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  1. #1
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    DHX RC4 for Solar Car - lowest spring rate?

    I'm on a college solar car team (NU Solar Car Team , we compete in the American Solar Challenge) and we're using Fox DHX RC4 bike shocks for our solar car, which has 2 wheels in the front and 1 wheel in the rear. Our car weighs 620lbs with a driver inside, and weight is rearward biased, but I don't know how much off the top of my head.

    Last summer we had 300lb springs in the front and a 450lb spring in the rear. The suspension barely moved (especially the front) and I think we need to experiment with lower spring rates.

    Looking around it seems like 300lb is already on the low end of the range. Does anyone know a good place to find springs which are lower than 250lb for the front? If not we might need to switch to RP23s (which we totally should have used in the first place since they're lighter)

    DHX RC4 for Solar Car - lowest spring rate?-dsc_9625.jpg

    Size is 7.875x2.25 I think, can go in and measure tomorrow.

  2. #2
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    i'm just speculating, but i think you'd be looking at needing a custom spring since anything that low would be completely useless on a bike.

    air shock seems the way to go

    but that is way cool!

    what are you gonna do with those?

    I am talking out my ass now, but I wonder if you could change the leverage rate of the suspensions design to match the springs you already have?

  3. #3
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    Thanks!
    These shocks are installed in a solar-powered car, we (college engineering students) design a car and race it against cars that other colleges made each summer.

    i'm going to get 250lb springs and see if those work, if not, we'll order air.

  4. #4
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    Those are pretty high tech shocks and I think you have a few different problem. Other than spring rate, the shocks are likely over damped for the the situation. Full downhill bicycle shocks in the 7.875x2.25 configuration are going to come with a high compression tune. You're likely going to need a much lighter spring (think 100-150lb range) if you want to get any usable travel out of the shock.

    I think you would be wasting money on getting the 250lb springs. The suspension must have something around a 1:1 leverage ratio to need such a low spring rate.(the damping would be set up for something closer to 3:1) Its just the wrong shock for your application. You are going to be better off getting a DHX air shock so the spring rate would be infinitely adjustable. The damping is mostly controlled by the boost valve chamber on the DHX air shock as well, so you wont have to worry about it being so over damped if you run the minimum pressure in the boost valve. It will also save you a few pounds in the process.

    If you go the air shock route, make sure to get a shock pump as well.

    Hope that helps

  5. #5
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    thanks for the advice, do you think that getting 7.875x2.00 RP23s would be different than getting DHX Airs? They're a lot cheaper and I don't think we'll need the DH features or durability since we're only traveling on paved roads.

    Nobody on our team is a mountain biker, and I didn't start riding until this summer, so I don't think anyone understood the differences between the shocks back when we were purchasing them...I heard that we just got the best/most expensive, haha.

  6. #6
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    For your application, There is no reason for a lock out feature on the shock. I would think a float R would be the best/cheapest option.

    Fox Float R Rear Shock | Fox Racing Shox | Brand | www.PricePoint.com

    It will still be over damped though, causing some issues with a slow return rate after it compresses. If you are willing to pay MSRP, you could contact fox and see if they would custom tune some new ones for you. either way, it will be an improvement over what you have.

  7. #7
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    Okay, thanks. I saw that earlier today and we might end up going with the one on that exact page for the front, then move one of the 300lb springs to the back. Need to do some more experimenting though, after the car gets reassembled...

    Do you think that setting for 20-25% sag, like is standard for MTBs, is a good way of determining whether our air pressure/spring rate is correct?

  8. #8
    moaaar shimz
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    Quote Originally Posted by mullen119 View Post
    For your application, There is no reason for a lock out feature on the shock. I would think a float R would be the best/cheapest option.

    Fox Float R Rear Shock | Fox Racing Shox | Brand | www.PricePoint.com

    It will still be over damped though, causing some issues with a slow return rate after it compresses. If you are willing to pay MSRP, you could contact fox and see if they would custom tune some new ones for you. either way, it will be an improvement over what you have.
    Even with low air pressure the shock will rebound pretty damn fast when rebound is set to full open.

  9. #9
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    Quote Originally Posted by tacubaya View Post
    Even with low air pressure the shock will rebound pretty damn fast when rebound is set to full open.
    I don't see this happening, my wife has a vanilla rc with a 2:1 leverage ratio and a 300lb spring on her bike and set at full open, the rebound was way to slow. I had to re shim the rebound to get it to speed up enough to be usable. The spring rate just wasnt enough to force the shock back to full extension in a reasonable time.

    op, I think setting sag to 20% is a great starting point. Adjust accordingly to get the feel you are looking for.

  10. #10
    moaaar shimz
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    Quote Originally Posted by mullen119 View Post
    I don't see this happening, my wife has a vanilla rc with a 2:1 leverage ratio and a 300lb spring on her bike and set at full open, the rebound was way to slow. I had to re shim the rebound to get it to speed up enough to be usable. The spring rate just wasnt enough to force the shock back to full extension in a reasonable time.

    op, I think setting sag to 20% is a great starting point. Adjust accordingly to get the feel you are looking for.
    Float R has 400 PSI nitro charge helping it rebound.

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