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Old 02-06-2007, 12:06 AM   #1
ehatcher OP
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Springs: Progressive vs straight questions

Having the forks on my BMW Dakar revalved with RT gold, the suspension guy stated that he wanted to put .85 springs in, but he could not source them for my bike. I mentioned that there were a couple of companies making progressive springs, he said that he felt straight springs would work better, and that he would rather me stick with the stock springs than go with progressives.

Most of my riding is backroads and gravel with some off road here and there.

So two questions: Anybody know where to get .85 springs for this bike? and, any thoughts on progressive vs straight in this particualr application?


Eric
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Old 02-06-2007, 01:29 AM   #2
PeterW
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Sonic. Wilbers. A few others, all around the same price.

Progressives have a better feel when you aren't riding 100%, straight work maybe a bit better IF you can get the damping right for the surface.
My gut feel is that progressives probably do make sense for a dual-sport bike - you don't know what the hell you are going to hit and you can never get the damping exactly right anyway.

I know I'm happy with the progressive rate spings in my bike and I'm doing everything from commuting, to near track like roads to gnarly dirt. What I'm not doing is fiddling around getting everything just right for the next 10km of road on a 1000km trip :).
FWIW - the stock shocks on the older F650's used to be progressive or at least dual rate - not sure what they are now.

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Old 02-06-2007, 01:33 AM   #3
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Somthing to keep in mind

is how much travel do you have overall.

If the progressive spring is too soft you will get into your sag really quick and eat up the progressive portion of your travel getting on the bike.

Bikes with more travel may benefit better from that type of a spring but shorter travel bikes like a DL650 or non Dakar BMW might have better overall performance with straight rate.

Just my .02 since I played with both on my DL's
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Old 02-06-2007, 05:46 AM   #4
Baggi'tard
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RT nor Lindemann Engineering will suggest progressive springs because you can't develop damping rates around changing spring rates. The only time that too-light initial rate might work well is when you are floating the front wheel off the ground, otherwise you'll sag right through it. On big hits the too-stiff final rate will just make the ride even harsher over big hits. And again the damping you decided works good for the middle 50% of the spring/ride will be to slow at the light rate and too fast at the stiff rate.

my 0.02 too.
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Old 02-06-2007, 06:08 AM   #5
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PeterW suggested Sonic, there website has an excellent tech article on progressive vs straight rate, which I have quoted below. Nice explanation of why the trend is away from progressive (thanks PeterW, I emailed them to ask about custom springs)
Over the last 20 years, one of the biggest changes in the aftermarket motorcycle suspension world has been the virtually complete switch from progressive rate springs to straight rate ones. I think itís safe to say that thereís not a single reputable suspension tuner who advocates progressive springs. The question is why? Whatís the problem with a progressive rate? Why are straight rate springs better?

One of the problems is that bikes, street bikes anyway, just donít have enough travel to take advantage of progressive rates. The soft initial portion gets blown right through, leaving a limited amount of travel for the stiffer portion to deal with. This results in less compliance, less traction and a harsher ride. Another issue is damping; Damping rates, particularly rebound damping, need to be matched to the spring rate. With a progressive (i.e. variable) rate, thatís impossible. Damping is always a compromise and a progressive rate just makes the balancing act that much more difficult. Adding to the problem is that modern forks actually have 2 spring mediums, the steel coil and the air trapped inside. The air is intrinsically a highly progressive spring. Adding a progressively wound steel spring to the mix is just making a bad situation worse.

http://www.sonicsprings.com/catalog/...ch_article.php
Downloaded 2/6/07 from Sonic website


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Old 02-06-2007, 07:13 AM   #6
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Hi Eric,

Nice job on your bike project.

I put in the Race Tech emulators in my Dakar forks, and they made a world of difference in terms of front end compliance. The back end still sucked though, it was very harsh, so I ended up upgrading the entire suspension to the TT - WP Kit. They are straight rate springs.

The tech article you referenced is dead on with medium to short travel bikes, and I would also tell you (having just gone through all this with my KTM 400) that the progressives are much more difficult to dial in.

A lot of the progressiveness of the suspension has to do with the linkage ratios for the shock mount and the internal dampening. Keep in mind that the MAIN purpose of the spring is to hold the bike up! It is NOT to dissapate the energy of the bumps - that's what the internal valving does - and why the inside of the shock gets hot (yes the springs certainly absorb some energy).

So, with this said, when setting up your suspension, you want about a 20 to 30% sag when sitting on your bike. You want less static sag (just the weight of the bike). I dont recall all the details, but there are some good online articles about setting up your suspension (or you can go to KTMTalk or ThumperTalk and browse the suspension sections).

The first task is to get the proper spring to set up your sag. Once this is done, all other suspension tuning is done to the compression and rebound dampening.

As mentioned before, with a progressive spring, the last part of your travel is harder (harsh) and the spring wants to return the wheel to the ground much faster - so it makes the rebound dampening much harder to dial in (think harsh pogo stick).

With this said, my recommendation (for your bike) would be to leave the stock springs in your forks (for now) and adjust the pre-load (for sag) with spacers (add or remove) in the top of your fork tube. You have reduced the weight of your bike, so there is less static sag now on your front anyway (20 to 30 lbs is a huge difference). I weigh 225, and the sag on my stock springs (on the Dakar) was about right - stock; it was just a poorly dampened system for heavy off-road use. So unless you weigh a LOT more than me, I would try adjusting the stock springs first.

I would also recommend changing the rear shock to the Ohlins. (I like my WP's, but adjusting the spring pre-load sucks on this shock - otherwise it is a beautiful shock).

Good luck.
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Old 02-06-2007, 12:32 PM   #7
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Quote:
Originally Posted by BuckRider
my recommendation (for your bike) would be to leave the stock springs in your forks (for now) and adjust the pre-load (for sag) with spacers (add or remove) in the top of your fork tube.
From what I am researching today, I think that is very sound advice. I can order custom springs from Sonic later if I have to. I weigh about 185, and the bike is lighter too (as you mentoned), so we will see how it works.

The rear shock is going to get a slightly stiffer spring and different oil, the tuner is looking into what else he can do. I use the preload adjuster quite a bit on the stock shock for luggage and two up etc, an Ohlins or Wilbers with that feature would be a lot of money, so for the moment I will see what can be done with the stock. The WP setup is not out of the question for me, but it may be overkill. If it turns out I am happy with a reworked stock setup, I will stick with it.

Eric
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