Spring rates?

**pritch** · 2nd December 2009, 11:47

There is a question been perplexing me for a while, and since I may not be the only one I'll ask it here.

As I understand it spring rate is measured in "Newtons", as in say
17 Newtons?

What I'd like to know, is how does that relate to rider weight?

If a fat bastard was 110kgs and lost 10 kgs thereby becoming a slightly less fat bastard, how many Newtons would the spring rate need to be reduced by?

Does this relate only to the spring on the rear shock? The springs on the front appearing to be measured differently?

There are other factors such as rider height go into the setup equation as well but to keep it simple let's ignore all that, and we'll leave BMW out of this as well...

And none of the foregoing is intended to offend anyone (else) who may, however unjustly, be considered short for their weight

**Cajun** · 2nd December 2009, 12:04

Originally Posted by pritch

There is a question been perplexing me for a while, and since I may not be the only one I'll ask it here.

As I understand it spring rate is measured in "Newtons", as in say
17 Newtons?

What I'd like to know, is how does that relate to rider weight?

If a fat bastard was 110kgs and lost 10 kgs thereby becoming a slightly less fat bastard, how many Newtons would the spring rate need to be reduced by?

Does this relate only to the spring on the rear shock? The springs on the front appearing to be measured differently?

There are other factors such as rider height go into the setup equation as well but to keep it simple let's ignore all that, and we'll leave BMW out of this as well...

And none of the foregoing is intended to offend anyone (else) who may, however unjustly, be considered short for their weight

this is the racetech guide to weight & spring level.

http://www.racetech.com

go to search -> Spring Rate Search, and find your bike

Then look for 'Custom Fork and Shock Spring Calculation and Available Rates' with a link, brings up a page like this
http://old.racetech.com/evalving/Spr...pringType=Fork

**Pussy** · 2nd December 2009, 12:14

What Cajun said!
The Racetech recommendations are pretty spot on... well they are for me, anyway.
You been eating less pies, Pritch?

**vifferman** · 2nd December 2009, 12:30

My understanding was the Racetech (and Sonic springs) spring rate calculators were very race biased.
F'rexample, for me, the Racetech calculator recommends a 1.00kg/mm fork spring rate; Sonic recommends 0.95, and Ohlins recommends 0.80. (Standard is 0.74).
I've tried 0.74 (too soft), 0.95 (too firm for the road, but would be great on the track), and 0.90 (feel cushy after the 0.95s, but are pretty good for our crappy roads).

**pritch** · 2nd December 2009, 13:27

Thanks for all that. I was interested to note that both front and rear rates were in kgs/mm. No mention of the revered Mr Newton.

**Cajun** · 2nd December 2009, 13:30

Originally Posted by pritch

Thanks for all that. I was interested to note that both front and rear rates were in kgs/mm. No mention of the revered Mr Newton.

i got this ohlins chart that does some matching i think

**Juzz976** · 2nd December 2009, 13:42

Newtons are a measure of force, in this instance mass x acceleration due to gravity
110kg mass exerts a downward force on earth = to 110 x 9.80665 and vice versa (earth's for on mass)
therfore your 110kg mass weighs 1078 Newtons on earth at sea level.

double the weight on the back doesnt always mean double the displacment, especially when you factor that they're preloaded.

I have seen springs with newton rating as force required to completely compress the spring otherwise its usually mass/displacement or k (spring constant) force divided by travel = k .

Rider height comes into it when you may sit further back if you have long arms and legs and where your centering your mass while leaning through curves.
The mass of your swingarm, wheel sprocket brake tires affect the dynamics too as a soft spring with hard shocks tends to float and other way round you'll bounce.

**Pussy** · 2nd December 2009, 15:09

Originally Posted by pritch

Thanks for all that. I was interested to note that both front and rear rates were in kgs/mm. No mention of the revered Mr Newton.

Aren't the shock spring rates in Nm on the Racetech site?

Edit: I see you are quite correct, Pritch!
Wonder if kg/mm is the same as N?

