Fat bloke needs help picking rear shock for 300 lbs rider (dual shock possible?)

HD_rider · Mar 15, 2016

I just found this forum via google. Totally new here so hello everyone.
I am making a heavy duty mtb/dh style bike. Almost ready but not sure what to do for rear shock. This build is to help me exercise more efficient and away from traffic and noise. I am changing my ways and working for a better tomorrow.

So down from 360 lbs to 300 lbs. Tired of biking on paved roads and as I now have less trouble breathing air while biking I thought it was time to get serious with building a new bike.

Did a custom frame, carbon copy of a used DH frame. With thicker tubing, extra triangle and thicker beefier swing arm. Geometry should be the same, only a little more heavy.

In my youth I loved riding single tracks, jumping, etc. But today I am still too fat to enter that style of riding. Yet. But I will do single tracks, and I will start to ride backcountry and gravel/dirt roads to get "out in the woods". And firelanes etc.

But as a big bloke I need good suspension. The fox 40 was rebuild locally with stiffer spring in the one leg. According to the shop it will work well for my weight now.

For rear shock I struggle finding a shock that can handle my weight. I know there are aftermarket Titanium springs available that can be custom build to handle more weight. But those where so expensive. And truth be told I have kind of blown my budget already without solving the rear shock/spring issue.

Rather then getting a custom Ti spring, could I get another Fox rear shock and with the help of two custom brackets install two shocks/springs on the swing arm and frame? I got the real estate to spare - just don't know if that will work.

It seems most rear shocks will work very well for 150 lbs riders. So easy peasy math if one Fox shock works for 150 lbs two shocks should work for 300 lbs rider?

I don't care about squat, efficiensy or all that. I just need a rear shock solution that work well for my weight. The extra weight of two shocks means nothing to me. All I am after is to avoid bottoming out and recking the shock.

So what do you think? Will dual rear shocks work well?

Ciclistagonzo · Mar 15, 2016

The only way to answer this, is to see what type of leverage ratio the frames suspension is using. Do you have a picture of the frame or any idea what the leverage ratio is?

charging_rhinos · Mar 16, 2016

First, congratulations on the progress! Much respect. So a lot of this will depend on what your frame will take. To clarify, were you saying that you are using a custom-built DH style frame? What is it a copy of? Like was mentioned before, that will help a lot. If it's a more common leverage ratio (often somewhere between 2.4:1 to 3.4:1) it should be pretty easy to find you something that will work. In your case, a lower leverage ratio will mean getting a lighter spring.

But in general terms, I'd do a coil shock all the way. There are plenty of heavy spring options for most brand-name shocks that will work for you. Just slap on what they recommend for your weight, and you'll be good to go.

The damping abilities of the shock will be a slightly more complicated choice. Being a rather brawny lumberjack of a guy myself, I really like the adjustability of the Cane Creek Double Barrel. You can tweak the damping to suit everything from a mosquito to a mastodon. They are pretty expensive and they have to be sent in to CC-authorized shops for service, but the performance is worth it for me. Fox coils didn't have fantastic damping range when I ran them, but that may have changed in the last 4 years. Not much experience with RockShox myself.

YetiBear · Mar 16, 2016

Lost 60 lbs.? Nice. Fat Bloke, no, Big guy, yeah!! Can't wait to hear your progress when you get out on the trails.

HD_rider · Mar 16, 2016

YetiBear said:
Lost 60 lbs.? Nice. Fat Bloke, no, Big guy, yeah!! Can't wait to hear your progress when you get out on the trails.

Thx. I m not tall. Just fat and small

The bike frame that was used for mockup was a specialized Big hit. A few years old, like 5-6 or so.

When the swing arm was custom made I made sure the linkage had room for extra bracket if going for dual shock. Same with the anchor point in frame.

Mockup looked like this, the custom frame is the same only stronger.

HD_rider · Mar 16, 2016

Not sure about the ratio, I just weld after I a sketch. Head tube angle and other angles are just as the bike used for mockup. Same for swing arm. Everything stayed the same but I have the possibility to make a custom bracket to allow for double rear shocks.

