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Discussion Starter #1
JB are on the game.......

adjustable steel sub frame, long travel A Arms.........

JB Racing KTM page

SX A Arms

JB LT arms have been by far the best I've tried on my Yamaha MX quad, sharp turn in bar none


apparently moving the lower mount in bumps wheel travel from 9.3 to around the 11' range.......... now we are talking........
 

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Man, they are on the ball,however could someone explain how only moving the lower mount in increases travel? Would it not reduce travel? Though it seems a more perpendicular shock compared to the a-arm would be able to work more efficiently, correct? Not necessarily increasing travel?
 

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In reality it doesnt change travel, it changes leverage. No matter what a arms can only move a certain amount up and down. By moving the mount lower though you can put on a longer shock. With a longer shock you can have a little more forgiving valving. Correct me if Im wrong...
 

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Discussion Starter #4
Simple maths........

moving the lower mount inwards increases the leverage ratio giving more wheel travel for the same shock travel.

Extreme example, shock is right out at the ball joint and vertical, wheel travel = shock shaft travel, motion ratio is 1 to 1.

Move shock to half way along the a arm, shock vertical, motion ratio = 2 to 1, wheel travel = 2 times shock shaft travel due to the lever arm effect.

Moving the lower mount inwards increases the lever arm effect and increases wheel travel for the same shock shaft travel.

Laying the shock over complicates the ratio deal, when done properly gives a rising rate effect. Think way back to the late 70's / early 80's when the suspension revolution hit 2 wheelers, long travel lay down rear shocks where the hot setup before rising rate single shock rear linkages came into being.........

the following is all very rough okay:

Stock KTM motion ratio is approx = 1.65 5.3" shock shaft travel give 9.3" wheel travel

MR = 9.3/5.3 = 1.65

same 5.3" shock, 11" wheel travel MR = 1.95 hello, closer to the magic 2:1 now !

having a low ratio means the shock has to be able to handle a higher range of velocities which creates heat and wear issues. Older school suspension guys favour a 2:1 motion ratio and near vertical shocks, nice linear actions that are then tuned by the spring stack and internal shim stack to give a progressive action.

1.65/1.95 = 0.85, very very roughly a 15% move inwards on the bottom mount bumps wheel travel from 9 to 11 inches, on the SX a arm this is something like a 1.75" move, due to the laydown probably less than that.


Alter the relative position to the a arm itself may give more up travel (to the limits of ball joint angularity, but thats been all altered too).

longer shock, only advantage is they holds more oil and have a longer length to mount multiple springs on. Shaft travel is the important component. Shock velocity is set by the motion ratio and overall leverage arm lengths. In general lower velocities are easier to tune.

This is very very simplified and a whole heap of maths and figuring will have gone into this.

move the lower mount inwards by an inch and a bit makes a big difference to the whole action..........

look at the castor setting, 5 degrees, man thats FAST..... will be a turning fool..... but a bit twitchy under brakes from high speed maybe (thats exactly what my Yamaha JB LT front is like, the carve up king but wiggly under brakes).
 

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I agree with everything. BUT no matter what math equation is involved and a arm can only move to its stops. You cant raise the a arms TOO high or your tires will be higher than the bottom of the frame. So when you talk ratio's, Motion and Leverage are two TOTALLY different monsters. BUT buy going with a longer shock will allow a longer stroke which will allow more travel if the a-arms is capable of its up and down free movement. So it all plays into factors somway or another. But what it all comes down to is the valving in the shock. A longer shock is more forgiving for internal valving adjustment. Where as a short shock needs the same valving but with fewer shims, so it needs to be more acurate.
 

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Discussion Starter #6
this is where having a ZPS spring setup comes into play, moving the shock mount in raises the nose unless the shock lower mount then hangs under the arm, you then get more up travel, whicih as you state is limit buy the chassis hitting the ground.

With a sag spring front end you lower the ride height to where ever you want it, you have the full travel over jumps and whoops but sit flat and hard into the turns.

The JB arms will likley need a full custom shock setup to take full advantage of what is on offer.

PEP have advise thaty can can get 10 3/4 to 11 3/4 out of the KTM front end by moving the lower shock mount combined with there front end conversion on the WP shocks so the smart guys in the business have got it figured.

A banshee, now that has a crap front end setup due to it's short arse little weeny shocks......

the KTM shock is plenty long enough 18 1/4 long with only 5 and bit shaft travel.
 

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Ah I see now, that clears it up, :D.
 
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