GS forks on a GT750

General discussion about Street two-stroke Suzuki motorcycles.

Moderators: oldjapanesebikes, H2RICK, diamondj, Suzsmokeyallan

User avatar
jabcb
Moto GP
Posts: 4240
Joined: Mon Dec 13, 2010 4:32 pm
Country: USA
Suzuki 2-Strokes: 69 T350 thru 75 GT750
Location: southwestern Pennsylvania

GS forks on a GT750

Post by jabcb »

Background...
Smoke Riders has some info about the stock GT750 forks: http://www.smokeriders.com/Technical/Su ... ocket.html
They said, "GT-750 fork on a stock weight machine has approximately 4" of travel. When the bike is resting on two wheels, up to 2" is lost. As one sits on the bike, another inch is taken up, and when the bike reaches higher speeds, 1/2 inch of heaviness is gained back. This leaves the rider with only 1 1/2" to work with, so at 150 mph, a small ripple will bottom the forks resulting in what is called taken slapper."

Cycle World provided fork improvements for the the GT triples: http://www.ozebook.com/compendium/suzi/mags/forks.htm
The GT750 info is here: http://www.ozebook.com/compendium/suzi/mags/forks4b.jpg & http://www.ozebook.com/compendium/suzi/mags/forks5b.jpg
Stock forks have 4.375" travel & 54 lb/in spring.
Modified forks have 5.4" travel & 38 lb/in spring. Also slid forks up 1" in the triple clamps.
Recommend to adjust preload so forks settle an inch when you sit on the bike.

Progressive Suspension's GT750 springs are progressive 35/50 lb/in. (# 11-1120)

My latest GT750...
The PO updated the front end with GS1100GK forks & triple clamps. Did a very nice job on this and other updates.
He slid the forks up 1" in the triple clamps & increased fork pressure to preclude contact between fender & lower triple clamp stuff.

GS1100GK forks have 6.3" travel and are about 1-1/4" longer than the GT750 forks.
GS1100GK is about 169 lbs heavier than the GT750.
GS1100GK standard fork pressure is 10 psi & the max is 35 psi.

The PO is a good bit taller than I am. Forks are way too stiff & the forks don't compress much from the combined weight of the bike & me.

My plans...
Use Progressive Suspension springs for the GS1100GK, which also are 35/50 lb/in. (#11-1129)
Use no fork pressure.
Lower the fork by 1". This provides 5.3" travel -- nearly the same as the Cycle World update.
Slide the forks up the triple clamps by about 1/4" to provide the same effective unloaded fork length as stock GT750.

Questions...
Will need to get 1" top out springs + washers. Any suggestions on where to get them?
Progressive has fork lowering kits that include 1" top out springs. Anyone know if they will separately sell the top out springs?

Any other suggestions?

Any thoughts on how much the forks should compress from the weight of the bike & from rider weight?
BAS (Bike Acquisition Syndrome) - too many bikes but have room for more

Suzuki:
GT750 2x75
GT550 72 & 75
GT380 72
T500 69 project & 73 project
T350 69 & 71
Honda 85 CB650SC & 86 CB700SC
09 Triumph Bonneville SE
User avatar
Suzsmokeyallan
Moto GP
Posts: 4326
Joined: Fri Oct 24, 2008 9:11 am
Location: Mostly Barbados, sometimes Florida and western Canada
Contact:

Re: GS forks on a GT750

Post by Suzsmokeyallan »

