Fork this bike!

[morpheus666]

Can any of you tell me what the difference is between the forks on the 80-82 gs1100e's? Are they interchangeable?

Thanks
~M

[Griffin]

The '80-'81 GS1100E forks are leading axle, the '82-'83 are center axle. The newer forks also have Suzuki's anti-dive feature on them.

I just replaced the forks on my '81 GS750E (one of them was pitted and tearing up it's fork seal) with forks from an '82, and consequently went from leading axle to center axle forks. It quickened up the steering a bit because there is less rake and more trail now.

Because the forks are now at a slightly different angle in relation to the wheel, the issues to take into consideration for this swap are:

-The front fender won't fit correctly, you'll need a fender (with the inner steel brace) from the same year as your fork

-The brake caliper mounts also have to be changed. The mounts from any center axle GS fork will do on any other center axle GS fork, I think (I used caliper mounts from an '85 GS550ES on mine).

-The '82 anti dive plumbing will have to be addressed.

[Larry D]

Can any of you tell me what the difference is between the forks on the 80-82 gs1100e's? Are they interchangeable?

Thanks
~M

I don't know. Whenever I have a interchangability question, I'll compare parts numbers on one of the online fische's. Not sure if this method is always accurate, but it's worked for me.

[Nessism]

I just replaced the forks on my '81 GS750E (one of them was pitted and tearing up it's fork seal) with forks from an '82, and consequently went from leading axle to center axle forks. It quickened up the steering a bit because there is less rake and more trail now.

Sorry Griffin if this sounds contrary, but the leading axle REDUCES trail, which makes the bike steer FASTER. The swap you made will SLOW the steering.

For many of the L model bikes Suzuki was going for that RAKED out look so they slackened the head angle a couple of degrees to achieve it. They then installed a leading axle fork to allow them to keep the raked out look but bring trail back to the same as the straight axle bikes. It's the trail that determines how fast a bike steers. I know this seems contrary to what you would think by looking at it but none the less, that's the way the physics works out.

[Griffin]

Sorry Griffin if this sounds contrary, but the leading axle REDUCES trail,

That's what I said. I went from the leading axle to the center axle, which increased trail. I clearly stated I increased the trail. It went from 103mm to 116mm.

which makes the bike steer FASTER. The swap you made will SLOW the steering.

It would if the wheelbase remained the same, but it didn't. I have shortened the wheelbase by 10mm, which helps quicken steering. This shortening of the wheelbase more than offsets the increased trail as far as affecting the handling characteristics.


From Cycle Magazine, April 1982 test of the GS1100E:

...Most important among the modifications is a switch to a center axle fork from last year's leading axle setup. The new axle position increases trail (about one-half inch over last year's GS) and decreases the wheelbase a like amount....

....All in all, we like the slightly quicker handling and quicker turn-in compared to previous GS1100s. It makes the '82 GS feel nimbler than last year's bike at all speeds, and the high speed handling characteristics are nevertheless blue-ribbon stuff.

There are several factors that affect steering quickness. When one is changed, it generally ahs an effect on others, which may change handling characteristics counterintuitively to what you might think.

[Nessism]

Griffin, you have it backwards; INCREASING trail makes the bike steer slower, not faster.

"Trail" is like caster on a car and determins how fast the bike turns. Trail is made up of two things: the steering head angle and the fork "offset".

A leading axle fork has more offset than a straight axle fork. By increasing the offset the trail goes DOWN. The reduction in trail makes the bike steer faster. Yes, the wheelbase increases but that is a minor change vs. the reduction in trail.

Note from the Cycle article you have quoted... "...Most important among the modifications is a switch to a center axle fork from last year's leading axle setup. The new axle position increases trail (about one-half inch over last year's GS) and decreases the wheelbase a like amount....

This increase in trail will make the bike steer SLOWER. I have no idea what they are attributing the quicker steering "turn in" to, but it's not due to increasing trail and again, small changes in the wheelbase don't fully offset the trail change.

[Nessism]

Check this diagram of rake and trail for a bicycle, motorcycle is the same but the "offset" is built into the triple clamps most of the time.

It's clear to see that a leading axle fork has more offset, which will reduce trail...which will speed up steering vs. a center axle fork.

[Griffin]

Griffin, you have it backwards; INCREASING trail makes the bike steer slower, not faster.

"Trail" is like caster on a car and determins how fast the bike turns. Trail is made up of two things: the steering head angle and the fork "offset".

A leading axle fork has more offset than a straight axle fork. By increasing the offset the trail goes DOWN. The reduction in trail makes the bike steer faster. Yes, the wheelbase increases but that is a minor change vs. the reduction in trail.

