A few thoughts about progressive fork springs... - .: XT660.com

Pleiades · # 1 12-10-12, 21:18

...well quite a lot of thoughts really!

Thought it might be worth sharing what I’ve discovered with progressive and linear fork springs on the Tenere, having now tried both Yacugar progressive and Ohlins linear springs back to back. I must point out that I am no suspension expert, although I have had some advice from both FTR and Maxton. What follows is just my humble opinion based on a bit of professional advice and some first-hand experience. This may be of help to someone out there thinking about changing their fork springs, but then again you might well have changed your fork springs and be more than happy with what you’ve found (in which case I wouldn’t bother reading on!)

I never intended buying two sets of fork springs (bit extravagant), but problems setting the suspension up to work properly for me after fitting progressive springs led from one thing to another and eventually to linear fork springs.

A tale of two springs…

I fitted Yacugar 0.51-0.79Kg/mm progressive springs (the starting rate being very similar to OE) and recommended SAE15 oil last winter. I was initially well impressed, mainly on shorter road rides and a few light, low speed green lane jaunts: Less brake dive, less bottoming, more direct steering and all-round sharper handling on the road. However, after a couple of longer rides (on road) I noticed I was finding things more tiring than usual due to a slightly harsh, jittery feeling, rattling over minor bumps and imperfections particularly as speed increased. I put up with this for a fair while and more miles, trying to remind myself that the trade of was better handling, but eventually some experimentation was needed.

Initially, increasing preload helped smooth things out but at the expense of sag (the bike riding too high at the front and the front wheel much more prone to leaving the ground!) Not happy; so changed the oil to SAE10 and reduced preload, with an immediate improvement in comfort (and correct sag again), but ended up with a bouncy ride over bigger bumps and rougher terrain. Adding preload now made things worse. This all suggesting to me that I was struggling to get the damping to match the spring rate at the various points in the springs travel; Compression damping being too great with the SAE15 oil giving a harsh ride, and rebound damping being a problem over large bumps with the SAE10 oil. So I thought stuff this, I’ll shell out �100 and see what linear springs can offer. Cue Ohlins 0.60Kg/mm (6.0N/mm) springs…

Fitted the Ohlins springs a couple of months ago with SAE10 oil, the standard air gap (all as recommended by Ohlins) and wow, what a difference. Sag easily set and spot on for my weight, firmer (than stock) but compliant ride, controlled damping and no harshness in the upper part of the fork’s stroke and no bounciness in the lower part of the stroke. Dive on the brakes is a little more than the progressives, but actually I think this gives a better, more consistent “feel” through the suspension when braking. Consistent being the key word, but then the springs would be, they’re linear!

So what's the problem with progressive rate springs then?

The first problem I think is that the initial softer part of the spring just gets used up straight away leaving a limited amount of travel for the stiffer portion to deal with, resulting in less compliance, less comfort and less traction. The second problem is setting the sag to any degree of accuracy is almost impossible. The third problem (the big one) is that damping rates need to be matched to the spring rates, in particular rebound damping; with progressive rate springs this is impossible. Damping is always a bit of a compromise but adding progressive springs to the equation just makes the job so much more difficult to achieve.

Here’s the rub with the XT, its damping rod forks. Being a very basic system (my 45 year old BSA has the same design!), it’s all a massive compromise between rebound and high/low speed compression damping; fitting progressive springs just makes the compromise greater. The high speed compression damping action is not sufficiently responsive or progressive by design and only just works with linear springs so doesn’t stand a chance with progressives, especially with heavier oil which won’t flow through the damping orifices as quickly, giving a slow response. There's not a great deal you can do to alter that other than change the oil weight/viscosity, and that mostly controls the low speed damping response... In real terms, the amount and speed of dive on the brakes rather than the ability of the forks to soak up sudden jolts and bumps in the road surface. This is why I guess the first thing that everyone (me included) notices after fitting progressives and SAE15 oil is that dive on the brakes is hugely improved – but it’s not all down to the springs.

So what did changing to lighter fork oil (with the progressive springs) achieve?

