Question about rear shock length

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Question about rear shock length
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Posted: Feb 15, 2009 at 16:18 Quote
Hi.

Just me thinking...

Anyway, I just took my shock out of my 2004 Stinky to clean it up, put in a stiffer spring, etc. I was looking at my eye to eye measurement (7.875 stock) and was wondering if I could put a longer shock in for a bit more travel. The wheel drops another inch and a bit after I take the stock shock out, and I did a theoretical measurement with the shock out. I could fit in a 8.5 inch eye to eye shock fairly easily I think. When I set the wheel into position so that the bolts are 8.5 inches apart, there is still some room before "topping out."

Would I be able to do this? It's a 2004 and I am not the original owner, so I am not worried about voiding the warranty, etc. Seems logical to me, but I am not a mechanic so maybe there is something I'm not considering. Looking at the 2004 Tech manual, the only suspension difference I see between the Stinky (6" travel) and the Stinky Deluxe (7" travel) was the eye to eye measurement of the rear shock (8.5 inches for the Deluxe).

Posted: Feb 15, 2009 at 16:37 Quote
A few years ago I recall reading a letter in one of the MTB mags (I want to say it was in Mountain Bike Action) where someone had asked basically the same question.

Basically the response was that "yes" you'd get an additional suspension travel but you won't actually be getting an additional rear travel.

I also recall that the editor discouraged doing this because it would most likely alter the BB height; making it higher. He also went on to say that the extra travel is there to help the shock "recover" and prevent it from packing up and the "correct" way to have an adjustable rear travel is to change the shock mount location.

Again, this was a few years ago (4+ years) and I don't know how much truth there is to this.

That said, if you have access to another shock, I'd say give it a try and see how it feels. I would think that you would have to run a lighter weight spring otherwise it will just be too stiff.

Posted: Feb 15, 2009 at 17:08 Quote
rkuhn wrote:
Basically the response was that "yes" you'd get an additional suspension travel but you won't actually be getting an additional rear travel.

Hmmm... I think that might depend on the style of rear suspension. On my bike the longer shock would push the end of the swing arm higher, moving the wheel lower (by about an inch), adding to the travel.

rkuhn wrote:
I would think that you would have to run a lighter weight spring otherwise it will just be too stiff.

Maybe... but I'm a fat bastard and need a 700 lb spring, I'd have to try it. Unfortunately, I don't have access to an 8.5 inch shock, hence my checking first.

Thanks for your input, sounds like it might work, but anyone else?

Posted: Feb 15, 2009 at 17:19 Quote
A longer eye to eye shock will do nothing for the amount of travel, it is the stroke that effects travel.

All in all, this is a bad idea. If you were to look at the frame specs for those two bikes, you would likely see different stay lengths, rocker lengths, all that good stuff, not just a different shock size.

If you put a bigger shock on your frame it will alter the geo in a rather bad way, it will steepen the head angle, raise the bb, and all kinds of bad things.

The only bike that I know of that you can just stick a bigger shock on is the older Specialized Big Hit Comp's. In 05 for sure, perhaps other years, the comp and expert frame are identicle in every way other than the shock, the comp comes with an 8.75x2.5" shock, and the expert comes with an 8.75x2.75" shock. In this case, you can actually stick the bigger of the two shocks on the comp and keep the exact same geo and the whole nine yards, but get that extra little bit of travel.

Posted: Feb 15, 2009 at 17:25 Quote
Don't do it. It will make your headangle steeper, and your BB higher. It will handle like crap, especially for DH and FR, if you screw with geometery.

Posted: Feb 15, 2009 at 17:27 Quote
jonbikes wrote:
A longer eye to eye shock will do nothing for the amount of travel, it is the stroke that effects travel.

Can you explain the physics behind that? Not sure I can wrap my head around it.

Posted: Feb 15, 2009 at 17:40 Quote
Yeah, sure.

The eye to eye is the over all length of the shock in it's fully extended state.

The stroke is the distance that the piston can move in one direction, so a stroke of 2.5" means that the shock can compress 2.5". Clearly your bike has more than 2.5" of travel however, so you arrive at your final travel of 6 or however many inches your frame has through the linkage, which gives you a leverage ratio, meaning that for ever one inch of travel you go through on your bike, your shock only compresses .3 or .4", allowing that 2.5" stroke to turn into 6" of travel at the wheel.

