Mountain bike suspension

Discussion in 'General' started by EdwinTheMagnificent, Jun 15, 2011.

  1. EdwinTheMagnificent

    EdwinTheMagnificent Legend In His Mind

    I have a question about the front suspension on mountain and hybrid bikes. Let me give you a little background first,so maybe my question will make sense !
    When I bought my first mountain bike in 2006 , I allowed the owner of the bike shop to assist me in getting the right size frame. That Mongoose was a pretty decent bike but after a couple thousand miles Kelly expressed an interest in riding ,so I gave it to her.
    Then I bought a Trek,but by this time I was a self-proclaimed "expert" and I determined the frame size all by myself (yes, I guessed). I have about 4600 miles on the Trek now
    (bought 8-9-09) and the front suspension is pretty harsh and stiff. I don't think the fork has deteriorated,I just think it has always been very stiff. When I go back to the Mongoose,I notice the ride is PLUSH compared to the Trek.
    Now my question : Do they put stiffer fork springs in larger frame sizes assuming that the larger rider weighs more ? How can I tell if this is the case ?
    Thanks for reading this inprobably long and boring post. Have a nice day !
  2. WriConsult

    WriConsult Super Moderator

    Depending on the bike, you could have either air suspension or coil springs. Nothing wrong with coil: the springs are cheap to make, give a progressive feel and require FAR less maintenance than air, but they also add a pound or two of weight.

    If you have air, you can adjust the air pressure with a shock pump -- or, since the problem is that it's too firm, just find the schrader valve for the positive air chamber (look for a little metal cap with a "+" on it) and let a little air out using your thumbnail. These air chambers are VERY small, so letting air out for even 1 second will make a big difference.

    Many shocks also have a negative air spring which can be adjusted to control rebound, so that may require adjustment too or it may not rebound sufficiently on stutter bumps. Come to think of it, I think insufficient negative pressure could also be a cause of the excessive stiffness you describe. Make sure there's at least SOME air in the negative chamber.

    If you DO indeed have a coil fork (entirely possible) then you should have a compression adjustment knob on the end (usually the top) of one of the fork legs. Back that out a few turns and you may be able to soften things a bit. This will offer some range of adjustment; on SOME forks, bigger changes can be made by swapping out the springs internally. By the way, anytime you open up a fork it's considered an "advanced" job by most bike shops and they'll farm it out to either the manufacturer or another suspension specialist.

    To answer your question directly, the bike manufacturer will generally NOT put different springs in the forks of different sized bikes even if different springs are available. Usually they'll just bolt on the fork manufacturer's stock model with the standard spring, and assume the range of the compression knob will provide enough adjustment for you.

    Chances are pretty good you or someone will have to contact the fork manufacturer for the spare part if you want to change springs. Normally that would be you or your bike shop, but with Trek's legendary customer service I would contact them first and see if the can manage the change for you. Make sure you have your bike model number handy, and it wouldn't hurt to have the make and model of the fork written down in front of you too, when you call.
    Last edited: Jun 16, 2011
  3. rfruth

    rfruth Well-Known Member

    Being a roadie myself AFAIK, Trek uses air suspension (air & oil but all the user is supposed to add is air) is the suspension close to what is recommend :confused:
  4. corvette

    corvette just 1:18 ;-)

    WriConsult said it all.

    For diagnose it would help telling us the bike model or suspension components to further determine settings. And I'm positive you'll find the how-to setup info in the user manual.
  5. EdwinTheMagnificent

    EdwinTheMagnificent Legend In His Mind

    WriConsult,rfruth,and corvette : Thanks for your good advice. Now I have a good idea where to start on this.
  6. phoebeisis

    phoebeisis Well-Known Member

    Inexpensive bikes generally have just a sprung fork- no damping no fluid like a shock.
    They always feel softer on smallish bumps than fancier forks.
    Now they aren't as stable feeling in actual offroad type riding, but most riding is done on road, so it doesn't matter.

    You can get plusher high end forks-the plusher forks more commonly have actual coil springs or polymer "squishy" springs. Air springs just aren't plush.The spring rate increases too fast because of the small chamber.

