Progressive rear spring on 640 Adv???
Next year I'm doing what is probably all of our dream trip, and riding London to Cape Town. I will be riding badly corrugated roads loaded with the usual junk on the back including TTech boxes, cooker, tent, the little carpet I'll pick up in the Moroccan markets etc etc...
Anyway, I rang a suspension expert here today, and asked what he'd recommend in the way of back springs. I figured he'd probably advise something like an upgrade from the 70/260 to an 80/260 - instead, after he'd rung the WP Suspension importer, he suggested that I go for a 70-90 progressive spring.
Firstly, anyone know how the 640 would go with a progressive spring? Does the linkage already make the spring rate progressive with a linear spring?
I figure that the loaded sag of the bike is going to be pretty high with a spring starting at 70 (sorry, I've got no idea what units the 70 is:shog . 70kgs/cm???), and would probably need a fair amount of preload to set it up right - would this create spring bind like it can on a mountain bike? Any other negative effects?
Anyone with any knowlegeable advice as to what I should put on will be really appreciated. I don't want to go fast, I just don't want my shock to blow up in in Timbuktu (literally). FYI - I probably weigh close to 100kgs fully kitted for riding (220 for those who think in old school).
One more thing - is is possible to change the spring from the shock body yourself, just by backing off the preload, or do you need the special spring compressor tool?
Get a hold of ZeroDog on the forums here.
He has supplied quite a few front and rear springs for 640E and 640Adv owners.
I have ordered some springs from him a couple of weeks ago and just waiting for them to arrive in NZ. The rears get made to order. About a 2 weeks delay.
ZeroDog sorted the springs for another forum member called bmwktmbill before he rode though europe/asia.
I had the suspension professionally reworked on my 640A and it works very well on all road surfaces, loaded or unloaded. It is a big improvement over stock especially on dirt or washboards and on pavement as well.
Both the front and rear springs are straight wound.
The correct spring is in play and ZDog is the man for the Adventure bike, having done the most research. My bike was lowered 4 inches so I am a special case in that respect. I ran a 76 spring and it wasn't enough. I weigh 70 Kg and the travel luggage was around 40Kg.
Aother member named Luke is doing some very special work right now from an engineering standpoint.
this winter the spring problemwill be solved.
My spring was fine for potholed gravel and small to medium bumps but was not enough for big G-out /whoops.
Get an accurate weight for yourself and the gear you are going to pack and give Rob a chance to work on it for you.
I also ran sub-tanks and a steering damper.
Rob reworked/revalved/rebuilt my shock.
It worked much better in corrugations and stutter bumps when he was done.
It als stood up to the pounding of Russian and Mongolian roads.
I saw half a dozen blown rear shocks.
The WP progressive springs the tuner is probably thinking of are slightly wider than the 640 springs, if a centering spacer isn't added, the spring will rub the shock body, which is bad.
Spring rates are quoted in either kg/mm or N/mm. kg/mm is the most common, but N/mm is the correct unit from a proper physics standpoint.
1 kiloram is 10 Newtons (sitting still on earth) so an 80N/mm spring is the same as an 8.0kg/mm The 70-90 spring is in N/mm. Oddly enough, 1N/mm is about the same as 1kg/cm.
Bill and I are still hashing over the numbers, but the first stab looks like the
linkage has way more progression than the recommended spring. As in, comperable to a 50 to 100% increase in rate. By comparison, the 70-90 spring has a 28% increase.
You need a tool to change the spring.
The progressive springs are designed for shocks with progressive damping- ie PDS.
On the practical side I'd recommend an 80 or 85N/mm straight rate spring in the back, depending on how much luggage you carry and what you change the fork springs to.
I'm trying to do this trip on a bit of a budget, so rebuild/revalves of shocks, and full setups of forks are luxuries that I'd prefer to try to avoid. This may well be something that I regret later, but these bikes are meant to be built for this stuff right? At least that's what the salesman in the shop told me at the time!
This is probably totally flawed thinking, but I wasn't going to touch the front. The vast majority of the weight being carried is on the back wheel, meaning rear wheel sag is going to be pretty high, and so the soft spring on the front would balance it out. Right? If it's diving too much under braking, then I'll just add a bit of compression damping. (I also have a steering damper fitted along with Loaded's submount)
My thoughts now then are that I run an 80N/mm linear spring on the rear with the stock fork setup at the front. Anyone think this is a really really bad idea? It's got to be an improvement on the standard spring...
Zerodog does this stuff professionally right? I didn't want to bug him, as I can source the springs and work here in England, and asking him what I should be buying from someone else might be stretching his altruism too far!
Don't skimp here...
Mate, you will need to do the front as well.
A well-sprung rear with a soggy front is not a recipe for a god time. And the front is only sprung for a 70kg rider.
Seriously, the biggest payback you will get for your bucks is getting the suspension sorted, springs AND damping, by a real expert. Added safety, much more comfortable, way better handling, less fatigue (you're not wrestling the bike on rough stuff), blah blah blah. Just do it.
A progressive rear spring would be for KTM's link-less suspension, not the linked like on the LC4. The linked "KINDA" makes it progressive.
I can't believe a WP guy would tell you that. WTF??? The starting rate is way way too light anyway. Even if it did fit.
I would say stick to the straight rate spring. The linkage does the progression on the LC4. The LC4 sping is also 59mm ID. The normal PDS springs for other KTMs is 62mm ID. It doesn't sound like a lot but it really is when you put it on the shock. The 62mm ones are all over the res.. I think even with a centering ring of some sort on the shock colar it would rub like crazy.
