Boxerworks.com

Major Softie wrote:

Chuey wrote:What means this word: "modern"?

Chuey

Post Bronze Age. You probably missed the transition.

No, mine is a five speed. I don't have a kick starter but it would be cool to have one, huh?

Chuey

Just to finalize this one......18/20mm preload was still far too stiff. I ended up with about 10mm and using 7.5w high spec oil, the forks do now work properly whether or not the bike is loaded for traveling.

Fun was right about stiffness being very sensitive to preload.

I would not do this again, I have a useless pair of racetech springs @£100, and the modifications to the damper rods means that I can't go back.

blitz wrote: If you go by the RaceTech recommendations, you get a spring rate MUCH too stiff (something like 0.8kg/mm) and a spring much too short. Why? Because the racetech system is set up for motorcycles with 5" of travel.

Your airhead has about 8" of travel. Short, stiff springs will limit you to about 5" of travel, and when that travel is used up, you get a mechanical bind (spring coils touching) instead of the hydraulic/bumper cushion. No good.

Since this thread has been revived, I was reading through it again. The above stood out. If Race Tech is selling to airhead owners, they should make some relevant instructions or products or whatever. To sell us stuff that is generic without informing us of the what's what, makes their product less than attractive to us. It could end up being downright dangerous in some cases. They should do better. Either make it relevant or say, right up front, that their product is not for airheads.

I'd love to see something posted here from Race Tech on this.

Chuey

Chuey wrote:

blitz wrote: If you go by the RaceTech recommendations, you get a spring rate MUCH too stiff (something like 0.8kg/mm) and a spring much too short. Why? Because the racetech system is set up for motorcycles with 5" of travel.

Your airhead has about 8" of travel. Short, stiff springs will limit you to about 5" of travel, and when that travel is used up, you get a mechanical bind (spring coils touching) instead of the hydraulic/bumper cushion. No good.

Since this thread has been revived, I was reading through it again. The above stood out. If Race Tech is selling to airhead owners, they should make some relevant instructions or products or whatever. To sell us stuff that is generic without informing us of the what's what, makes their product less than attractive to us. It could end up being downright dangerous in some cases. They should do better. Either make it relevant or say, right up front, that their product is not for airheads.

I'd love to see something posted here from Race Tech on this.

Chuey

I'd love to know if blitz actually knows what he's talking about. How is a spring made stiff? Fewer winds. A shorter stiffer spring does not usually coil bind with less travel. In fact, it usually has more travel. I'm not saying blitz is wrong, I'm just saying that I'd like to know if he knows this from actual experimentation, or if it is a supposition.

Here's the basis for my claims above. You can decide whether i know what i am talking about.

Number of coils is just ONE factor in spring stiffness. Spring OD and wire ID also play a role.

K, the stiffness of a helical spring (force/distance) = d^4G/(8*D^3N), where d = spring wire diameter, G=modulus of rigidity (E = 2G(1+mu)), where E = Young's modulus (30x10^6 for steel), and mu = Poisson's ratio, ~0.3 for steel),
D= (spring OD+ spring ID)/2, and N is the number of coils in the spring.

This is all you have to play with.

NOW, design your spring with your desired rate, which will be constant over the entire length of the compression of the spring. In the case of an airhead, you want ~8" of travel. How big the OD is dependent on the ID of the fork tube; your spring OD can't be bigger than the ID of the fork tube. Limit 1.

Now, you want a particular rate. Your tools now are d (wire diameter), and number of coils: TWO variables. We have FOUR remaining design parameters: compressed length (bound), free length, spring rate, and maximum allowable stress in the spring.

How do you play the game? If you want a high rate, you need few turns (small N) and a bigger d. If you use TOO FEW coils (small N), when you fully compress the spring, you get very high stress levels, which produces PLASTIC deformation. You compress the spring all the way, you won't get your spring to return to its prior free length. Thus, stress in the spring gives you a lower limit on the number of coils you can have if you don't want plastic deformation at full compression.

The other alternative is to increase d, but if you increase d, then you reduce the amount of compression that spring can handle before it bottoms out (binds). (i.e,, as you increase d, you decrease the difference between free length and fully compressed length).

Ahhh, optimization.

You also have limits on free length and compressed length. You want a spring rate which gives you ~28-30% sag. Given the length of the fork (and space for your spring), front fork loading for desired sag, you can compute your rate. You have to check your rate against free and compressed length (and stress) to see whether you can fit that spring in your fork.

For an airhead, the best you can do is a spring of rate of about 0.7kg/mm if you want to use your full 8" of travel and get reasonable sag. (That's the tradeoff of N and d). Any more than 0.7kg/mm gets you too high of a spring stress level at full compression. (Stock springs are ~530mm in free length, cause that's the space we have available in the fork). You can do the math.

