Rear Shock Length (and how it changes the ride)

[MikeCR]

Hello Again,

I thought this was going to be an easy thing to figure out but I have found some really different suggestions around the forums here as well as over on triumph rats forums.

I am trying to better understand how rear shot length changes the dynamics of the bike.

I find it hard to believe that rear shock length dramatically changes the length of the trail but I could be totally wrong.

From what I understand the standard length is 12.5" or 317.5mm. I have found people talking about generally increasing the length, and all the way up to 370mm.

I understand that this also increases the seat height which I am keeping in mind.

I'm not really sure what the shock length changes other than maybe riding position as your seat rises more weight gets passed to your shoulders...

Is there some other benefit of going with longer shocks? Should the intention be to go as long as possible while still putting your feet on the ground? Or is there a sweet spot one should aim for?

Thanks in advance!

Edit: Just to clarify, I am looking to increase the rear shock length but don't want to go overboard and end up with a twitchy highway ride.

 

[1olbull]

Imagine your motor sitting upright on your garage floor.
Now imagine you hook a lift to the frame at the top shock mounts.
As you lift the rear up (front wheel remains on the ground) what is happening to the angle of your steering head and front forks? That OEM 32° rake is becoming smaller.
Trail is measured from where a perpendicular line from the axle meets the ground to where a line through the center of the steering head meets the ground.
Shorter the trail, quicker the steering and lees the stability at speed.
Longer the trail, slower the steering and greater the stability at speed.
Trials bikes have less trail for more maneuverability at slow speeds. Ride one over 30 mph and your sphincter will surely pucker! Sport bikes are designed for a combination of handling and speed and will have more trail than the trials bike. Cruisers and Touring bikes will have greater trail for better stability (less demands on control) at highway speeds. They also will be more difficult to control at low speeds where counter steer goes away (parking lot stuff).
Remember that Motorcycless are produced for the lowest common denominator of their purchasing public with liability being a huge factor.
Once we purchase a motor, we are pretty much stuck with its frame geometry. The enthusiast wishing to improve the handling of his motor to best fit his abilities will experiment with rake & trail.
The diagram below is a comparison of the OEM Roadster (red) and how just changing tire sizes can noticeably affect handling. As you can see just a .62" reduction in trail will result in a large difference by quickening the handling.

 

[barbagris]

It does indeed affect the steering angle which has the knock on effect of trail alteration. And it need not be a dramatic change to affect the steering to a notable extent.

What tyres are you running. Yes - it makes a difference - calling @1olbull

Edited - he beat me to it.

 

[1K9]

Hello Again,

I thought this was going to be an easy thing to figure out but I have found some really different suggestions around the forums here as well as over on triumph rats forums.

I am trying to better understand how rear shot length changes the dynamics of the bike.

I find it hard to believe that rear shock length dramatically changes the length of the trail but I could be totally wrong.

From what I understand the standard length is 12.5" or 317.5mm. I have found people talking about generally increasing the length, and all the way up to 370mm.

I understand that this also increases the seat height which I am keeping in mind.

I'm not really sure what the shock length changes other than maybe riding position as your seat rises more weight gets passed to your shoulders...

Is there some other benefit of going with longer shocks? Should the intention be to go as long as possible while still putting your feet on the ground? Or is there a sweet spot one should aim for?

Thanks in advance!

I confess that I am less than a novice at this motorcycle geometry stuff and that my opinions are all based on personal anecdote. My experiments are just that, experiments and most of my mileage has been in a straight line, until I met this gang.

Shortening the shock length should be done out the necessity to reach the ground with your feet. If you do so, lower the front too or the bike will not handle well in the turns. It will want to straighten up unless you work hard on the handlebars with pressure. The HD crowd that like their bikes low rider style... don't do well in the twisties. 15 degrees of lean and thei'r dragging pegs and think thei'r raaad. If you are carrying any speed in your turns, you don't want to drag anything.

When you enter a turn the rear compresses "seating" the rear of the bike a lot. This makes the bike want to get out of the turn line. I had the same problem of not understanding how lowering the rear of the bike so little would be bad, until I installed a 225 55 15 Raptor and later replaced it with an Exedra Max which is about an inch taller. Maaan, what a difference holding the line in a turn. You want your bike to at least stay level in the turn. That's why front trail braking helps in some turns. Most people don't even cover their front brake in the turns. My front brakes were so bad it did not matter. Once I replaced seals and flushed all the old junk in there the index finger stays on the brake lever near it's pivot point to modulate the turns. It sounds weird but it works. Now I need a stiffer less divey front. Have to install the front Progressives and a little heavier fork oil. Then I might be able to ride the back without loosing the pack.