Note from the Cycle article you have quoted... "...Most important among the modifications is a switch to a center axle fork from last year's leading axle setup. The new axle position increases trail (about one-half inch over last year's GS) and decreases the wheelbase a like amount....

This increase in trail will make the bike steer SLOWER. I have no idea what they are attributing the quicker steering "turn in" to, but it's not due to increasing trail and again, small changes in the wheelbase don't fully offset the trail change.

Jesus, read what I wrote. Twice. I don't disagree that I increased the trail with center axle forks. I wrote that in my first post. And my second post. In my second post, I also agreed that increasing trail slows steering. I never said it didn't. But shortening the wheelbase quickens it.

You have no basis to state the last half of your sentence. Shortening the wheelbase also affects CG, front/rear weight bias, ride height, and probably other factors that affect handling sharpness. On some bikes it might make a difference, on some it might not. On these particular bikes, the results are what they are. Maybe the forks are slightly stiffer now due to the change in axle mounting. Who the Hell knows? Why does it really matter anyway?

By the way, Motorcyclist Magazine in their June '82 issue noticed that the handling of the GS750E was quicker as well, with the only change up front being the forks and addition of anti-dive, just like on the 1100. I could go through and compare what Cycle World and Cycle Guide also said about the changes in steering geometry on these bikes, but there's little point in it. I'm sure they'll state the same thing

I'm done with this. The original poster asked a simple question about fork interchangeability, it was answered in the first reply. The rest of this thread is pretty much crap.

[Nessism]

Jesus, read what I wrote. Twice. I don't disagree that I increased the trail with center axle forks. I wrote that in my first post. And my second post. In my second post, I also agreed that increasing trail slows steering. I never said it didn't. But shortening the wheelbase quickens it.

You have no basis to state the last half of your sentence. Shortening the wheelbase also affects CG, front/rear weight bias, ride height, and probably other factors that affect handling sharpness. On some bikes it might make a difference, on some it might not. On these particular bikes, the results are what they are. Maybe the forks are slightly stiffer now due to the change in axle mounting. Who the Hell knows? Why does it really matter anyway?

By the way, Motorcyclist Magazine in their June '82 issue noticed that the handling of the GS750E was quicker as well, with the only change up front being the forks and addition of anti-dive, just like on the 1100. I could go through and compare what Cycle World and Cycle Guide also said about the changes in steering geometry on these bikes, but there's little point in it. I'm sure they'll state the same thing

I'm done with this. The original poster asked a simple question about fork interchangeability, it was answered in the first reply. The rest of this thread is pretty much crap.

Geez, take it easy.

I've seen a LOT of people that don't understand rake and trail, and these people often mix up things like trail and offset. I accept that you understand.

That said, this statement that you made is wrong...

This shortening of the wheelbase more than offsets the increased trail as far as affecting the handling characteristics.

Trail changes trump wheelbase changes by a far margin; changing from an offset fork to a center axle design will slow the steering, wheelbase be damned.

I've changed the fork offset on different bicycles I have and the steering change always follows the trail change, not wheelbase. On one bike I changed the fork from having 55mm of offset to one with 45, and the steering slowed down significantly despite a reduction in wheelbase.

[posplayr]

Cycle Mag April 1982

I found the write up from Cycle magazine

Nice write up on Physics of Bikes.

http://en.wikipedia.org/wiki/Bicycle...cycle_dynamics

From the shop manuals comparing 81 GS750EX and 82 GS1100EZ trail went from 103 to 116 respectively while turning radius dropped from 2.8 meter to 2.7 meter respectively.

In theory better turn in (of the fork) due to lean with a shorter wheel base.

What I'm realizing is that the change from a leading axle fork to the center axle fork also involved a change in the triple tree offset.

Pos

P.S. Whoops looks like I got back a little late. I started this post a few hours back.

Nessism: Even reading the link above it is somewhat contradictory as to what the overall effects and feel are of the additional trail. I know for myself, that my GS1100ED has a nicer roll-into curves (it almost feels like it just drops in) and at the same time stable in the turn. If the head tube bearing is not properly adjusted it also seems to have more of a tendancy to have fork wabble. At rest the fork seems to flap side to side easier. All of this is compared to the GS750EX.

The key element as described in wikipedia is:

In traditional bike designs, with a steering axis tilted back from the vertical, trail causes the front wheel to steer into the direction of a lean, independent of forward speed.[4]

This would explain the tendancy to drop into a corner better.

