Laying Down!

[austini]

Just resurecting one of my ty175's and have just purchaced some new falcon shocks and the sammy miller footpegs, although i'm not aiming to create a museum piece I do want her to be competetive in the twin shock class.

I have noticed that one of my 175 frames has been modified, although cruedly to enable the rear shocks to be layed down to a variety of positions.

Is it worth swapping frames and if so what is the ideal distance from the original top mount to adjust the shock too? What are the advantages/disadvantages of laying down the shocks and at what angle do you stop?

[jaylael]

The most critical point to consider is the effect on steering head angle. I set mine to 24 degrees with an angle finder, which are available at most hardware store very cheaply. Once you have set the fork angle to 24 degrees, the shocks get mounted where ever the spring rates and length of the shocks dictate. The fork angle is ten times more important than shock configuration in a good working trials bike. JL

[jamie stokes]

:agree: :agree: :agree:

[ask greeves]

This is a technical bit; Lets say the shock was upright 12 o/clock, for every inch of wheel movement the shock moves 1 inch. If you could lay the shock down to 3 o/clock ie. parallel with the swinging arm, for every inch of wheel movement the shock moves virtually zero. Ok; so halfway between the two, shock angle 45 degrees, for every inch of wheel movement the shock moves approx half inch, therefore more spring required, travel increased and the damping action virtually halfed as opposed to the upright shock. What i'm saying is, it's not just a point of laying a shock down, there's more to it.

Edited January 5, 2007 by ask greeves

[martinm]

Might seem a bit obvious - but take a look at the Majesty pics in the "Twinshocks prices" topic and lay em down at the same level as these.

[westyfield]

Back in 1978 my old mate Walt Keeling and I layed the shocks down on our TY175's as discussed and then John Shirt did his bit to the motors with 200 cc conversions and the bikes were transformed fom being just "OK" into "quite competitive".. untill the Fantic came along anyway!!

(This was pre MAJESTY don't forget) The motor was amazing with the big bore conversion but the ride over rocks and the grip in mud were much better too, so I would say "go for it"

[austini]

OK thanks for that guys I was just about to paint my original frame until Westyfield put in his 2 bobs worth!!!

Now at the end of the 70's and into the 80's shocks started to angle their way down the frame, I myself converted my 250 to a godden frame which was far superior to the yam. This was of course due to the revised geomentry, lighter frame, girlings gas shocks and Bernie's handlebars (ah where is she now!!!!).

What makes a bike ride better with angled shocks? Westy reakons it transformed his TY, Jay informs me that I must start cutting and welding my headstock to get the desired effect and Mr Greeves quite correctly points out that more angle means more spring less dampening, which in theory I would think was worse for your rear end.

Now I know laying down the shocks is better, it must be because every manafacturer did it!!!! or was it just a gimmick. I remember seeing a Yamaha promo in slowmo pointing out the superior grip of a DT175 mono against a DT175 twinshock, but today only my little Scorpa follows that trend all other bikes having a more vertical shock. A Scorpa gets excellent traction however the rear end doesn't flick around as easily as say a Beta is this the same theory for twinshocks???????

[ask greeves]

Leverage and fulcrum points are how the Egyptians built the pyramids. As your only moving your shocks probably 10%, your springs and dampers are being altered by a similar amount. As most production parts have to cater for a wide selection of rider weight, your probably still within the window for everything to work. What is interesting , that laying down , seems to work better, that would indicate that standard shocks, in standard position, are probably over damped and/or over sprung. Fortunatley things have progressed, customised shocks with lighter or heavier damping are now available. Monoshocks usually have damping adjustment, very few twinshocks have damping adjustment, by moving the shock angle, your in actual fact getting damping adjustment and spring rate change (at the rear wheel).

Edited January 6, 2007 by ask greeves

[feetupfun]

Following on from the previous post about leverage, here is how to calculate how much you are changing the leverage if moving one or both of the mounting points or the compressed or extended length of the shockie or the length of the swingarm.

First calculate the existing leverage ratio:

Measure swingarm pivot to axle centre. (distance 1)

At half travel position, measure from swingarm pivot centre to centre line of shockie at 90 degrees to shockie centreline. (distance 2)

Divide distance 1 by distance 2. You should get a ratio of about 1.05 for a standard TY175 (Ratio 1)

Now calculate the leverage ratio for the proposed changes:

Mock up the proposed changes and do a similar set of measurements.

If you move the top mount of standard length TY175 shockies to halfway down the plastic sidecover with no other changes, the leverage ratio will be something like 1.25 (Ratio 2)

Now you can work out how much extra loading you will be applying to the shockies by the proposed changes.

Divide Ratio 2 by ratio 1. (1.25 divided by 1.05 = 1.19) ie the new loading will be 119% of standard ie a 19% increase.

Jay informs me that I must start cutting and welding my headstock to get the desired effect

Jay is telling you to make sure a suitable steering head angle is retained when you start fiddling around with the height of the rear of the bike (changing the height of the front or the rear of a bike changes the steering head angle).

It is usually possible to achieve the ideal steering head angle when you do shockie mods without doing any mods to the front part of the frame.

[nigel dabster]

Are the laying down of shocks moying one point to there fore give a rising rate and instead of a staright line it now becomes a parabola????????????

[feetupfun]

Are the laying down of shocks moying one point to there fore give a rising rate and instead of a staright line it now becomes a parabola????????????

The leverage ratio on twinshock type suspension never stays exactly the same throughout the travel. The mount locations on standard bikes are usually well chosen to minimise the variation in leverage ratio throughout the travel. When people modify only one mount location on a standard bike, the new arrangement can produce the desired increase in wheel travel but becomes less ideal as far as leverage variation is concerned.

Laying down shockies by moving only the top mount forwards (or similarly by moving only the bottom mount forwards) usually causes the opposite of rising rate ie the leverage ratio on the shockies increases as the swingarm moves upwards.

[jaylael]

Jay informs me that I must start cutting and welding my headstock to get the desired effect

Jay is telling you to make sure a suitable steering head angle is retained when you start fiddling around with the height of the rear of the bike (changing the height of the front or the rear of a bike changes the steering head angle).

It is usually possible to achieve the ideal steering head angle when you do shockie mods without doing any mods to the front part of the frame.

Brilliant explanation! Exactly what I was trying to say. JL

Edited January 7, 2007 by JayLael