**warewolf** · 2nd December 2009, 17:03

No, kg is not the same as N.

Newton is a unit of force.
Kilogram is a unit of mass.

They are related as Force = Mass x Acceleration. Usually, we talk about kilogram "weight" where the forgotten acceleration is due to this planet's surface gravity, approx. 9.8m/s/s. So 1kg is ~= 9.8N, roughly a factor of 10.

Spring rates are expressed as N/mm or kg/mm; the former being rougly 10 times the latter. eg my KTM fork springs are 4.6 N/mm, the shock spring is 70 N/mm. Or at a factor of 9.8 (say 10), they can be expressed as roughly 0.46 kg/mm and 7.0 kg/mm.

Road gear is about twice that rate, commensurate with the heavier bike weight the springs are supporting.

A 110kg rider on say a 250kg (wet) bike = 360kg load. Lose 10kg rider weight, it's still 350kg load - not a very big change at all. A little preload adjustment would likely correct for this. Different story on my 95kg dirt bike with 70kg (wet) rider... KTM recommend a different spring rate every 10kg rider weight.

**Robert Taylor** · 2nd December 2009, 17:06

Suspension nerds ( like me ) bandy around spring rates in ''Newtons'' when in fact it is an abbreviation. For example if an Ohlins TTX36 has a rear spring with ink writing 21040-31 / 95 the 3rd digit ''95'' is the spring rate, being 95N/mm.

Newton millimetres is the most correct metricated unit of force to rate springs in. Race Tech and most of the oem Japanese manufacturers rate their springs in kg/mm. That is not a purist way because technically kg is a unit of mass. ( any scientists/ physicists reading this please correct me if required ) In truth though there is very little difference in rate between a 10 ''newton'' rear shock spring and a 10 ''k'' rear shock spring, its very close.
A word of warning about spring guides, Race Tech, Sonic and Ohlins. All of these companies are based in first world countries that have a much much higher percentage of smooth roads than we have here. Our roads are bumpy and its much more of a challenge to build suspension that will absorb these bumps and still provide excellent overall control.
Vifferman got caught out with using the Sonic spring calculator and the suspension was too harsh, requiring further expenditure on springs softer than Sonic recommended. Like many overseas companies and resellers they are not familiar with our roading conditions!!!!
Very often we will choose spring rates softer than what these companies recommend, so that road going bikes will actually absorb the bumps, although its also more than about spring rate. And here and now I will reiterate what many of our CKT customers already know. If we deliver with spring rates that in the end event dont quite suit you, we will exchange foc excepting freight recoveries.
One mans meat is another mans poison. You can have two riders of exactly the same weight and height and often they will prefer different spring rates and base preload. Its all about personal preference, no internet spring rate calculator can allow for that!!!

**Pussy** · 2nd December 2009, 17:42

Originally Posted by warewolf

No, kg is not the same as N.

Newton is a unit of force.
Kilogram is a unit of mass.

They are related as Force = Mass x Acceleration. Usually, we talk about kilogram "weight" where the forgotten acceleration is due to this planet's surface gravity, approx. 9.8m/s/s. So 1kg is ~= 9.8N, roughly a factor of 10.

Spring rates are expressed as N/mm or kg/mm; the former being rougly 10 times the latter. eg my KTM fork springs are 4.6 N/mm, the shock spring is 70 N/mm. Or at a factor of 9.8 (say 10), they can be expressed as roughly 0.46 kg/mm and 7.0 kg/mm.

Road gear is about twice that rate, commensurate with the heavier bike weight the springs are supporting.

A 110kg rider on say a 250kg (wet) bike = 360kg load. Lose 10kg rider weight, it's still 350kg load - not a very big change at all. A little preload adjustment would likely correct for this. Different story on my 95kg dirt bike with 70kg (wet) rider... KTM recommend a different spring rate every 10kg rider weight.

This I understand, and was basically what I was getting at.
Good explanation!