reduced offset triples for faster turn-in??

[Chuck78]

I had read an article on a racer by the name of Art Robbins from the early '80's who had a very well built up GS1000. The article on the bike mentioned that it had the original custom made billet aluminum triples, which were made with less offset to enable quicker turn-in.
http://www.supershowevents.com/magazine/articles/fb2.html

I was looking around for 37mm triple clamp sets to swap in to upgrade to 37mm forks, and I was lured in to some GS1100 triples, as it looked to me like the lower triple was aluminum vs the cast steel on my 77 gs750. Then as I searched various models of gs1100e's, g's, etc, I noticed the g and earlier e models appeared to have noticeably less offset in the triples from the steerer to the fork stanchion holes. I remembered the 1st 2 years of gs1100e actually had leading axle fork legs (was a surprise to me, for a performance oriented bike). Maybe this reduced offset was used in conjunction with the extra offset of the leading axle forks?


11980-1981 GS1100E:

1982-1983 GS1100E:

Would it be feasible to run these reduced offset triples with straight leg forks for the same effect that the Art Robbins/Frank Leitner bike had? Or would this produce other ill-handling traits, and require a steering stabilizer, as the Robbins bike has fitted??? This will be a street ridden bike built for riding in the Appalachian twisties & mountains, and will have had a large amount of thought put into the chassis/brakes/suspension.

 

[John Kat]

Yes, it's a good idea to reduce the offset to get a quicker steering.
You can easily calculate what the new trail will be by using the formula that's included in one of my old posts:
Having installed the GSXR 11 K front fork on my GS 1000 ST, I wondered to what extent the rake and trail had been modified?
The OEM specs for the GS 1000 are:
Rake: 27?
Trail: 116mm
First, I'll assume that the bike's ride height remains constant by adjusting the fork tubes in the triples.
This means that the rake will remain constant at 27?.
The formula that links the different variables is:
a=R*sin(rake angle)-d
a is the trail
R is the radius of the front wheel
d is the offset between the fork tubes and the steering axle
So with the 19" front wheel that has a radius with tire of 344 mm, and the GSXR 11K triple that has an offset of 32 mm, the trail comes out at 124 mm.
That's a lot and the bike will feel heavier to turn.
Second, if we lower the front end by 2.5 cm the rake will decrease by 1?.
With this new rake of 26? the formula will give us a new trail of 118 mm
This is almost the same as the OEM spec!
Is this achievable with the GSXR 11K fork legs?
The answer is yes as the loaded length of the fork tubes goes from 745 mm on the GS 1000 to 715 mm for the GSXR 11K resulting in a shortening of 30 mm that translates in a reduction in height of 26.7 mm
In other words a GSXR 11K fork will give you the same trail as the OEM bike while the rake will decrease by 1? which is good.
You loose of course 26.7 mm of clearance in the front that should be negligeable.

If you wish I can go through the maths if you give me the offset of the triples

 

[Chuck78]

So if keeping the same ride height /fork length, but installing 37mm forks with reduced offset triples, I would be then keeping the rake pretty similar, but the trail would be decreased a pretty fair amount. What affect would thus substantially reduced trail have? This would be going on a GS425 frame with GS500 forks & probably 1980 GS1100E triples, the top two photos in the first post. I am assuming the GS425E stock triples for the 33mm forks were similar in trail to the 82-83 GS1100E photos in the lower two photos in the 1st post.

Basically the question is - will you get a better sporty handling ride improvement when reducing the trail considerably, but keeping the rake the same or maybe 1 degree less? I may end up with slightly taller rear shocks, & could possibly drop the triples on the forks to install clip-ons above the triples if the handling wouldn't suffer, but I estimate that the handling would become very twitchy if doing so, therefore drag bars and stock height fork are probably going to be the ticket.

John Kat, it seems as if you had forks with substantially more offset in the triples, so dropping the front end height substantially brought the trail back in closer to the steering bearing centerline, to make it steer closer to stock specs aside from the 1 degree decrease in rake.

 

[John Kat]

John Kat, it seems as if you had forks with substantially more offset in the triples, so dropping the front end height substantially brought the trail back in closer to the steering bearing centerline, to make it steer closer to stock specs aside from the 1 degree decrease in rake.