    You could put higher volume tires(2.1-2.4") on your bike-this will allow lower pressure in the tires- and it will soften the ride a bit.
  7. GrnHrnt

    GrnHrnt Well-Known Member

    I agree. Running your tires at lower pressure may mitigate the problem. What pressure are you running them at now?
  8. phoebeisis

    phoebeisis Well-Known Member

    Slight aside

    You might be able to change out the fork fluid-make it thinner 3wt maybe- this will make it a bit more compliant.This might be a little tricky-just something to think about.

    Changing Tire pressure(and tire volume-new tires )-and making any adjustment in compression damping you can are much easier.Many forks now don't have a compression damping adj nob.

    Normally you can drop 26" tires down to 30 psi or so-depends on how tall they are-you drop the pressure too low and you'll bottom them out-get a flat.
  9. EdwinTheMagnificent

    EdwinTheMagnificent Legend In His Mind

    GrnHrnt , I am running the 26x1.95 tires at 60 psi. I'm trying(like any good hypermiler) to go further on less energy. But I ran the same pressure in my Mongoose ! It doesn't appear that the Trek fork has any air fittings,but I will look closer at the forks on both bikes.
    Phoebeisis , thank you for your advice. I will try to change the compression damping. And they are both inexpensive bikes . The Mongooose was $260 in 2006 , and the Trek was $340 in 2009. It seems like I can ride the hell out of a bike, but I have lots to learn about how they work.
  10. msirach

    msirach Well-Known Member

    It will be a sprung only suspension on the Trek. I have a Giant Cypress DX and it is spring only. Supposedly it takes a couple of hundred dollar set of forks to get adjustment. Hardcore bikers don't want suspension as part of the energy goes into the compression and not into the pedals.

    I run my tires at 85 psi on streets and a chat bike trail. It is much easier rolling with the extra 20 psi in them. It does fine but I am used to more harsh suspensions.
  11. phoebeisis

    phoebeisis Well-Known Member

    Ha,ha-I forgot we are an efficiency forum.
    Yes, dropping to 30 psi will increase rolling resistance by quite a bit.
    If it is strictly a sprung fork- no damping adj and no hydraulic fluid to change out.
    You can probably buy a fork with adj(my Manitou Black Elite for example $120 delivered)-
    But it would be a some work and some $$.

    Why not just "steal" your girlfriend's(wife?? Kelly) fork??
    If the steerer tubes are close to the same length(1/8" or so) you could switch them out-give her the stiff one-you take a soft one.

    If you have a grinder you can get some extra effective length(maybe 6mm or so 1/4") by grinding either side of the stem.

    Plan B is to use this as an excuse to upgrade your bike again.
    You can get a very nice -used- hardtail (front suspension only) for $500. Technology driven sports like MTB mean the "last years news bikes"(roughly 5 years old) are worth maybe 25% of new price(especially in the winter in cold climates- not so much in NOLA where I am).
    Wait until winter and you can get a $1500(new in 2005) hardtail for $400 or so.

    See how easily I spend other folks $$
    PS I'm also a wimp-I prefer a cushy ride-but I do it with high volume low pressure tires now.
  12. WriConsult

    WriConsult Super Moderator

    I'm a big believer in big volume low pressure myself -- for genuine mountain biking. I run 29 x 2.4-2.5" tires (yes, they finally exist now) and rarely run them over 20-25psi offroad. In fact, for a while I was using a 2.5" downhill tire on the front, with barely any air in it whatsoever -- the sidewalls were stiff enough I could get away with that.

    I usually run a suspension fork in the summer when conditions are fast and dry (and the bigger descents aren't covered in snow), and a rigid fork in the winter when it's sloppy and slow. But I'm enjoying this setup so much I'm not sure if/when I'll swap in the springy fork this year.

    It does come at a price in terms of rolling resistance though. I don't mind the rolling resistance of low pressure tires when I'm on technical singletrack, but on gravel or pavement I air them way up.

    If you want to soften the ride with less of a speed penalty, then you'll need to do it through the suspension. If you don't care, then big soft tires work just great.
  13. EdwinTheMagnificent

    EdwinTheMagnificent Legend In His Mind

    D'oh ! I had a long reply nearly finished and ready to send , but I lost it all when I tried to read the comments on the 2nd page. I will write more later today when I get back from my Father's Day breakfast. You guys have been SO helpful.
  14. EdwinTheMagnificent

    EdwinTheMagnificent Legend In His Mind

    WriConsult , you're more of a mountain biker than I'll ever be ! I do very little off-road these days,and I do not go in the snow. I try not to use gas to get to where I want to bike,but sometimes it can't be helped.

