ATC 250SX Rear Shock Alternatives

[barnett468]

To calculate compressed spring rate of straight wound spring.

Measure the extended length of the shock from the center of each eye.

Compress the shock until it just touches the bumper.

Measure the length.

Measure the length of the bumper and divide by 3.

Take that number and multiply it by 2.

Take that number and subtract it from the compressed length of the shock. This will be your approximate fully compressed length [within 1/4"].

Measure the length of the spring.

Subtract the total compressed length of the shock from the length of the spring.

Multiply the spring rate by that number.


EXAMPLE

Spring free length 12” minus shock total compressed length of 8” = 4” X spring rate of 300 lbs. per in = 1200 lbs. when shock is fully compressed.

[barnett468]

Hello Flyingw;


As you may know, if you have a dual rate spring that is 400 lbs. on the high rate for example, it could be anything on the lighter rate such as, 150, 200 or 250 etc..

Also, if you have two equal rate and equal length springs like 150/400 lb. from different mfg’s etc., one might have 2 inches of 150 lb. spring rate while the other might have 3 inches of 150 lb spring rate.

The one with the longer light spring rate will have a smoother transition from the light rate to the stiff rate. This is because the longer light rate spring provides a wider “transition” phase than the other. It will also transition to the heavy rate at a later time in the travel.

We tested one dual rate spring on the 85 T3 prototype that had such a big difference in spring rates on a dual rate spring, and such a short light spring, that it felt like a rock when it hit the transition phase.

If the free length and compressed length of both springs is the same, the one with the longer light spring rate will have less compressed rate than the other. It will also get stiffer later in the travel as mentioned.

Now, if one adds any type of progressive ratio suspension to the equation, it complicates calculating the "curve" of the spring rate much more.

The easiest way I know of to determine the installed [with shock extended], and compressed rate of any dual rate spring is simply to compress the spring to those distances using the method I described previously.

[ps2fixer]

I kinda figured that reading the thread, because you didn't mention having to add nitrogen to the first red/gold shock purchased. Now after adding nitrogen to the second one you got you mentioned it helped along with adding preload...is that still the case that the second is now comparable to the first or has
performance diminished with time in the saddle? I'm also a heavier rider than you at 240lbs with all my crap on. I'm in the middle of converting to a TRX swinger and since I have to fabricate a lower shock mount I might as well upgrade so the added leverage will, all other things remaining equal will make the shock seem softer still. Thoughts?

You have a lot of options when your making your own mount, besides being able to change the length of shock within reason, also the point at which it is mounted to the swing arm. Every inch out will effect how soft the shock feels due to the changes in leverage from the pivit point of the swing arm (great old days of math class lol). In theory if you liked your old suspension and want it to act similar, you would want to keep the location at the same ratio from the pivit point as the OEM swing arm. Here is a quick example, I don't have the measurements at hand.

Say the stock swing arm is 30 inches long from center of the pivit point and the center of the rear axle, and the center of the shot mount bolt hole is 8 inches from the pivit point, the ratio would be 26.66% out from the pivit point. Say the TRX is 4 inches longer, so 34 inches length would give your mounting point at about 9.07 inches from the pivit point. This isn't a perfect way to figure where to make the mounting point, but atleast gives you a good idea. If you want it softer, say "10%" softer, my logic says you could multiply the raito by 10% and get your new ratio (26.66 * 1.1 = 29.33%). Just remember, not only does it effect how soft it rides, but also effects travel distance on the shock vs swing arm, farther out makes you loose distance unless your shock is also longer... makes everything extra complicated lol.

Just to point out, I don't specialize in math/suspenion or anything like that, so my math could be way off, but in my head the logic is right.

[sledcrazyinCT]

Did anyone buy one of the Kazuma dual rate shocks? It makes sense to avoid a remote reservoir even though the battery box mod is straightforward, but losing the tool box on a big red too much of a trade off.

Next time a see one of these junk china chassis on craigslist local I might have to talk them into selling the shock cheap

[Flyingw]

I don't know enough about shocks to speak intelligently about how they work so I defer to guys like Barnett and PS2fixer. I just want to know what shock we could buy and put on our SXs that gives much the same ride. A little softer would be nice. The Chinese shock I put on my SX works pretty good and makes for a nicer ride but as I said, that shock is a fluke. I do know the length is critical for the SX so that's a factor to consider.

[ps2fixer]

I actually don't know much about shocks , just put forward some math or physics into the logic.

From my understanding, the actual shock its self is what softens the ride, and the spring is the load ability and load range the shock can handle. With a serviceable shock where you can change the fluid, I think they work about like the front forks where you can put in thinner oil for a softer ride, and thicker for a stiffer ride. For a direct bolton interchange, besides the shocks referenced in this thread, I don't really know of any with out buying/risking it not working.

[GSUwhiskers]

Sooooooo, there isn't an aftermarket shock that can be bolted straight onto the SX?

