Ducati Sporting Club UK > DesmoDue Race Series > DesmoDue - General Questions and Chat > FAO Tonio600, Fork spacer

[skidlids]

And for anybody else wanting to do something about the springs in their Class B bikes front forks

Sping is a Ohlins single rate spring as supplied for a Honda VTR1000 Firestorm

Spacer is 18mm long with a OD of 35mm and a ID of 27mm (4mm wall thickness)

[Tonio600]

Thanks Kev, that's brilliant

[Tonio600]

Kev, did you get rid of the black plastic spacer? Shouldn't it at the top of the springs?

[skidlids]

Yep did away with the black plastic spacer, as the Ohlins springs are longer and therefore a 18mm spacer as per the pic adds the right amount of preload

[Tonio600]

Thanks Kev.

Now I don't understand what's the importance of the length of the spring... What does a longer/shorter spring means?

On one hand there is Kev who has found a solution using (much) longer springs and much shorter spacers than standard, and on the other hand there are those Ohlins springs sold by a mate which almost match the size of the standard springs but they are 20mm shorter. So I will need to add more spacer (about 20mm I'd say....).

What's the difference between those 2 solutions? Which one will be easier to ride for me? It's either more spring less spacer or more spacer less spring...

Thanks guys.

Tonio.

[Paul James]

Springs can be pretty complex things Tonio, there are quite a few aspects to consider. You have spring rate, number of coils per unit of linear measurement, preload, whether the spring is a constant or variable rate etc. The wire diameter will have an influence on all of these things of course. The spacers will give some effect on the preload. When you consider how little actual preload travel you have on adjustable forks relative to the free length of the spring you can see how critical it can be.

I'm not a suspension expert, in fact I'm looking to take my 996R to someone who is very soon as the front end is just not right as it is and I'm not too sure how to set up the combination of spring setting and damping. That said I have done a lot of engineering design work on springs in the past.

Where the spring coil pitch is constant you have constant rate; so if your spring has a rate of lets say 5N per cm you will compress the spring 1 cm for every 5N of force applied, if variable rate the "softer" part of the spring will come into play first then the harder section giving you a variable suspension rate hardening up during compression. If you had a spring of this rate and by your selection of preload spacer had say 20 cm of preload you would in effect have 100N "pushing" up the forks, to then compress the forks by 20cm you would have to apply another 100N of force. (Just simple figures to illustrate the theory, not likely to be relative to your fork springs)

If you want to really stiffen up the front forks get a higher rated spring not just a longer one, it might actually be shorter and require a longer spacer. Theoretically you could end up limiting the fork travel if you used a spring of small pitch if you put on too much preload. You can simply work out the available spring travel by multiplying the spring wire diameter by the number of coils, take the product of that calculation from the free length and you have the maximum travel available, take off any fixed preload length you apply to give you the actual travel available before the spring becomes coil bound.

If you know the spring rate and the length of fork travel you can calculate the force required to compress the spring to any given length. If you know the data for a range of springs you can calculate the effect each would have.

If you want to find out the rate for your spring get a known weight, measure the free length, apply the weight, measure the compressed length, take the compressed length from the free length and relate it to the weight you have applied and you have it !!! This is usually expressed in Newton/Metres.

You could do that with the bike all assembled I guess. Put it on the rear paddock stand, stick a set of bathroom scales under the front wheel and note the weight displayed. Apply a weight to the top of the forks, doesn't matter what you use as the scales will tell you the difference. Measure the fork deflection from unloaded to loaded to give the travel. Divide the weight by 2 (cos both fork springs are having equal effect here) and you can work out the spring rate of the ones you've got in there now without even taking the bike to bits. I guess this is slightly flawed as to be precise you would need to raise the back of the bike to overcome the rake angle of the steering head and ensure that the forks were acting in a truly vertical plane. A certain a,mount of riction would come into play as well but oif you "bounced" the front slightly once compressed it should overcome that.

Then you come to the mysteries of damping which is influenced by oil viscosity, oil volume and the rate at which it can pass through the damping valves.

Well that's the theory, how the hell you apply it all in practice depends largely on how you like the bike set up I guess.

Probably best to go back to the basics on the spring bit, decide what you are actually trying to achieve then see what you have now and in which direction you want to take it to make the desired changes.