    Corvette , the fork is a Suntour SR 2025.

    Charlie , did you call me a wimp ? Ha ha ha, that's okay. I actually really liked your suggestion that I steal Kelly's fork from the Mongoose. But I'm more likely to buy another fork. Can you suggest a good online store for that ?

    Msirach , you are absolutely right. This fork is spring-only. I don't even think there is any preload adjustment,and certainly no compression or rebound damping. But you know what surprised the heck out of me ? They only put a spring in ONE LEG of the fork. The left leg is empty ! Is that the way they do it with cheap forks ? And as far as pressure, my tire sidewalls suggest a range of 45-65 psi , so I run them at 60.
  15. WriConsult

    WriConsult Super Moderator

    I think I've heard most expensive forks are done that way too.

    Take a look at the Lefty forks from Cannondale. They don't even bother with a right fork leg!
  16. corvette

    corvette just 1:18 ;-)

    It's normal to have spring just in one leg. The top of the line forks ($1000+) have the same principle, usually it's left side air/coil spring and right side damping.

    The fork listed is an entry level, so adjusments I'm afraid won't be possible with turning the knob - or at least easy to do. Maybe you could ask your bike store or some good mechanic, best bet could be replacing the spring for softer one.

    About the tires: as soon as you leave smooth surfaces, the hypermiling principles about pressure do not apply anymore. Comfort will suffer a lot. Plus the rolling resistance actually increases when driving off-road or on trails with extra high pressure. The German magazines have proved that with some measurements.

    For starting point I'd recommend 20-30 PSI for front and 25-35 PSI for rear no matter the tire or manufacturers recommended side wall. Usually the lowest point is waaay to high to have comfortable ride (but surely you won't get flat tire :)). That is of course, if the bike is ridden on anything but smooth surfaces - for tarmac you'll want to bump the pressure a little go get better RR.

    But as said, YMMV, this is from mountainbiker's perspective who often runs 7.5 PSI in the front to achieve the absolute max. traction on the way down. There are tires burly enough to allow that, but weight almost 3,5 lbs a piece.

    Feel free to check some MTB passion from the Alps:

  17. phoebeisis

    phoebeisis Well-Known Member


    I'm 60 y o,so,
    I have a very high wimp quotient- I like a nice soft ride.

    Jenson USA- they sometimes advertise here- usually has the best online prices.
    You can frequently find OEM forks(forks meant to full bikes that somehow were taken off or never were installed on an entire bike) that they sell for very reasonable prices.
    You should be able to get a pretty good for for under $200.
    Ebay is also a good choice- used stuff is even cheaper.
    There is a downside to ebay since it is a pig in a poke-sometimes you get something with a defect (usually a leaky seal).
    Make sure you measure your steerer tube length.Getting one too long is fine(just hack saw it, or use spacers-the spacers are the best bet initially-you might find you like the longer tube more upright ride)
    Don't get one too short.

    A $200 fork is probably more-much more- than your bike can be sold for- but you can always take the fork off and put it on a new bike- if someday you decide to upgrade your entire bike(one more reason to get a too long steerer).
    Don't toss your old fork-you can reinstall it when you sell it.

    Keep in mind you can buy a pretty good-excellent really- MTB- used- for $500(roughly 2.5X the fork). I just sold a Giant NRS for $400. In 2002 it would have sold-new- for $1800 or so.
    I really like to spend other folks $$$. If you do decide to entirely upgrade-wait until the weather tuns cold-and snowy if it snows where you are).

  18. EdwinTheMagnificent

    EdwinTheMagnificent Legend In His Mind

    I just got a new fork ! It's not a high-dollar unit , but it's much better than the frozen-up Suntour fork I had. It's a Rockshock with 100 mm travel. This works MUCH better and makes riding a lot more pleasant. I'm "planning" to keep this bike for a while , but I did find myself looking at the sweet 29" mountain bikes when I was at the bike shop. I think I will resist temptation for another year at least.

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