The lighter oil flows a bit better and gives an improvement by lessening high speed compression damping (rapid small movements), giving a more pliant and plush ride (the perceived improvement in comfort that I felt). Unfortunately, that brings you into conflict with the rebound damping requirements (you’re not going to have enough) which is even more of a problem with progressive springs as spring rebound is not linear (the bounciness that I felt at the bottom of the stroke).

My guess is that Hyperpro/Yacugar (and others) recommended SAE15 oil to bolster the rebound damping in order to better control the return stroke with from within the higher rate portion of the spring. The downside is that it seriously hampers the compression damping (particularly as the OE fork damping orifices are designed to work with much thinner oil), making it much more aggressive in the upper part of the stroke controlled by the lower rate portion of the spring, giving a harsh ride; in effect you’re riding on the oil (compression damping) rather than the spring!

Conclusions?

If you’re an average weight rider (90Kg kitted out) or slightly less (like me at 85Kg) I‘m really not sure progressive springs are the answer, due to the fact that you aren’t going to make best use of the higher rate part of the spring and you’re going to be rattling about in the over-damped (with SAE15 oil) softer part of the stroke. I guess, maybe progressive fork springs are better suited to the heavier rider? Then again, if you’re heavier I’m still not sure they’re the ideal because to get your sag right you’re going to have squashed out most of the soft part of the spring and be riding on the higher rate part of the stroke, nullifying much of the progressive effect of the spring, so you might as well have gone for a stiffer than stock linear spring in the first place?

After trying out both spring types I’m not convinced the progressive springs really work (for me), mainly because you can only achieve the correct damping for either the higher rate or the lower rate part of the stroke, but crucially not both (or for that matter the bits in between). Adjustable high/low compression and rebound damping would be much more effective and would have cured most of woes (but unachievable with XT forks), but even clever cartridge/USD forks can’t vary damping within a single stroke to match a progressive spring. As mentioned earlier, I think the improvement that most people who fit progressive springs notice first is reduced brake dive, but I reckon that’s more likely down to the original springs that were replaced being too soft in the first place and the change to heavier oil, not the fact that the replacements are progressive.

Basically I reckon progressive springs might be an issue if…

• You’re heavy, they’ll blow past the soft part early then ride hard and too low, so you’ll attempt to increase preload to get the ride height back and make the ride even harder.
• You’re light, the damping will be too great in the soft part and ride harsh (plus you’ll never use the stiffest part anyway), so you’ll attempt reduce oil weight and end up with too little damping in the hard part and bouncing.

Of course, everyone is different, every bike and the way it’s used is different. You may think what you have just wasted the last five minutes reading is a load of twaddle! On the other had you may not? You may, perhaps, have fitted progressive springs to your beloved Ten and love them, but for me, this is what appears to work best…

• Linear springs correct for rider weight/conditions and damping oil that exactly matches the spring rate (and matches what the damping orifices in the forks can cope with at speed).
• Use the air gap/oil level (not the springs) to control bottoming resistance and progressive stiffening at the end of travel.

Or better still fit WP forks (if you can afford them)!

Notes:

Whilst asking around I found that apparently Yacugar and Hyperpro fork springs are one and the same product, made in Holland in the same factory, but supplied in a different box (and also I’m informed that Hagon and Wirth progressive springs are the same as each other too.)

All the fork oil I used was Fuchs-Silkolene, so I could make direct comparisons, also Fuchs is one of only a handful of oil companies that use SAE grading which is comparable between other brands that also use the SAE designation ( a lot use just # weight, # w and Ohlins their own numbering system all of which aren’t really comparable, although Ohlins do supply a comparison chart to SAE spec).

AndyXTR · # 2 15-10-12, 07:08

Wow, that's quite a post! Given that I'm waiting on my Hyperpro progressive springs at the moment I'm wondering if I've shelled out my cash for nothing! I've got Kev's spacers as well so before fitting the Hyperpro's I might just try the spacers and a change of fork oil first and see what happens. What volume of fork oil were you using?

Cheers.

Kev · # 3 15-10-12, 08:55

Great feedback on your experience.

Unfortunately you can't paint all progressive springs with the same brush, there are many different rated springs out there. 80% of suspension setup is a personal thing.

