URT suspension design revisited

Discussion in 'Mountain Bikes' started by Robocam, May 14, 2013.

  1. Robocam

    Robocam New Member

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    I came across this thread from a Google search.

    http://www.cyclingforums.com/t/29174/urt-sucks

    It started getting interesting after this post but it looks like it quickly went off-topic after that.

    http://www.cyclingforums.com/t/29174/urt-sucks/15#post_186630

    I'm curious for more input from people with strong physics backgrounds or a strong understanding of physics.

    In the original thread, people were claiming that the rear suspension is inactive when the rider is standing, and I find that claim to be erroneous. The only way you're going to lose your rear suspension is if you're standing directly over the rear wheel (directly on the hub bolt). Otherwise, the triangle can still pivot.

    It seems to me that the reason people feel a loss of rear suspension activity when standing is because they're transferring their weight towards the front of the bike, so naturally, less weight is placed over the rear wheel, so the rear suspension feels less active. Maybe tuning the suspension with the rider standing would alleviate this problem?

    What I am curious about is what exactly are the problems with URT, and how have modern suspension designs overcome those problems? If the loss of rear suspension activity is in fact due to a weight shift, how does a change in suspension geometry make a difference? The weight shift is still there isn't it?

    Here is a link showing a few suspension designs.

    http://ffden-2.phys.uaf.edu/211_fall2010.web.dir/Michael_Stanfill/UnifiedRT.html

    What I've found so far is that one con of the URT is the varying BB to seat distance.

    What do you all think? The reason I'm wondering about this is because I've recently decided to upgrade my URT bike, and I was wondering how important a frame upgrade might be. I mainly use my bike for light trail riding (no jumping), so I think I'm probably ok with leaving my frame alone but I am very curious about the physics.
     
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  2. vspa

    vspa Active Member

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    as a road bike cyclist, where every bike manufacturer and every wheel manufacturer (and even shoes brands) strive to offer the best rigid products possible (without sacrificing other variables), for the simple reason that all the power produced by the rider should be converted into motion power pushing you forward, it is not easy to be attracted to a dual suspension bike, i do have a road bike friend that made a transition into freeride mountain biking where he uses a dual suspension bike of course. im sure modern mtb with URT are amazing to ride on offroads, but they are also quite expensive,
     
  3. singletrackmac

    singletrackmac New Member

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    If you have a URT frame then your bike sounds like its's from the late 90's or early 2000. I would get a new frame. Anything new or newer (mid 2000 or later) will be noticeably better. 
    The only manufacturer (not counting department store bikes) that I could find that still makes a URT type design is mongoose. $2,500 for their top of the line  ride. http://www.mongoose.com/usa/teocali-expert-19699 Not too expensive for mtbs these days. However it is not a true URT. The only place i know of that you can get a new true URT designed mtb is Walley-World, K-Mart and similar stores. This alone should tell you all you need to know about the URT design. As far as the suspension only working when sitting, i think people say that because there is no shock obsortions to the pedals when the rear tire hits a bump or rut, no matter if your sitting or not. When you are standing you feel every bump and shock from the rear wheel in the pedals. When you sit, you still feel every bump and rut in the pedals but your ass doesn't. So it tricks you in to thinking its full suspension when you are sitting. To me a URT bike is not a full suspension bike. It is a faux-suspension bike. The reason I say this is because there are 3 points of contact on a bike. And a true full suspension bike obsorbs shock on all 3 points. A URT only provides shock obsorption to 2 of the 3 points, the handlebars, the seat, but not the pedals. This is because the BB is part of the the rear triangle, not the main frame. No shock absortion to the pedals is sort of a big issue when taking corners on rough terrain and trying to get your center of gravity low. In order to coner hard and fast as possible, i am sure we all agree that you need to use the right technique. As we all know this involves pushing down on the inside side of the handlebar, rotating the pedals so the outside pedal is at the lowest point and then shifting as much weight as possible onto that outside pedal. When taking a corner on rough terrain using this technique on a URT design bike, the back end bounces all over the place since your outside pedal weight is not suspended. And when the rear end bounces around the pedals follow since the BB is part of the rear triangle. Take a look at the picture from the link you posted, http://ffden-2.phys.uaf.edu/211_fall2010.web.dir/Michael_Stanfill/UnifiedRT.html Imagine the rear axle hitting a 4" rock. When this happens the rear axle moves up 4" forcing the BB to move violently as the triangle rotates around the pivot. How violently it moves depends on how far the BB is placed from the pivot. But no matter where it is, if the BB part of the rear triangle and not the main frame, then the peddles do not have shock absorption. It's like mounting a car engine directly on to the axle.  Another issue is that the rear triangle eliminates all chain growth. This means it cannot be used to reduce pedal bob. The pedal bob is also a major issue with the URT design.  Today's advanced single pivot designs use chain growth to their advantage to reduce pedal bob as well as (on single pivot designs) dig the rear tire into the ground for more traction than a hardtail or any other design when climbing in the lowest gears. Chain growth can cause pedal feedback, but most people naturally stop pedaling when hitting bumps large enough to cause noticeable pedal feedback, so this really is not an issue.  Hope this helps.
     
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