Sorry to hear of this but it all could have been avoided if he had researched this properly and just used the early GS650 or 850 forks that would have slipped right into the stock GT triple trees.
Being the weights are almost identical on these two bikes to the Buffalo the spring rates would have been decent for regular riding. The big benefit on these over the buffalo ones would have been slider bushings in the lowers and a larger damper rod with way better dampening and no stiction.
You would have had to raise the tubes up a tad in the trees by about an inch or so because of their length but thats about it.
It might be cheaper to just look for some of these forks instead on ebay and sell the 1100 ones. The altered rotors and calipers will swap over if he used the snowflake style rotors as well as the GS 1100 calipers.
To use these you will need to get another set of GT750 set of triples as well for the retorfit. These lowers are identical to the Buffalo ones except one side does not use the lower cap design and the caliper mounts are slightly lower for the smaller diameter GS discs, the fender mounts and other dimensions of the lowers are the same as the Buffalos.
Heres a set at present to consider, you want the ones with no offset on the axle.

http://cgi.ebay.com/ebaymotors/Suzuki-G ... ccessories" onclick="window.open(this.href);return false;
Two strokes, its just that simple.

69 Suz U70
69 Suz T500
72 Suz GT750 cafe
74 Suz TS250
74 Suz GTXVR project
75 Suz RE5
75 Suz GT750
76 Suz TS400
76 Suz GT750
81 Suz GSX1100
86 Suz RG500x2
88 Hon CR500
93 Hon CBR900RR
98 Suz GSF1200x3
15 Kaw Ninja H2
smoketriples
On the main road
Posts: 115
Joined: Tue May 17, 2011 8:38 am
Country: UK
Suzuki 2-Strokes: GT750

Re: GS forks on a GT750

Post by smoketriples »

Mine's PO was even nuttier. 88 CBR 600 front end...triple trees, wheel, brakes, forks, etc.

Handles suprisingly well but I haven't tried a good bump yet at speed. I don't particularly like high speed tank slappers.

e
Smoketriples
current stable:
'75 Zooki GT750 (rider)
User avatar
tz375
Moto GP
Posts: 6204
Joined: Mon Nov 03, 2008 10:47 am
Location: Illinois

Re: GS forks on a GT750

Post by tz375 »

Lots of good data there. Fortunately those forks (if I have the right ones in mind) don't have those leading axles.


That's a lot of travel and it might be better to reduce that somewhat. A spacer under the damper rod will shorten the fork but won't reduce travel per se. With a spacer, the fork could bottom out. You may be able to do the opposite of the FORK FIXES article and reduce the length of the damper rod.

With either a spacer or modified damper rod, check the oil orifice position are to be sure you are not interfering with damping functionality.

The 1100 forks appear to have a top out spring under the damper rod already, so I wouldn't expect to add another.

I would see if it's possible to modify the damping and fit gold valve emulators from Race-tech or YSS.

For static sag with the rider plus bike weight the nominal amount is in the 25-35mm range or 1/3 of suspension travel.

Do your forks have anti-dive? If so clean that out and check it. If it is in the Closed position, it makes the forks much stiffer.
User avatar
jabcb
Moto GP
Posts: 4240
Joined: Mon Dec 13, 2010 4:32 pm
Country: USA
Suzuki 2-Strokes: 69 T350 thru 75 GT750
Location: southwestern Pennsylvania

Re: GS forks on a GT750

Post by jabcb »

Thanks for the insight. Didn't know it could have been that easy with early GS650/850 forks.

Think I just need to get 1" top out springs + washers. (Also called rebound springs.)
Am not yet sure what to do about preload.

Here is a pic of the PO's work. Bike has GS wheels, rear disk brake & expansion chambers. Looks quite nice.
GT750s.jpg
You do not have the required permissions to view the files attached to this post.
BAS (Bike Acquisition Syndrome) - too many bikes but have room for more

Suzuki:
GT750 2x75
GT550 72 & 75
GT380 72
T500 69 project & 73 project
T350 69 & 71
Honda 85 CB650SC & 86 CB700SC
09 Triumph Bonneville SE
User avatar
Suzsmokeyallan
Moto GP
Posts: 4326
Joined: Fri Oct 24, 2008 9:11 am
Location: Mostly Barbados, sometimes Florida and western Canada
Contact:

Re: GS forks on a GT750

Post by Suzsmokeyallan »