Gas_Up_Lets_Go · # 4 15-10-12, 09:16

Quote:

Originally Posted by Kev

80% of suspension setup is a personal thing.

I fitted Hyperpro's to the front, the difference was immeasurable. I've not experienced the same harsh front end as you have. However, I did the rear at the same time - maybe something in that ?

Pleiades · # 5 15-10-12, 10:30

Quote:

Originally Posted by Kev

80% of suspension setup is a personal thing.

+1 I quite agree Kev.

I hope I made that clear in the OP, if I didn't, then just in case...

Quote:

Originally Posted by Pleiades

Of course, everyone is different, every bike and the way it’s used is different. You may think what you have just wasted the last five minutes reading is a load of twaddle! On the other hand you may not? You may, perhaps, have fitted progressive springs to your beloved Ten and love them, but for me, this is what appears to work best…

I just found it quite interesting experimenting with the front end and seeing what could be achieved (or not). I think the long and short of it is; whatever your suspension choice/preference/setup is, there's much to be gained by having a fiddle and trying a few things out. It's surprising how much you can change/influence, even with an essentially "non-adjustable" fork setup like on the Tenere!

Kev · # 6 15-10-12, 11:01

+1

Pleiades · # 7 15-10-12, 14:01

Quote:

Originally Posted by glassona

Wow, that's quite a post! Given that I'm waiting on my Hyperpro progressive springs at the moment I'm wondering if I've shelled out my cash for nothing!

Don't get me wrong, any replacement fork springs (progressive or linear) are a massive improvement over the OE items!

Quote:

Originally Posted by glassona

I've got Kev's spacers as well so before fitting the Hyperpro's I might just try the spacers and a change of fork oil first and see what happens. What volume of fork oil were you using?

Far be it from me to tell you what to do! However, IMHO trying a fork oil change and the spacers would be a good, sensible and cost effective starting point.

As for oil volume... It's actually more to do with setting the the air gap correctly (space between top of oil and top of the fork tube with the fork compressed and spring out), the oil volume is a rough guide to get approximately the right amount in the fork to work from. With the progressive springs I started off with an 30mm larger air gap (less oil) as recommended by Yacugar in their instructions (this is mainly to take into account the larger diameter of the spring's windings (which displace more oil than the originals). I then experimented with small adjustments in level, keeping a careful note of what I'd done. With the linear springs I started with Ohlins recommendation of sticking to the OE spec air gap (which I can't remember off hand).

Just remember the golden rule of tuning suspension (or anything else for that matter) - Change just one thing at a time and record every change you make.

waynovetten · # 8 15-10-12, 21:52

I'd go way beyond 80% on the suspension thing,it was a nice read,and for anyone that hasn't got one here's my favorite piece of kit.

http://yambits.co.uk/advanced_search...tion=1&x=3&y=2

Kev · # 9 16-10-12, 02:04

I always set the air gap by measuring the oil height, as I found setting the oil level by volume = mm never equals the correct air gap height.

AndyXTR · # 10 16-10-12, 05:16

Quote:

Originally Posted by Pleiades

Don't get me wrong, any replacement fork springs (progressive or linear) are a massive improvement over the OE items!

Far be it from me to tell you what to do! However, IMHO trying a fork oil change and the spacers would be a good, sensible and cost effective starting point.

As for oil volume... It's actually more to do with setting the the air gap correctly (space between top of oil and top of the fork tube with the fork compressed and spring out), the oil volume is a rough guide to get approximately the right amount in the fork to work from. With the progressive springs I started off with an 30mm larger air gap (less oil) as recommended by Yacugar in their instructions (this is mainly to take into account the larger diameter of the spring's windings (which displace more oil than the originals). I then experimented with small adjustments in level, keeping a careful note of what I'd done. With the linear springs I started with Ohlins recommendation of sticking to the OE spec air gap (which I can't remember off hand).

Just remember the golden rule of tuning suspension (or anything else for that matter) - Change just one thing at a time and record every change you make.

Excellent advice - thanks for that. I've never played around with forks much before (haven't really needed to on my dirt bikes as the adjusters generally seem to do the trick) so looks like I'm in for a bit of a steep learning curve - not a bad thing!

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