Front Spring Rates - Wrist Twisters
 
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post #1 of 4 Old 01-29-2012, 01:00 PM Thread Starter
McTavish
 
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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.
Attached Images
File Type: jpg Front Ends - spring rate chart 1.jpg (43.3 KB, 27 views)

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post #2 of 4 Old 01-29-2012, 01:03 PM Thread Starter
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The attachment file was too big
I need to shrink and try again
Stay tuned


Now fixed, pasted it into the original post.

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post #3 of 4 Old 01-29-2012, 02:54 PM Thread Starter
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Here are some spring rate conversion factors, which will make things easier for you if you are comparing springs or selections from different sources.
N/mm X 0.102 = kg/mm
kg/mm X 56 = #/inch
N/mm X 5.710 = #/inch

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post #4 of 4 Old 01-31-2012, 12:16 AM
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Learned this one the hard way. Jumped on a good deal for used springs that were too heavy for my weight. I wanted to stiffen up the front & reduce brake dive...i figured if stiff was good, "stiffer" would be better. Too much of a good thing ended up bad. Went too stiff and the ride was too harsh. Went from the mushy stockers (too soft) to 1 kg springs (too hard) and am now happy with .925's.

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