In fact the GSXR 11K triples have much LESS offset than the OEM GS 1000's.
Less offset in the triples generates MORE trail and I had to compensate that by reducing the rake.
So in your case you will be INCREASING the trail with the 1100 triples...

 

[Chuck78]

Aha...I was thinking of trail being the difference of the headstock centerline and the some distance measurement from directly below the headstock to the tire contact patch. This clears it up:

So using less offset would still be a good gain in my case? But less high speed stability??? More offset & leading axle forks would reduce the trail, but less offset gives you more trail???

 

[blowerbike]

any leading axle triples will have less offset.
these will shorten the wheel base.
we used to run leading axle forks on E model clamps to add around an inch extra of wheel base for drag racing.
not sure if this helps but...

 

[Chuck78]

Yeah I don't plan to ride over the "ton" very often, & 115 is probably the fastest I'll desire to go unless on a closed/safe race course (ECTA/dragstrip/Nelson Ledges/Mid Ohio Sports Car Course).

So how would running the leading axle 80-81 gs1100e lesser-offset triples with gs500 straight fork legs affect the 50-110mph stability of a lightweight 330lb GStwin with a much longer than stock GS1100E alloy swingarm??? Maybe it will do nothing more than bring it close to the specs of a more modern sport bike's geometry???

 

[Nessism]

So using less offset would still be a good gain in my case? But less high speed stability??? More offset & leading axle forks would reduce the trail, but less offset gives you more trail???

Less offset = more trail = more high speed stability = slower turn-in

This is a generality of course. I wouldn't use less offset if you want quicker steering though.

 

[Chuck78]

So from reading the article which that drawing is from, it states that less offset creates more trail (as evidenced by drawing). It says a higher # rake angle or more trail both increase high speed stability& minimize effort needed from the rider to keep the bike stable at higher speeds. It says less rake (steeper headstock) or less trail will make the bike turn much quicker & easier, but will require more effort to keep the bike stable at higher speeds. Am I reading this correctly?

That all would translate to the lesser offset triples on straight forks requiring more effort to turn in, not what I was looking for. Correct???

http://bikearama.com/theory/motorcycle-rake-trail-explained/

 

[GregT]

I can answer both on big bike and the small twin...
On my 450 racebike, i used marzocchi forks (ex Ducati pantah) which had about 3mm MORE offset than OE 450. In conjunction with a 17inch front and longer than stock rear shocks it became a VERY quick steerer...Stable enough not to need a damper but yep, a bit twitchy. i never measured the trail but I'd estimate it at around 94- 96mm. Nice on track, but i don't know if I'd want it on the road...

Our GS1000 racebike uses longer than stock shocks and 17 inch wheels, std triple clamps. measures up at 26deg rake and 96 - 97 mm trail. Still very stable, handles very well indeed. BTW figures are very close to a current Triumph Sprint...but the GS is longer wheelbase.

 

[Chuck78]

http://www.ridermagazine.com/motorcycle-features/understanding-motorcycle-rake-and-trail.htm/

Another article confirming. Seems as if I should keep the stock offset, & if needing quicker steering, drop the front slightly & maybe adjust the rear shock ride height... Maybe I will look into dropping the triples & running clipons. I wanted GS500 forks because they are the same height as stock for the small twins, they have a built in fork brace, they fit the popular tokico twinpot calipers with 310mm rotors, & they are 37mm... With emulators & spring mods, the best budget upgrade for the little twins.

 

[John Kat]

You are on the right track by keeping the std triples.
Remember that lowering the front end by 1 inch decreases the rake by one degree ( with a corresponding decrease in trail).
The same applies of course if you lift the rear by 1 inch.

 

[limeex2]

Sorry about my stupidity,but I dont understand this.I have a 80-81? gs1100e with a leading front axle forks.I have a extra set of forks off of a 82-83 gs1100e(NO leading axle) What will changing the forks from a leading axle to a std axle do? Quicker steering or more stability?Also what is your opinion on the leading axle type forks? Do they work better than the std gs1100e forks off of a 82-83.One has a leading axle the other has anti-dive and in my opinion both are butt ugly,but if one works better than the other thats what I would go with.