[posplayr]

Counter Steering and trail

If you watch the video it appears that the turn dynamics can be understood fairly easily (when you consider the effect of trail).

http://en.wikipedia.org/wiki/Image:Bike_weaving.gif

In order to initiate a turn and the necessary lean in the direction of that turn, a bike must momentarily steer in the opposite direction. This is often referred to as countersteering. This brief turn moves the wheels out from directly underneath the center of mass, causing a lean in the desired direction. Where there is no external influence, such as an opportune side wind to create the force necessary to lean the bike, countersteering happens in every turn.[14]

If you look at the weaving video above and understand the concept of trail then it tends to come together.
Remember trail causes the front wheel to turn into the direction of a lean angle when the bike or motorcycle takes on a lean angle. This is true based on simple static kinematics (I just tested my bike in the garage).

You will try it, you will notice the contact patch moves as the forks rotate. The Bike CG is also moving (watch the engine case move as you turn the bars) This is definately more complcated that I'm going to make it here.

In a turn the front wheel has a track further out than the rear wheel. Like a car the rear wheels always take the shorter path. To get the front tire on that outside path you steer the front wheel to the outside of the turn. This tends to rotate the bike about the cg toward the center of the turn (remember the front wheel is trying to go to the outside of the turn). The trail then kicks in and causes the tire to turn automatically back to make the proper turn.

I tested the trail on my ED; it is more pronounced than that on the GS750. Hold onto the tank or the engine and lean the bike side to side without touching the handle bars. The bars will turn toward the lean. Careful you have reduced leverage down and you will need to keep the bike upright which is harder this way.

Seems to make sense.

Pos

[posplayr]

Fork Trail

Now I was getting worried about my 88 GSXR conversion. Did not want to upset the handling too much.

I just looked up the GSXR Gen 1 (86-88) 1100 it is also 116 mm of trail just like the 82-83 GS1100EZ/ED/ESD.

On the other hand the 2nd Gen 1100's (89-92) have much less trail only 91 mm. These are the USD forks.

The 81 GS750 was 103 mm and the 2nd Gen 1100 forks are 91mm should make the front end much more twitchy in the turns I would gather (responsive???). This would explain why the front end on my 86 GSXR 1100 with 91 GS750 front end seemed to need more force to keep it down in the turns.

The ED lays smoothly into the corner and doesn't need much pressure to stay down. I like that.

Pos

[chef1366]

That'll put an end to the bickering Jim.

[posplayr]

Chef

U saying "Stick a fork in it???"

Just some food for thought; documenting some thoughts as I was forking around.

Pos

P.S. Some trail is good, but not too much.

http://davesbikeblog.blogspot.com/2007/05/trail-fork-rake-and-little-bit-of.html

I remember writing an article for Cycling magazine in the 1970s; someone wrote to me saying my theories on trail were wrong, and sent me an early 1950s article from Cycling to prove it.

The old theory was that if you had the front wheel’s point of contact behind the steering axis, when the steering was turned 90 degrees the point of contact was then on the steering axis line. Therefore, the front end of the bike had dropped slightly, and to straighten up again, the steering had to lift the weight of the bike and rider; thus sluggish handling.

http://www.sportrider.com/tech/146_0402_susp/index.html

Trail: Technically speaking, trail is a horizontal measurement from an imaginary point where a line through your bike's steering head meets the ground and then back to the front tire's contact patch. This measurement is important for general handling because trail determines steering quickness and stability. Less trail will quicken steering but sacrifice stability, while more trail will make steering heavy but add stability. You can change trail by using a front tire of a larger or smaller diameter, triple clamps with different offsets or by altering chassis attitude using ride height.


The description of slowing down steering might be a little mis leading. Here is what I'm thinking. Less trail allows the front wheel to be turned without leaning the bike. In other words there is less coupling between front wheel steering angle and the body lean when trail is low. Basically it feels like you can turn the steering wheel without waiting for the frame to right.

On the other hand a moderate amount of trail tends to make the front wheel turn with the body lean and so actually provides a quick roll into turns. To much trail and it probably feels heavy as the fork steering won't come out till the body lean comes out.

Anymore thoughts?

[Nessism]

Hey Jim,

That Wikipedia article is LONG, a bit too much for me. My understanding of two wheel steering geometery comes from the bicycle side. I build road bike frames and went through a LOT of different frames experimenting.

It's a bit simplified to only look at trail and try to judge how a bike will handle, there are many other characteristics that come into play. Trail will help judge how fast the bike will steer but it won't tell you how stable the bike will feel in a turn.

Regarding your GSXR, if you like the handling with the old fork, try to find one with the same offset. This may be difficult since more modern race type bikes usually have very steep head angles and short offset so as to get enough trail. Putting one of these short offset forks on your older style bike may make it steer kind of slow.

Not sure this is helpful so I'll just stop now. Good luck.