Measuring Rear Travel

Postby VERT on Mon 4/Apr/05 9:51am

I have always wondered how rear travel is measured, obviously it would be measured differently to front travel as it wouldn't be how much the shock compresses itself but something to do with the combination of the frame movement and shock?

Can anyone explain this a bit better or have a diagram explaining this?
VERT
Member for: 15 years 11 months

Postby Soba on Mon 4/Apr/05 10:02am

my old ghetto method was to pull the spring off the shock, lie the frame down on the ground without the rear wheel. Then make a chalk mark on the ground at the rear axle with it at full extension and another at full comperession and measure the difference.
i suspect most manufacturers work it out with maths instead
Soba
Member for: 15 years 1 month

Postby p2 on Mon 4/Apr/05 10:03am

take the spring off your shock and re fit it then out the bike on your dh rack so that it is level and measure the distance from the axle to the ground then push the wheel all the way up and measure it again the big number minus the small number is the travel


dont forget to refit your spring before your next ride
p2
Member for: 15 years 3 months

Postby VERT on Mon 4/Apr/05 10:06am

I know how much travel my rear has but I am asking in general how it is measured (maybe with this thing some call maths:))
VERT
Member for: 15 years 11 months

Postby Stun on Mon 4/Apr/05 10:08am

Jeremyb has 6 inches of rear travel - until he bottoms out.


Hang on... is this not in the pit?
Stun
Member for: 15 years 10 months

Postby p2 on Mon 4/Apr/05 10:11am

yeah its called the leverage ratio between wheel movement and the stroke of your shock
p2
Member for: 15 years 3 months

Postby Stun on Mon 4/Apr/05 10:14am

If you're running a coil over shock, and are attempting to do the 'remove spring' method...
Don't forget that the bottom out bumper robs you of the last bit of travel... and if it's half an inch worth of bumper - that could equate to 1 1/2 at the axle!
Stun
Member for: 15 years 10 months

Postby jeremyb on Mon 4/Apr/05 10:23am

Measure distance from seat to ground with no sag, take shock out and replace with a piece of zip tie and set it to the length of the shock minus the shocks travel, measure from the seat to the ground again :)
jeremyb
"www.derailled.co.nz - New Zealand Cycling Forum"
Member for: 15 years 10 months

Postby phunk on Mon 4/Apr/05 10:30am

VERT wrote:I know how much travel my rear has but I am asking in general how it is measured (maybe with this thing some call maths:))

As has been mentioned, bike manufacturers will design a bike with a certain leverage ratio, say 1:3, 1:4, etc so for every 1unit of shock travel the rear wheel will move 3, or 4 units respectively, is that what you are getting at?
So if you had a 1:3 ratio and a 2" stroke shock = 6" of travel.
Working out the ratio is beyond me, perhaps when Dougal is back online...
phunk
User avatar
Member for: 14 years 8 months

Postby CroMo on Mon 4/Apr/05 11:33am

phunk wrote:
VERT wrote:I know how much travel my rear has but I am asking in general how it is measured (maybe with this thing some call maths:))

As has been mentioned, bike manufacturers will design a bike with a certain leverage ratio, say 1:3, 1:4, etc so for every 1unit of shock travel the rear wheel will move 3, or 4 units respectively, is that what you are getting at?
So if you had a 1:3 ratio and a 2" stroke shock = 6" of travel.
Working out the ratio is beyond me, perhaps when Dougal is back online...


That is good for a rough estimate but if the ratio changes through the stroke then it won't be very accurate. The leverage ratio on a D8 changes from 4:1 to about 2.5:1. Manufacturers will model the bike during the design process, and plot the suspension curve on a graph. This is the plot of wheel travel versus shock travel and is usually a curve - not a straight line. The curve is telling you that the leverage is changing through the stroke (eg rising rate), and a straight line is not (a linear rate).

When making a prototype, or a custom suspension design, I have made a model and then plotted the shock travel (1mm at a time) and measured the resulting wheel travel. It is a bit of a tedious task but the suspension curve is important and must be right before any cutting or welding.

Hope this helps ;)
CroMo
User avatar
Member for: 14 years 10 months

Postby Dougal on Mon 4/Apr/05 5:44pm

The easiest way to measure is to take the spring off the shock and turn the bike upside down.

Then just measure from the axle to the ground at full extension and compression.
Dougal
Member for: 15 years 6 months

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