All these nuts here are telling you the truth. "Don't lower the rear of the bike" Lower the seat or get tall boots or something if you are inseam challenged. As far as raising the rear with shocks, I have not gone there. I recently added preload to prevent the bike from seating under loads even if I give up comfort. I would like a set of 13s just to try.

Have fun. For me jacking with the bike is half the fun. If you don't jack with the bike, leave it stock. At least get a rear Exedra.

 

[MikeCR]

@barbagris I am on the stock metz right now but will go exedra max (which i understand is taller) when they need replacement (next year if not sooner).

@1olbull That graphic is awesome, I did not think altering the rear shock would so dramatically effect trail but obviously its because I just hadn't done the math. You basically got an 11% decrease in trail with a 2% increase in rear height (half of tire radius) which is WAY more significant than I thought it would be. While rear shock length isn't 100% represented in vertical difference I can see how it will affect is more than I had thought.

@1K9 Indeed! My first ride was a lowered (didn't know that when i bought it) Yamaha Roadstar... that thing was a while in the curves. I was always dragging something, I learned to just take my foot off the floorboard. My intention is actually to increase the length of the rear shock in this case.

 

[1olbull]

I confess that I am less than a novice at this motorcycle geometry stuff and that my opinions are all based on personal anecdote. My experiments are just that, experiments and most of my mileage has been in a straight line, until I met this gang.

Shortening the shock length should be done out the necessity to reach the ground with your feet. If you do so, lower the front too or the bike will not handle well in the turns. It will want to straighten up unless you work hard on the handlebars with pressure. The HD crowd that like their bikes low rider style... don't do well in the twisties. 15 degrees of lean and thei'r dragging pegs and think thei'r raaad. If you are carrying any speed in your turns, you don't want to drag anything.

When you enter a turn the rear compresses "seating" the rear of the bike a lot. This makes the bike want to get out of the turn line. I had the same problem of not understanding how lowering the rear of the bike so little would be bad, until I installed a 225 55 15 Raptor and later replaced it with an Exedra Max which is about an inch taller. Maaan, what a difference holding the line in a turn. You want your bike to at least stay level in the turn. That's why front trail braking helps in some turns. Most people don't even cover their front brake in the turns. My front brakes were so bad it did not matter. Once I replaced seals and flushed all the old junk in there the index finger stays on the brake lever near it's pivot point to modulate the turns. It sounds weird but it works. Now I need a stiffer less divey front. Have to install the front Progressives and a little heavier fork oil. Then I might be able to ride the back without loosing the pack.

All these nuts here are telling you the truth. "Don't lower the rear of the bike" Lower the seat or get tall boots or something if you are inseam challenged. As far as raising the rear with shocks, I have not gone there. I recently added preload to prevent the bike from seating under loads even if I give up comfort. I would like a set of 13s just to try.

Have fun. For me jacking with the bike is half the fun. If you don't jack with the bike, leave it stock. At least get a rear Exedra.

Ricardo,
I agree, do NOT lower your bike.
Lower the seat, grow some leg or wear elevator boots.
Just two words of yours that cause me pause . . . car tire!

 

[1olbull]

@barbagris I am on the stock metz right now but will go exedra max (which i understand is taller) when they need replacement (next year if not sooner).

@1olbull That graphic is awesome, I did not think altering the rear shock would so dramatically effect trail but obviously its because I just hadn't done the math. You basically got an 11% decrease in trail with a 2% increase in rear height (half of tire radius) which is WAY more significant than I thought it would be. While rear shock length isn't 100% represented in vertical difference I can see how it will affect is more than I had thought.

@1K9 Indeed! My first ride was a lowered (didn't know that when i bought it) Yamaha Roadstar... that thing was a while in the curves. I was always dragging something, I learned to just take my foot off the floorboard. My intention is actually to increase the length of the rear shock in this case.

Don't forget, I also reduced the front to a 140/75R17.

 

[MikeCR]

@1olbull ah right so basically you changed the levels by almost 1 inch total. What was you overall opinion is it right where you want it? wish you had gone farther or not so far? What kind of riding are you doing and is this your roadster or classic?

 

[1olbull]

@1olbull ah right so basically you changed the levels by almost 1 inch total. What was you overall opinion is it right where you want it? wish you had gone farther or not so far? What kind of riding are you doing and is this your roadster or classic?

Works so well, I needed to make changes to increase the lean angle because I was dragging the pegs, frame and even engine cases.

 

[1K9]

Don't forget, I also reduced the front to a 140/75R17.

I've been schooled well Obi- Wan. Now you need to show me how to tap into The Force