Now, compare that spring (0.7 kg/mm, 530 mm free length) to what Race Tech recommends for an airhead (at my weight of 180 pounds) gives you a spring of 0.87kg/mm and a free length of 425 mm. This means that you have to use a SPACER of 100 mm (4") to get a free length of 530mm. You CAN'T compress a spacer, but you can compress a spring.

If you don't use the spacer, you've lost 4" of travel right off the bat. I don't want that. Do you?

Thus, Racetech limits the amount of travel you're going to get out of your airhead to about 5" before you get the spring to bind (again, you can't compress that spacer). They have to use a bigger d (wire diameter) and increase the number of turns to get the stress levels at the reasonable rate. Bigger d and more N means the spring binds sooner. They use the higher rate 'cause they know they've reduced the amount of travel you're going to get before you fully compress the spring. For a given N (and all other things being constant), a shorter spring gives lower stress at full compression. That's why race tech sells you short springs and a spacer. Rate is OK, but they limit travel (compression) to limit stress. The tradeoff is that they limit suspension travel.

Race tech manufactures a specific set of springs, and when they do rate calculations, they assume a fixed maximum stress level. They sell you a particular common spring, and have you space it for your application. It's cheaper for them that way.

So, they give you a rate, a spring free length, and a spacer. They design the spring so it doesn't produce plastic deformation at full compression, but they also give you the spacer which eats up travel as it doesn't compress.

As with anything, there are tradeoffs. I prefer my full 8" of travel, and a lower spring rate which gives me a decent sag setting and good response to what i see on the road. Maybe a racer who races on smooth race tracks wants a higher rate spring and less travel 'cause he/she doesn't need more. Modern street bikes seem to work just fine with 5" of travel. A new Kawasaki Ninja 1000 provides 4.7" of suspension travel. However, modern street bikes have much more sophisticated damping cartridges, and are able to control the attitude of the vehicle with significantly more precision that either a damping rod fork or one with an emulator valve.

Hopefully, that helps.

Blitz, I think I understood everything you're saying (I know I understood all the generic part that would apply to all springs and applications), but I missed where the info is that shows the Race Tech spring's compressed length is such that it will bind after a certain amount of travel, rather than why you think it must be so.

The stress factor is not a constant. Advances in metallurgy have led to springs of quite different design than the ones being designed when Airheads were new. My 06 Duc had fork springs with fewer winds spaced further apart than anything I had ever seen - not even remotely like anything I'd ever seen before. Admittedly, that fork also had room inside for a much larger OD spring (which, as you pointed out, is an important factor), but it was still a very different design than vintage fork springs.

Perhaps I misread your post, but it appears to be an argument showing the knowledge base that led to a supposition, rather than showing that you actually are certain that the supposition is true.

If I missed something, I apologize and will be happy to have you point out where it was.

Ooops. Sorry. I failed to provide a piece of information.

Please go to http://www.racetech.com/page/title/FRSP ... %20Springs and search FRSP 2643, the spring recommended for airheads.

Under column "min travel," you'll see the number 137 mm. That means that their spring will support at least 137 mm of travel. Maybe more, but at least 137 mm.

That's 5.4 inches. Airhead fork travel is 8 inches (~200 mm).

I decided not to buy these springs as they support only 70% of the maximum travel of our forks. I prefer using all of the travel. I can't comment on N, d, and material properties of the FRSP 2643 because i didn't buy them. Their specification was enough to dissuade me.

This is not to disparage RaceTech. I have Paul Thede's book, and have used his gold valves and springs in my older dirt bikes. His spring will "work," but it is not optimized for our airheads. I chose a spring optimized for the application.

I suppose someone who did buy the race tech FRSP 2643 could provide measurements, and we could compute. I only used the information provided by RaceTech to make my decision.

My supposition is that Race Tech's number is correct. I assume that the manufacturer would provide an accurate specification.

blitz wrote:Ooops. Sorry. I failed to provide a piece of information.

Please go to http://www.racetech.com/page/title/FRSP ... %20Springs and search FRSP 2643, the spring recommended for airheads.

Under column "min travel," you'll see the number 137 mm. That means that their spring will support at least 137 mm of travel. Maybe more, but at least 137 mm.

Okay, you've got me completely on-board now.

In the end I removed the emulators, even with cutdown BMW springs, I could not remove the jarring ride over potholes at low speed.

But even after going back to stock I still have issues and my problem might be more with 110 year old stanchions, taking a bit of a shape after all those road miles.

So my spec is late Ate fork dampers with the stiffening ring fitted, RT springs And I will start with aereoshock oil. For those of you in the UK Wemoto sell Italien made stanchions for a lot less than Motobins.

chasbmw wrote: For those of you in the UK Wemoto sell Italien made stanchions for a lot less than Motobins.

I have purchased Honda bits from Wemoto and they happily and quickly ship overseas.