You can update anything but you have to cover all the angles and not fit on what 'looks' right. If the bike is mostly stock and you just want a better front end performance wise then the GS 650 and 850 is the ticket to better brakes and better fork action all the while retaining the general stock look.
If you want 'high tech' then the skys the limit but then so is the budget to get the updates to work with what you want.
Theres too much work and cost involved for altering the 1100 forks to work on the Buffalo when the 650 or 850 forks will give you better performance right out the box.
Ok now I see the update he didn't even use the better calipers either. The ones he should have used are the types using the rectangular pads which have more brake swept area.
So you have a few choices since the bike is mostly stock, one of those would be to to get the 650 forks, the buffalo triple trees and the calipers to make it correct and you will be set.
Or another one of your choice.

Heres the style of calipers he should have bought, these were even used on the GS1100E of 1981 as well and carry the larger rectangular pads:
http://cgi.ebay.com/ebaymotors/1981-Suz ... 4aab61cbeb" onclick="window.open(this.href);return false;
Two strokes, its just that simple.

69 Suz U70
69 Suz T500
72 Suz GT750 cafe
74 Suz TS250
74 Suz GTXVR project
75 Suz RE5
75 Suz GT750
76 Suz TS400
76 Suz GT750
81 Suz GSX1100
86 Suz RG500x2
88 Hon CR500
93 Hon CBR900RR
98 Suz GSF1200x3
15 Kaw Ninja H2
User avatar
tz375
Moto GP
Posts: 6204
Joined: Mon Nov 03, 2008 10:47 am
Location: Illinois

Re: GS forks on a GT750

Post by tz375 »

Suzsmokeyallan wrote: There's too much work and cost involved for altering the 1100 forks to work on the Buffalo .................
I don't follow the logic here. The bike already has 1100 forks fitted and they work - just not as well as he'd like. Starting with a bike fitted with a set of stock750 forks that makes complete sense though.

So is it easier to massage the current set of forks that are on there rather than go out and buy another complete set and then add springs, damping changes etc? I haven't tried either conversion so I have no strong opinions on the technical merits of either. Are you saying that there is something intrinsically wrong with the 1100 forks or much better on the 650 forks? If not, I'd suggest he works with what he's got in this case.

There are many ways to achieve a certain outcome, depending on where one starts the journey. There are few Correct or Perfect answers to modifying bikes. There will always be compromise and subjective differences.

One of my GT750s has stock forks, fitted with Progressive springs, modified damper rods and Race Tech emulators with special spacers and I'm not happy with that set up either.

More to the point, do you happen to know if offset and trail are the same on 1100 and 650 forks? That could potentially be an issue with the 1100 triples.

Do you have a link to the excellent fork thread you posted earlier showing the differences?
User avatar
jabcb
Moto GP
Posts: 4240
Joined: Mon Dec 13, 2010 4:32 pm
Country: USA
Suzuki 2-Strokes: 69 T350 thru 75 GT750
Location: southwestern Pennsylvania

Re: GS forks on a GT750

Post by jabcb »

Thanks for your input -- you gave me a lot to think about.

Checked out GS650 & GS850 on http://www.suzukicycles.org
The GS850 forks with its chrome fender would definitely be the best match looks wise.

Except for the 1" top out (rebound) springs + washers, I've already purchased the parts to modify the GS1100GK forks.
Other than that, the additional cost would be shop time -- I'm going to have the work done at a shop that works on older bikes.

PO also rebuilt the brake calipers, replaced the pads & used stainless steel brake lines.

Starting from scratch, I would definitely go with your solution. But at this point I'm basically looking at installing a fork lowering kit.
Just need to get the 1" top out (rebound) springs + washers, but I don't know where to get them yet.

