GSXR 1000 K2 Front Suspension Very Stiff - Suzuki GSX-R Motorcycle Forums Gixxer.com
 
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post #1 of 8 (permalink) Old 06-18-2017, 01:57 PM Thread Starter
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GSXR 1000 K2 Front Suspension Very Stiff

Hi all Suzuki GSXR Newbie here.

I have a bought a K2 with K4 forks fitted but find them very stiff. Previous owner weighed approx. 170 lbs and I weigh 240 in full gear so I expected to have to adjust preload to reach correct "rider sag". To my surprise found 25mm with no preload - great for smooth roads and bends but horrendous when bumpy and after approx. 50 miles over various road conditions including heavy braking found only using approx. 50 percent of that available even with compression damping reduced to zero.

If I understand correctly that means at best forks are over-sprung so need "softer" springs. At worst I may have a compression damping issue?

Should standard Suzuki front end be good enough for me alone - I will not be riding with pillion?
Any advice would be extremely welcome and much appreciated in advance.

Last edited by oldninjaron; 06-18-2017 at 02:06 PM.
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post #2 of 8 (permalink) Old 06-19-2017, 04:19 AM
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Re: GSXR 1000 K2 Front Suspension Very Stiff

The low sag at zero preload might be explained by longer than normal internal spacers or stiff springing. The bad internal preload wouldn't explain the harsh ride, or using half the available travel, so stiff springing sounds more likely. Another, perhaps more likely, possibility, which also falls in the "stiff springing" category, would be too juch oil in the forks. This would reduce the air pocket inside them, causing the resulting gas-spring effect to be much more pronounced. If the first part of the suspension travel seems softer, but then it quickly stiffens to the point of locking, then consider just changing the oil making sure to get the level right, and see if that helps.
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post #3 of 8 (permalink) Old 06-19-2017, 03:02 PM Thread Starter
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Re: GSXR 1000 K2 Front Suspension Very Stiff

Thanks for the info - I did not realise that oil height/air gap could have that much of an impact on performance but if combined with high rate springs it is understandable. By the way I am currently riding with no compression damping, fully out with mid range rebound damping.

I have what have been described as standard original fork internals for K3/K4 forks including original fork springs etc as a back up. My idea is to send forks in for oil swap and examine internals and if it turns out that indeed over sprung then could fit original fork springs at the same time. That is of course if standard springs would be suitable for me at 240lbs in full riding gear, no pillion, for road riding only?

Last edited by oldninjaron; 06-19-2017 at 03:05 PM.
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post #4 of 8 (permalink) Old 06-19-2017, 06:31 PM
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Re: GSXR 1000 K2 Front Suspension Very Stiff

At that weight (which is close to what I weigh), I find it unlikely, both statistically and practicality, that you would get a heavily oversprung suspension. For my K4 I went with 10N/mm springs and it was the stiffest K-tech had on offer for that model. Race Tech suggest about 1.1kg/mm springs for your weight and the stiffest available is 1.2kg/mm.

Oil height on the other hand, can have a very large contribution to total stiffness, regardless of spring rate. Normally, this should only be the case when the suspension is near the end of its stroke, but a seemingly small change in oil level can change that.
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post #5 of 8 (permalink) Old 06-21-2017, 01:19 AM Thread Starter
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Re: GSXR 1000 K2 Front Suspension Very Stiff

Performance on the road which is hellish, you come back from a ride in pain because suspension at the front is incapable of absorbing even the slightest bump and the back is too soft and totally out of synch with the front - then you find the only smooth bit of road in the area and its heaven. That is my main issue of course but my inabaility to set sag for my weight etc has to be springs as with the bike static surely oil level should not play a part IMHO?
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post #6 of 8 (permalink) Old 06-21-2017, 04:11 AM
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Re: GSXR 1000 K2 Front Suspension Very Stiff

Quote:
Originally Posted by oldninjaron View Post
That is my main issue of course but my inabaility to set sag for my weight etc has to be springs as with the bike static surely oil level should not play a part IMHO?
That depends. The air doesn't have anywhere to go, so it has to compress as the suspension compresses. As one can see from operating a hand-pump with the outlet blocked, a relatively big volume of air, is very easy to compress initially, but gets progressively harder (exponentially). A smaller volume of air will start out a bit stiffer and get much stiffer, much faster. So it all depends on how overfilled it is. If it were full, the suspension would have to compress the oil to sag, which it can't, so you'd have essentially a strut. If it's not entirely full, it might be, that there's little enough air, that it can support yours and the bike's weight, without having to compress enough, to get proper sag.

Is it likely? Well, I suppose it's not unlikely. I'm not sure of a figure for the proper air volume in there, but it's not much and, in any case, all it takes, is a botched up fork service job. On the other hand, a linear spring would have to be pretty stiff to support you at a sag of 25mm with no external preload, unless either the internal spacers, or the springs are significantly longer, than they should be. I don't think you'd be able to find one. Can you compress it much farther than the sag point, by pushing down on it, or does it more or less lock up soon after 25mm of sag?
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post #7 of 8 (permalink) Old 06-21-2017, 02:39 PM Thread Starter
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Re: GSXR 1000 K2 Front Suspension Very Stiff

Bike forks use up approx. 45mm, on the road, approx. half of remaining during ride as per cable tie on fork. Effectively my weight plus ride uses up 70mm (25+45mm). Compressing forks with brake on can get relatively close to cable tie within about 25mm.

Measurements as follows for Rider Sag: No preload.

Front wheel in the air: 115mm
Sat on bike in full gear: 90mm
Rider sag forks: 25mm

Remainder theoretically available 90mm of which 45mm used including v bumpy roads and heavy breaking - probably great for nice smooth track.
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post #8 of 8 (permalink) Old 06-21-2017, 06:58 PM
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Re: GSXR 1000 K2 Front Suspension Very Stiff

Those numbers don't fit the wrong oil level hypothesis, as far as I can see. This looks more like a somewhat too stiff and too long spring, or perhaps a way too stiff spring, but I think you'll have to open the forks and have a look.
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