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Essential GPX250 Modifications

Discussion in 'Modifications and Projects' at netrider.net.au started by jd, Jul 29, 2012.

  1. When I bought my GPX I really only intended it as a cheap commuter, and didn't really forsee seeing spending any money on it other than the usual maintenance and running costs. That plan didn't survive for long....

    A lot of the deficiencies in the GPX are somewhat understandable given that it was made to a price, and for new riders with no experience of anything else some of the problems may not be that noticeable (at least not until they upgrade to something bigger and get stuck with the perception that all 250s are crap).

    So I thought I'd share some of the modifications that I think should be made to all GPXs, even if people are only planning on keeping it for the 12 months or so of their restrictions. I'm not going to bother with details on how to do them, as this pretty well covered elsewhere (search for Ninja 250 FAQ on the 'net).

    1. Suspension
    This is easily the the biggest deficiency with the GPX, the front suspension in particular being truly awful. The fact I could fully compress the front suspension with minimal effort just stopped at the lights was certainly worrying, though it does at leadt allow you to do a fairly good impersonation of a rocking horse to freak out the car beside you :). There is heaps of information around suggesting things like heavier fork oil or packing washers/coins in to compress the springs, but these only mask the problem. The only real fix is to replace the front springs with ones which have a higher spring rate (ie require more force to compress). I sourced mine from Sonic Springs in the US and would certainly recommend them, service is good and they come with instructions on fitting as well as how to adjust the oil level in the fork to suitably change the degree of dampening. Price was around $130 delivered. Other brands may also be good, but just make sure you're getting "linear" springs, not "progressive". Installation is no more complicated (or expensive) than changing the fork oil - which is something that needs to be done periodically anyway. The improvement is definitely noticeable, as it makes the front end feel a lot more stable through corners and reduces the amount of diving under hard braking.

    The rear suspension is not quite as bad as the front, though it's shortcomings will become far more noticeable if the front end is upgraded. The ultimate upgrade is to source a new shock unit from someone like Hagon, but you do pay accordingly (around $500-600 which I consider overkill for a road bike). There are a variety of shocks from larger bikes which can fit, but sourcing these could be tricky. Personally I just went for a rear shock from a current model Ninja for a number of reasons: It's designed for a bike virtually identical to the GPX, it fits perfectly without any modification (two bolts and only takes 20-30 minutes to fit), it adds preload adjustment and a stiffer spring, are reasonably easy to find with low kms, and is a realtively low cost solution (I paid $70 for a near-new shock, and another $70 in shipping but only because I was impatient). As with the front this certainly improves the stability of the bike, especially under acceleration on uneven surfaces (like inner city streets).

    2. Brakes
    The standard brakes on the GPX I consider to be quite adequate for road use given how little it weighs. However given that every GPX is now at least 5 years old if yours still has the original brake lines then you should give some serious thoughts to fitting braided brake lines. If purchased from somewhere like HEL they are still road legal, but are also cheaper than the OEM rubber lines AND offer an improvement in braking performance (they also last longer than the rubber lines). Installation is pretty straightforward, the only tricky part is bleeding them. Price is somewhere around $80-100, though hard for me to remember as I bought braided lines for my car at the same time (also money well spent).

    3. Tyres
    It is possible to fit slightly taller tyres to a GPX to significantly increase the range of options available, but personally I wouldn't bother unless it's a track bike. Dunlop GT301s were designed specifically for the GPX and I certainly can't find any problem with them. If your GPX has Dunlop K630s however you should get rid of them immediately for two reasons: they're what was fitted in the factory and may now be over 5 years old, and they're complete crap.

    4. Aftermarket Mufflers
    Are a complete waste of money for road use. Even if you can get the jetting right all you'll achieve is a very slight increase in top-end power, most likely at the expense of useability. If you want faster acceleration - change the sprocket ratio. If you want a higher top speed - rent a Commodore.

    5. Engine
    See 4. If it's running okay the leave it (and the carbs/intake) alone. You'll more likely to make it worse than better.

    6. Ignition
    Iridium plugs are worth considering, although I consider their claims to increase horsepower to be complete crap (unless comparing new iridiums with used copper plugs). However I also find it a complete pain in the arse getting the plugs out of a GPX engine, so if the iridiums mean I can avoid doing that anytime soon the money will be well worth it. A full set of Iridium plugs can be sourced from overseas for around $25 - which is not bad given I've seen places here charging $8-10 for standard copper plugs (and ridiculous amounts of money for identical Iridiums).

    That's all I can think of at the moment, though I will come back and update this post if I find more things I think are worthwhile (I have fitted a luggage rack and am currently working on fitting driving lights - but certainly wouldn't consider these essential). Feel free to add anything you think I may have missed, as long as it's not something purely cosmetic. I'm actually also curious to hear any stories of modifications people may have made which actually made the bike much worse.

    With that much text I think this thread needs a picture :)
    • Like Like x 1
  2. I miss my GPX. Good list of thinking rider mods there JD.

    I seem to recall that the springs in the GPX were meant to be a variable spring rate jobbie - the rate going up the more they were compressed. They were supposed to get stiffer. Was there any hint of that when you had the removed spring in your hands?
  3. Stock springs had an even spacing between the coils, so barring some sort of really sophisticated heat treatment I'm going with linear :).
  4. What's the difference between braided lines and the OEM ones?
  5. Wouldn't that suggest a variable spring rate? An even number of turns per inch would be a linear spring rate. My spring theory is very sketchy though... 25yrs since Mech 101 at uni.

    Unbraided rubber lines swell with pressure, so some of the lever effort goes to the swelling, giving a little bit of spongeyness to the braking feel. This gets worse with age.

    Braided lines on the other hand are quite stiff so all the lever effort goes to the caliper.
  6. That's great jd, thank you for the tips!

    What spring rate did you choose, and for what weight/load on the bike? Not too stiff now?

    A small suspension upgrade sounds worth the effort and I'm going to look into it for my GPX (same colour & trim as yours!) :)
  7. :-s Surely an even number of turns per inch is the same thing as an even amount of gap per inch? Either way that's what the stock springs have.

    Progressive springs are these sort:
    Where the amount of gap (or number of coils) changes to produce a variable spring rate - which means the dampers only really work well over a very tiny range of fork travel.

    I went with 0.7kg/mm based on the result I got from the spring rate calculator on the Sonic Springs website:
    Wet weight for a GPX is 350lbs, but you'll have to work out your own weight and riding style (I'd really only choose between normal or relaxed though - keep in mind that it was designed for all bikes and "agressive" on a 250 is not the same thing as it is on a 1000).

    The increase in stiffness does make bumps in the road slightly more noticeable, but on the other hand it also goes a long way to eliminating the "loose" feeling in the front end (ie when the steering feels light yet has little effect over where the bike actually goes). Of course if you go too stiff it can make the bike twitchy and potentially quite dangerous - but Sonic do apparently make springs less than 0.7kg/mm on request if you're a really light rider.
  8. Arrrrrgh, I read "an even" as "uneven". Sorry for the confusion. :/

    That's interesting then that the spring wasn't progressive. It used to be a selling point of the GPX. There you go.
  9. awesome list of mods. well thought out and brimming with common sense too.

    Kudos for putting suspension first... that's the biggest "failing" of the GPX250 as I see it, and the first thing anyone should look to fix if they plan on keeping their gippy for a reasonable amount of time.
  10. Great summary jd, and much of it would be applicable, at least in principle, to any older bike (y)
  11. Fixed.
    No offence to anyone over the age of 30 with one or a sports wagon.
    But other than that. We can not be internet friends!</3