do with all this?
It's not really that bad.
If you weigh up to 180 # before gear, and are not selecting for touring gear and/or a passenger, regardless of whether you do low key road riding or lots of track time, your rate is suggested as best being in the 0.875 to 0.925 kg/mm band.
In my mind and experience do date on my stuff, anything over 0.95 should start bells ringing, as should anything less than 0.85.
Bumpy roads mean less rate should be used.
A desired softer ride means less rate should be used.
Use oil level and technique to correct for brake dive, not spring rate.
Suggested internal preload is 15 -20 mm max.
A strongly track oriented selection can be a bit stiffer (say + 0.025 kg/mm) but with the internal preload reduced to 10 mm.