[dreux13]

Ok...my '86 SX shock just s*** the bed. Does anyone have any updates on this to simply get a shock that may even be comparable to stock? If I had a choice of buying used shocks on eBay 'hoping' I get a good one and using a Chinese one that is known to be close, I'd be happy.

Hoping someone has some info on what has worked and thanks Jim for bringing the conversation this far!

Here are a few I'm looking at...also, do any of the other trike shocks work?
http://www.ebay.com/itm/221496765351
http://www.ebay.com/itm/291096911242
http://www.ebay.com/itm/221409252497

-Dreux

[dreux13]

Ok...my '86 SX shock just s*** the bed. Does anyone have any updates on this to simply get a shock that may even be comparable to stock? If I had a choice of buying used shocks on eBay 'hoping' I get a good one and using a Chinese one that is known to be close, I'd be happy.

Hoping someone has some info on what has worked and thanks Jim for bringing the conversation this far!

Here are a few I'm looking at...also, do any of the other trike shocks work?
http://www.ebay.com/itm/221496765351
http://www.ebay.com/itm/291096911242
http://www.ebay.com/itm/221409252497

-Dreux

In doing more research, it appears a 300ex 97- present may work. I used this works fitmgnt guide as a reference with some google searches. Looks like works still makes them for the SX if you're willing to spend a ton of money!
http://www.worksperformance.com/pdf/app_guide/atv.pdf

[Flyingw]

The SX shock is 365mm eye to eye with 10mm eyes. I have a Chinese shock on my SX and I love it but this one I have was an anomaly. I bought another just like it and put it on another SX and it was way too soft. Also, the gas res needs to be on top facing to the rear putting the battery box in the way so I had to move my battery box back. What I have works well but it took some work. I tried a 340mm shock with eye extenders getting me to 360mm but it to was too soft.

[dreux13]

The SX shock is 365mm eye to eye with 10mm eyes. I have a Chinese shock on my SX and I love it but this one I have was an anomaly. I bought another just like it and put it on another SX and it was way too soft. Also, the gas res needs to be on top facing to the rear putting the battery box in the way so I had to move my battery box back. What I have works well but it took some work. I tried a 340mm shock with eye extenders getting me to 360mm but it to was too soft.

Thanks Jim...I've been following the thread and am aware of your freak one-off shock! I have a lead on a used 300ex shock locally for 45 and based on research, all I need to do is make a special bushing to make the 12mm holes 10mm. Based on reviews, this should give me a as close to oem as possible.

[dreux13]

Just an update. I went to pick up the 300ex shock and the guy had them in a bin.....lots of different shocks and I'm pretty sure that the one I got isn't from a 300ex since it was only 14" eye to eye with 10mm eyes. The shock fit, but the SX sagged immediately! The guy is looking for another shock in his inventory.

While he's doing that, I came up with some alternative options that should match up to the SX. I'm 98% sure that a 300ex shock will fit once I actually get one. I read that the blaster shock will fit a 300ex and I also stumbled on the Suzuki ozark rear shock which is the same dimensions as the SX.

So many options! I'll post an update once I get something to try.

[jeswinehart]

Having recently done a 300EX quad to trike conversion I wanted to post up how a 2004 300EX stock shock mounted looked + measured at.
The trike is strapped down but not in the way it pulls on suspension. I have had 300EX shocks on my 84/85 Tecates and it is a nice upgrade,,,, not perfect but a heck of a lot better then any stock Tecate shock in fine condition IMO.
These pics are center to center bolt hole with normal machine weight on the shock.
Hopefully this will be of some aid since this conversion is the result of me giving up on a better suspension for my 85 Bigred so hopefully there will be something to gain for those wanting to apply to the 250ES Bigred since they share many parts. I have had SX's in the past and again IMO, they are a tick better suspension wise then the ES Bigred, still has to be a stock bolt on improvement w/o spending several hundreds of dollars for a custom shock.
Definitely following this thread, very interesting this topic got kicked back into the spotlight !

john

[dreux13]

Having recently done a 300EX quad to trike conversion I wanted to post up how a 2004 300EX stock shock mounted looked + measured at.
The trike is strapped down but not in the way it pulls on suspension. I have had 300EX shocks on my 84/85 Tecates and it is a nice upgrade,,,, not perfect but a heck of a lot better then any stock Tecate shock in fine condition IMO.
These pics are center to center bolt hole with normal machine weight on the shock.
Hopefully this will be of some aid since this conversion is the result of me giving up on a better suspension for my 85 Bigred so hopefully there will be something to gain for those wanting to apply to the 250ES Bigred since they share many parts. I have had SX's in the past and again IMO, they are a tick better suspension wise then the ES Bigred, still has to be a stock bolt on improvement w/o spending several hundreds of dollars for a custom shock.
Definitely following this thread, very interesting this topic got kicked back into the spotlight !

john

Thanks for the pics John....man that thing is looking sweet!

[Flyingw]

That shock looks promising