You seem to be quite an expert on these front ends. Any suggestions on improving my planed mods?
BAS (Bike Acquisition Syndrome) - too many bikes but have room for more

Suzuki:
GT750 2x75
GT550 72 & 75
GT380 72
T500 69 project & 73 project
T350 69 & 71
Honda 85 CB650SC & 86 CB700SC
09 Triumph Bonneville SE
User avatar
tz375
Moto GP
Posts: 6204
Joined: Mon Nov 03, 2008 10:47 am
Location: Illinois

Re: GS forks on a GT750

Post by tz375 »

Your forks should already have a rebound spring under the damper rod. If you have spacers on top of the springs, cut I'' off the spacers and use that to extend the top out springs overall fitted length. I'd add emulators and modify the damper rods. Allan may have some other ideas too.
User avatar
Suzsmokeyallan
Moto GP
Posts: 4326
Joined: Fri Oct 24, 2008 9:11 am
Location: Mostly Barbados, sometimes Florida and western Canada
Contact:

Re: GS forks on a GT750

Post by Suzsmokeyallan »

Tz375 wrote
I don't follow the logic here. The bike already has 1100 forks fitted and they work - just not as well as he'd like.
Richard do you think rebuilding those 1100 forks to work with the emulators and a set of altered springs could be done for under the cost of a set of used GS 650 or 850 ones found on ebay for under $100.00.?
In this case the stock 1100 forks will not work without major work to the internals, compared to the 650 and 850 forks that are more in the ball park to the Buffalo, and with them you can play with some top spacers, oil viscosity and the air pressure if you get the version with the aircaps.
The big updates compared to Buffalo forks are the internals, especially the pair of slider bushings in the lowers and the larger damper rods that hold more oil internally, there is no comparison no matter how much you modify a stock set of Buffalo forks, they will pale in comparison.
The way I see it, those 1100 fork springs will need to be changed to a longer and softer rate spring as the compression and rebound rates now are too stiff, mainly because they are designed for a heavier bike.
An update is going to set you back at least $60.00 or more in cost for a set of springs, then the emulators you plan to use will be more money onto that figure.
Then there's the stock damper rods, they will need to bleed off more oil to match the new longer travel softer rate springs, if you drill too big of a hole it will 'pogo' on rebound, too small of a hole and it will still 'pack down' on rebound.
Since you are in unknown territory it make take two or three up-sizing in small increments of the damper holes to get close to what you need.
Remember, every time you do this mod to the damper rods the forks have to be pulled down and the oil changed etc, lots of fun times ahead, and you have to know what you are doing and looking for in a performance result.

Its basically a stock bike, the 650 or 850 forks with some proper oil will be more than adequate for what hes looking for, and worlds above the stock Buffalo forks, besides someone here on the board will have a set of Buffalo trees he can get for free if not a pittance.
It not worth the time or the money for what the end result will bring, I'd sell that 1100 front end back on ebay and get the 650 or 850 forks, then play with the air pressure and oil viscosity.

I actually have two sets of 650 front forks as well as the calipers and discs for this update, so I know just how its going to work as an update to the stock front end.
Two strokes, its just that simple.

69 Suz U70
69 Suz T500
72 Suz GT750 cafe
74 Suz TS250
74 Suz GTXVR project
75 Suz RE5
75 Suz GT750
76 Suz TS400
76 Suz GT750
81 Suz GSX1100
86 Suz RG500x2
88 Hon CR500
93 Hon CBR900RR
98 Suz GSF1200x3
15 Kaw Ninja H2
User avatar
jabcb
Moto GP
Posts: 4240
Joined: Mon Dec 13, 2010 4:32 pm
Country: USA
Suzuki 2-Strokes: 69 T350 thru 75 GT750
Location: southwestern Pennsylvania

Re: GS forks on a GT750

Post by jabcb »

Stock GT750 fork spring rate is 54 lbs/in. Stock GT750 fork travel is 4.375". Stock GS1100GK fork travel is 6.3".

Progressive Suspension sells fork springs for the GT750 & GS1100GK -- they have the same spring rate (35/50 lbs/in).