When riding with backpack, clothes and the weight of frame, rims and tires I think we can say sprung weight is 320 lbs or so. I was talking to a guy in the local shop about the CC double barrel, he said he would not trust longevity and performance of that for more then max 250 lbs rider.

I got one Fox dhx4 shock with standard spring, and I can get a second one exactly the same but used for cheap. I am thinking I just pick that extra shock up and give it a try.

Does anyone know what is the most weight to put on a standard fox dhx4?

Ciclistagonzo · Mar 17, 2016

So it has a linkage driven shock. The information we need now is, what length Shock and Stroke of the shock you plan to use and how much travel you expect the bike to have. It's hard to tell in the pics but it looks like the are using a roughly 2:1 travel to shock ratio so it's still well with the operating parameters of a quality coil over shock. A Fox Vanilla RC with a 600 lbs spring should work.

I come from experience, I once clocked the scales above 300lbs and used a bike with 2.6:1 ratio. I used a 650 lbs spring and it worked well.

HD_rider · Mar 17, 2016

To be frank I don't understand the concept of ratio in this setting. Is the ratio the relation between actual swing arm movement and rear shock movement due to linkage? How can I measure the ratio? Let me know and I will measure up tomorrow.

I know the front Fox has 8" travel. I think rear shock was 9" x 3" if that is what you asked for. But I have no clue for suspension travel for the rear. Can this be measured too?

Remember that this frame is not aluminum, it is steel so much heavier then the original frame. That combined with heavy rims and tires and a heavy rider I am not certain a single shock can take the punishment.

charging_rhinos · Mar 18, 2016

You're on the right track with your definition of ratio. It's a mechanical advantage ratio based on the distance from your axle to the linkage's main pivot, versus the distance from that main pivot to the shock. Kinda. It's hard to explain without drawing it out. The leverage ratio is sometimes a bit hard to measure. On most bikes, the leverage ratio actually changes a bit as the suspension is cycled. So the rear wheel is hanging out there, away from the main linkage pivot. That distance acts like a lever that can offer X-amount of torque. In a 2.8:1 ratio, the other side of the linkage that compresses the shock would only have about 36% of that X-value (1 divided by 2.8), because its lever arm is much shorter. So in order to balance the shock's force with the leverage of the rear triangle, you would need (in theory) to have a shock spring rated to 2.8 times more than the total weight being supported.

That might not be a good explanation, I apologize. But it's all I've got at this late hour. So if you've got major concerns about the shock not holding up, I'd just give Fox a call. Tell them you have a Big Hit with a 3:1 leverage ratio, and you want to know if 1) you can get a heavy enough spring for your weight, and 2) if the shock will stand up to the abuse. They will be able to tell you what you need.

butthead · Mar 18, 2016

I've always understood leverage ratio as the vertical wheel travel divided by the shock travel. It's a generalized number used to tell you how hard your shock is working. The higher the ratio, the heavier the spring you will need for a given rider weight. A higher number also means that the shock internals are moving relatively faster. In the case of the Specialized you have pictured, it has 7.5" of rear wheel travel and a 2.5" stroke shock, giving it a 3:1 ratio. These are the numbers I found in a few posts about the Specialized BigHit. I couldn't find any official information on their website.

My guess is that you should be okay with a 650 pound spring to get the sag set correctly. As far as which shock, you're probably okay with most manufacturers. From what you described, you aren't going to be going out and pushing it to its absolute limit right away. You're looking to build a burly bike that will hold up to a big rider on fire roads and easier trails until you've shed a few more pounds. I would think any shock would be up to that task. If you've kept the pivot points the same, you should need an 8.5 x 2.5 shock. Again, these are numbers from a few message boards, not official information from Specialized. Like charging_rhinos said, it may be a good idea to contact shock manufacturers or a suspension shop like Push Industries and see what they recommend.

Fat bloke needs help picking rear shock for 300 lbs rider (dual shock possible?)

HD_rider

Ciclistagonzo

charging_rhinos

YetiBear

HD_rider

HD_rider

Ciclistagonzo

HD_rider

charging_rhinos

butthead

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