So technically you could have a shock that is 8.5" long, with a 1.5" stroke, a 1.75" stroke, a 2" stroke, and so on. The two lengths are not dependent on each other.

Posted: Feb 15, 2009 at 17:58 Quote
jonbikes wrote:
Yeah, sure.

The eye to eye is the over all length of the shock in it's fully extended state.

The stroke is the distance that the piston can move in one direction, so a stroke of 2.5" means that the shock can compress 2.5". Clearly your bike has more than 2.5" of travel however, so you arrive at your final travel of 6 or however many inches your frame has through the linkage, which gives you a leverage ratio, meaning that for ever one inch of travel you go through on your bike, your shock only compresses .3 or .4", allowing that 2.5" stroke to turn into 6" of travel at the wheel.

So technically you could have a shock that is 8.5" long, with a 1.5" stroke, a 1.75" stroke, a 2" stroke, and so on. The two lengths are not dependent on each other.

Thanks... That part I already know, though... It's just the stroke length adding to the travel and not shock length that I can't seem to grasp. How would just adding stroke length increase the travel? To me, the longer eye to eye would cause the rear wheel to drop down further, creating a lower starting point for the travel. The stroke would be lengthened as well (stock size is 7.875 x 2.0, I was thinking 8.5 X 2.5), so the travel upward would increase as well.

As for slacking the head tube, I'm already running a 170mm fork (Super T) as opposed the stock 150mm (Drop-off), so wouldn't it cancel that out. It's not that I don't believe you, I'm sure you know more than I do, I'm just trying to wrap my head around the whole thing. Physics and Geometry were not my forté in High School. Redface

Posted: Feb 15, 2009 at 18:11 Quote
OK, lets remove the stroke. Without the stroke you could have a ten foot pole in there and get no travel.

Try thinking of it like a fork, you can have two forks that are the same axle to crown height with different amounts of travel. You can't get travel out of something that doesn't compress.

Posted: Feb 15, 2009 at 18:11 Quote
Discussion is good, so to add to the debate...

I just re-checked the frame specs for that year, I have already researched that part.
Here they are:
3006956

3006956

Stinky and Stinky Deluxe are on the very bottom. Seat stay and chainstay sizes are identical, except for thickness.

Posted: Feb 15, 2009 at 18:16 Quote
Could I bother you to post a picture of both bikes, preferably from the manufacturers website, and both the same year?

One could say I am lazyWink

P.S. in case you are wondering why, this would be so that we can get a decent comparison of the rockers.

Posted: Feb 15, 2009 at 18:18 Quote
jonbikes wrote:
Could I bother you to post a picture of both bikes, preferably from the manufacturers website, and both the same year?

One could say I am lazyWink

P.S. in case you are wondering why, this would be so that we can get a decent comparison of the rockers.

Sure... give me two minutes. In the meantime here's the specs showing fork, rear shock size and leverage ratios.

3006976

3006976


Posted: Feb 15, 2009 at 18:22 Quote
Here's a page from the 2004 Catalog. Thanks for your help...

3006984

3006984


If you look at the rockers, it appears they are the same, it's just the shorter shock on the Stinky creating a steeper angle on the rocker compared to the Dee'Lux and Primo, which both have an 8.5 x 2.5 shock.

Posted: Feb 15, 2009 at 18:27 Quote
Hmm, this is a tough one. Everything seems to be the same frame wise, but generally if you could go from one shock size to the other, you would only be required to change the stroke and retain the same eye to eye. I can not tell from looking at the pics if the rockers are any different or not, but something is not adding up here to say that you can just swap shock dimensions.

Posted: Feb 15, 2009 at 18:28 Quote
jonbikes wrote:
OK, lets remove the stroke. Without the stroke you could have a ten foot pole in there and get no travel.

Try thinking of it like a fork, you can have two forks that are the same axle to crown height with different amounts of travel. You can't get travel out of something that doesn't compress.

Gotcha... thanks!

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