My thinking was that the GS1100GK uses air pressure to deal with the additional weight. Going with no air pressure, I'd have the same spring rates as Progressive Suspension picked for the GT750 with stock forks.

You got me with the emulator & damper rods. What is an emulator? What do you mean by 'pack down' on rebound?
I would be decreasing travel from 6.3" to 5.3". Does that make a difference on modifying the damper rods?

Have already purchased all the parts needed to do this. The 1" section cut from the spacer was the only thing I needed.

Thanks for the info. Looks like I should have asked you guys about this before buying the parts.
Am currently working on the T500. Will have to think about this while I finish that project.
BAS (Bike Acquisition Syndrome) - too many bikes but have room for more

Suzuki:
GT750 2x75
GT550 72 & 75
GT380 72
T500 69 project & 73 project
T350 69 & 71
Honda 85 CB650SC & 86 CB700SC
09 Triumph Bonneville SE
User avatar
Suzsmokeyallan
Moto GP
Posts: 4326
Joined: Fri Oct 24, 2008 9:11 am
Location: Mostly Barbados, sometimes Florida and western Canada
Contact:

Re: GS forks on a GT750

Post by Suzsmokeyallan »

You are focusing on the travel, but in reality its more than that, the frame geometry also has a lot to do with it. Can you use it on another bike without changing the attitude of the bike when sitting at its static state.
Remember, the way the frame is designed determines how high the lower tree is from the ground and so this determines how long the forks need to be given both bikes have the same head tube angle.
In other words, putting on forks with more travel onto a Buffalo will raise the front end simply because they are longer, what you want to do is get more 'usable travel' without jacking the front up to a point the steering becomes slower with a shallower headtube angle.
The simplest way for this to happen on a buffalo is to have a longer damper rod that allows an increase in travel but then you need to push the tubes up into the top tree so the front ends attitude does not change.
If the wheels touch the ground when the bikes on the centre stand, you are almost out of room to play with, and then you will be going into the realm of making additional modifications to the rear suspension and the centre stand.

Emulators are basically a stack of thin shims and a fixed piston assy, they act as a flow rate device to alter the way the fluid travels when the fork moves up and down. Its a modern way of making an 'old school' fork act like a modern cartridge design.
http://www.racetech.com/html_files/GVWHYGV.html" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;

Damper rods are the hollow tubes at the bottom of the forks, they carry a piston and ring at the top and an oil control piece at the base. They use holes in them to channel the oil in such a way to slow the dampening of the fork when the upper leg is moved.
Its very basic and considered the simplest way of making a hydraulic fork work, the problem is its a fixed rate device and cannot work outside of its design, such as in high speed compression and rebound situations.
'Pack down' is a term used when the rebound action is at such a level where it does not allow the forks oscillations to recover fast enough and so with every compression action the fork travel gets shorter and shorter. Only when the repeated force is stopped can they slowly extend back up.
This happens mostly if the rebound dampening is too great, the oil used is too thick, or the springs are too soft.

Back to the Buffalo, if you can add an inch of internal travel to the front end and make it fully usable you will have solved a lot of the design issues. To this end you must have progressive wound springs which are worlds above the fixed rate springs and use proper oil of the correct viscosity.
Since a shop is going to do this work for you, you need to have an idea of what parts they plan to alter/buy and what they are hoping to achieve for you with the 1100 forks in terms of cost.
Two strokes, its just that simple.