[skidlids]

Tonio heres my basic discription of spings

think of a spring as a leaf spring as used on older car suspension and on vans and trucks. ie a long piece of tempered steel.

If you want to make it softer you could either make it thinner or make it longer. If you then want to make it into a coil spring, think of winding it around a round former.
If using a thinner piece of steel you will end up with a coil of a certain length with a certain amount of turns.
if you used a longer piece of steel (but thicker) then you could either end up with a coil spring of the same length but with more turns and with less gap between each individual turn (care required here in case these coils are to close together and the spring becomes coil bound when the suspension travel is to much for it)
or it can be wound with the same distance between coils as the spring made from the thicker material in which case you end up with a longer spring.
Progressive springs usually rely on the more closely wound springs beconig coil bound which then effectively shorterns the overall spring length to giver a stiffer spring for the remainder of the suspension travel

[skidlids]

I do have a formula that I have used for 15 years where by you can work out a single rate spring from its measurements,
you need to input the spings OD, the OD of the wire the spring is formed from and the number of free turns.

I also have another 15 year old formula I use for selecting springs rates, mainly used when I was building specials.
This uses information
Such as the bikes weight bias and overall weight of bike and rider, amount of suspension travel available (shock or fork stroke), linkage ratio (plotting the curves is fun) and g force the wheel is likely to encounter (size of bump and speed at which it hits it)

[Paul James]

Originally Posted by skidlids

I do have a formula that I have used for 15 years where by you can work out a single rate spring from its measurements,
you need to input the spings OD, the OD of the wire the spring is formed from and the number of free turns.

I also have another 15 year old formula I use for selecting springs rates, mainly used when I was building specials.
This uses information
Such as the bikes weight bias and overall weight of bike and rider, amount of suspension travel available (shock or fork stroke), linkage ratio (plotting the curves is fun) and g force the wheel is likely to encounter (size of bump and speed at which it hits it)

That is fine if all the springs are made of the same steel Kev but if the actual material spec is different it puts the calculation out of the window if you merely use the physical dimensions within your calculation. That's why I suggested some simple practical tests of the springs you guys may have access to.

It is a minefield when you start to add on the imponderables like reaction time, severity of bump etc. so like most things in life the theory is interesting and gives a starting point to work from, in reality its always going to be what it feels like when you are out there riding it innit

[skidlids]

Trouble is paul unless you are prepaired to experiment and invest in several sets of springs to find what will work for you on each bike, you are better asking somebody that has done it. people like Maxton, Dave Parkinson, KAIS, K-Tech, etc i always used Justin at WPS until he got snapped up by Red Bull, but when it came to sorting one off specials I learnt to do it myself.
Tonio currenlty runs the standard progressive spring that he doesn't seem to be happy with and therefore wants a change. He has been recommended a couple of different options. I know his budget for DD is tight and therefore he doesn't want to waste money on the wrong springs so he is trying to understand how the options differ.
personnally I try not to race with anything other than single rate fork springs hence me recommending the Ohlins Option, No doubt these springs can be supplied in different rates and that will be down to how you like your bike setup (front/rear weight bias) and your body weight. As Tonio looks to be about the same weight as me I would suggest trying 0.85Kg/mm but its his money and therefore his choice.

I agree the Shear Modulas of the steels used to form springs varies over a range and I settled on a fixed value between the upper and lower figures to use in my equations and therefore my calculations have always been an approximation close to what is required, but as my spring supplier provides me with rear shock springs in 25lb/In steps then I usually go for the one closest to my calculation and at just over £15 a sping having to buy a second after testing the first and deciding i need slightly harder or softer is no big deal after spending £6k to £8k on building a one off special or a personalised Harris Magnum. Its always harder when you start with thin air and I have learnt to always go for a shocck with as long as stroke as can fit in the gap and get the linkage to go through a curve as linear as possible.
The linkage and shock that Harris provided for my Magnum 4 was awful, the ratio went from 2:1 to 2.5:1 and then the shock didn't have enough Stroke to cope with it. As mine was one of the first Magnum 4s later ones came with a Ohlins shock tailored to suit them and a rocker that only gave a ratio range of 2:1 to 2.1:1, Dallas had one of these later ones and it handled a lot better at speed than the early one I had, although over time i did get mine sorted