The stock springs are appropriate for a 100lb dude. I don't know why KTM ships them this way and doesn't even offer springs in high enough rates.
For your weight I would say you need around an 8.4 in the back. But with the load an 8.6 would probalby be ideal. And a .52 for the front. I know this sounds stiff. But it will make your bike ride really nice and also not bottom every time you hit a dip or cross a wash.
If you spring the back you need to do the front. Your bike will ride horribly with only doing the back spring. It will make the head angle steeper and cause head shake at speed. Also as far as the weight being on the back wheel? The 640 A is really front biased. They dive like crazy under braking because if it. So at the very least you need the correct springs for your weight. If you really want to go for it you need to build a real midvalve in the fork. This is the best mod I have done to adventures to help control the dive on and offroad. You notice it the most when going down a steeep hill offroad. With stock valving the front tends to ride low. The bars want to turn to one side or another and it is hard to control. With the midvalve mods the front stays up and you can ride it more like a real dirtbike and charge down a steep hill.
Sorry for going on and on. Anyway I can get you springs if you need them. If you are out of the country it just takes a while and the shipping kind of sucks. But the rear springs I do for the Adventures are custom length and rates to fit the application. If you source the spring to a spring maker where you live let me know and I can give you some specs to get what you need.
One thing Zdog/Rob doesn't say here is that if you can rebuild your carburetor, you can alter the valving in your fork. It just isn't that hard or that complicated. If you ever changed fork seals you are experienced, if you haven't now is the time to learn.
Rob has published a midvalve shim stack(in other threads) that really works.
Shims and WP fork oil are available from MX Tech, they are friendly and the service is superb.
I will caution you to ask them fr the correct shims, it is easy to get confused.
You need a CHEAP micrometer (I bought one at Sears for $10) and the usual hand tools.
WP forks are really fun to work on, they are simple. The work is clean and you can do it in the living room but if you spill fork oil on a light carpet...you will replace it.
Mine was shot anyway.
The rear shock is a different story and needs to go to the pro for a revalve and nitrogen.
Rob set mine up with a regular tire type air valve so I could rebuild it and air it up in the third world but it was never necessary(the air we breathe is 70% nitrogen).
The WP system is the best there is for toughness, you just have to massage it a little for peak performance.
The fork is worth the hassle. After it s done you forget about it.
BTW I talked to the WP factory engineers t the KTM festival in Budapest this fall.
They are good guys but they don't know anything about the Adventure and made the same wrong spring recommendation to me.
The spring issue is a frikkin mess.
KTM knows the answers but the information is locked up in their racing department and so far they refuse to talk with anyone about it.
How the pivot suspension works is also not understood by most of their factory engineers, it is not understood by any of the dealers I have run in to.
God knows how KTM raced the Dakar with 30-40 Kilos of extra fuel.
I tried at the Festival and I could not get it out of anybody or find anyone who knows the specifics.
This is really frustrating because they won, they know.
Somewhere there is a block in releasing the information.
This just doesn't make any sense since the new PD racer is so different.
But damned if I know what shock the 690 PD bike is using.
Or for that matter the 690 Supermoto.
Since it's in this country we will be able to get a look at the shock and spring and the pivot if it has one.
If anyone is near a dealer would you take a picture of the rear suspension on the Supermoto and post it.
I am going to start another thread asking for any information on and pictures of the new suspension.
As others said, the progressiveness is built in to the stock suspension linkage. If you look at where the bottom of the shock attaches, it isn't bolted to the swingarm, it's bolted to a little network of levers. The levers are there to create the progression.
The stock springs will work. Most people never touch them. A stiffer front end will help most on downhills and in deep sand. A stiffer back end will help with a heavy load. It's also important to keep the springs balanced- being soft on one end and stiff on the other is probably worse than being soft on both ends.
Since you're on a budget, I'd say start with the stock springs, load up the bike and try to ride it in as many different conditions as you can. If you don't like how it handles in a given situation, fix it. Get all the free adjustments (preload and damping adjusters) dialed in, then if that isn't good enough start changing parts.
BikeDirt should be able to check his spring rate when he checks his sag eh? If the shock spring rate is good for his weight (including loaded luggage if appropriate) then he will be able to set the static and dynamic sag to specs. If not, then he will know its time to change the rear spring.
Preload won't work. He will end up with two inches of preload and the shock still won't give right numbers when it is fully loaded with gear and full of gas, and it will ride like a truck. It won't work right in the stutter bumps or as the Brit calls them...corrugations. It will be harsh, in the woops it will bottom and it will headshake in heavy gravel and sand.
What is the real pisser is that Touratech charges three thousand dollars to put WP suspension on the BMW 650 GS.
Since we already have the premium suspension really need to make it work right. It might be a little work and money but nothing compared to what the other brands require.
To have a suspension with the high quality of WP and that won the Paris Dakar race but won't work on a rough gravel road at 60mph/100km/hr does not make sense.
And to have WP provide incorrect information or stonewall us witholding information or just fail to take an interest in our problems is counterproductive and dangerous.
KTM does a great job for the racers but the adventure rider so far does not get the same level of support even though we must be a big part of their sales.
Can anyone explain that?
|Times are GMT -7. It's 01:36 AM.|
Powered by vBulletin® Version 3.8.5
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
Copyright ADVrider 2011-2014