69 Suz U70
69 Suz T500
72 Suz GT750 cafe
74 Suz TS250
74 Suz GTXVR project
75 Suz RE5
75 Suz GT750
76 Suz TS400
76 Suz GT750
81 Suz GSX1100
86 Suz RG500x2
88 Hon CR500
93 Hon CBR900RR
98 Suz GSF1200x3
15 Kaw Ninja H2
User avatar
Suzsmokeyallan
Moto GP
Posts: 4326
Joined: Fri Oct 24, 2008 9:11 am
Location: Mostly Barbados, sometimes Florida and western Canada
Contact:

Re: GS forks on a GT750

Post by Suzsmokeyallan »

Here you can see a fork leg from an RE5 versus the GS 650 fork leg. The RE leg and parts is at the bottom and is interesting. Suzuki fitted longer damper rods into these forks and an additional spring at the top to help the issues of the RE5 having more weight over the front of the bike. Notice the size of the GS damper rod at top, its much longer and of a larger diameter so it can hold more oil within its dimensions. The parts on the left end of the GS leg are the seal, washer and the two slider bushings.
The buffalo legs were identical to the RE5s in length except the damper rods were shorter and they did not carry the small top spring seen here.

Image



Here below you see the various damper rods for the Buffalo and RE5:
From the top, the stock GT750 rod,
The middle, the longer RE5 rod
The bottom is the 'trickit' rod that was offered back in the 70s as an upgrade to help cure the lack of travel the Buffalos had. It has a floating aluminum piston thats somewhat adjustable for freeplay and is longer as well.
This required the forks to be raised an inch in the top triple trees to regain proper static stance.
The small holes just under the piston are the bleed orifices, blocking one of these off on stock Buffalo forks is a well known modification to slow the rebound dampening, and improve the forks pathetic 'pogo' action.
The middle RE5 rod has been modified to this effect as you can see the upper hole is covered.

Image
Two strokes, its just that simple.

69 Suz U70
69 Suz T500
72 Suz GT750 cafe
74 Suz TS250
74 Suz GTXVR project
75 Suz RE5
75 Suz GT750
76 Suz TS400
76 Suz GT750
81 Suz GSX1100
86 Suz RG500x2
88 Hon CR500
93 Hon CBR900RR
98 Suz GSF1200x3
15 Kaw Ninja H2
User avatar
jabcb
Moto GP
Posts: 4240
Joined: Mon Dec 13, 2010 4:32 pm
Country: USA
Suzuki 2-Strokes: 69 T350 thru 75 GT750
Location: southwestern Pennsylvania

Re: GS forks on a GT750

Post by jabcb »

Wow, thanks for all this insight. You've given me lots to think about.

I had thought about spring rate, travel & ride height. Had completely missed the potential issue of having too much damping.

This is the Progressive Suspension spring I was going to use.
ProgressiveSpring.jpeg
Would give a progressive spring rate of 35 to 50 lbs/in.
GS1100GK forks allow up to 35 psi air pressure. Was planning on no air pressure.

GS1100GK forks are about 1-1/4 inches longer that GT750 forks.
Lowering GS1100GK forks by 1" would shorten them by an inch and reduce travel to 5.3".
This adds an inch to the GT750 travel. Was expecting to get about the correct ride height via sliding the forks up the triple clamps a little.
Suzsmokeyallan wrote:Back to the Buffalo, if you can add an inch of internal travel to the front end and make it fully usable you will have solved a lot of the design issues. To this end you must have progressive wound springs which are worlds above the fixed rate springs and use proper oil of the correct viscosity.
Since a shop is going to do this work for you, you need to have an idea of what parts they plan to alter/buy and what they are hoping to achieve for you with the 1100 forks in terms of cost.
Looks like I got close to this, but perhaps more out of luck than knowledge.

On damping & oil viscosity, I was just going to use the standard viscosity specified for the GS1100GK. Given everything you mentioned, I obviously need to think about this more.

On parts & shop work... Have purchased all of the needed parts. Only modification needed is shortening of the spacer.

Definitely would agree that starting from scratch, the GS650/GS850 forks are the way to go. Switching to GS650/GS850 forks looks to be the cheapest solution. But in my case, all the parts are already paid for.

Thanks again.
You do not have the required permissions to view the files attached to this post.
BAS (Bike Acquisition Syndrome) - too many bikes but have room for more

Suzuki:
GT750 2x75
GT550 72 & 75
GT380 72
T500 69 project & 73 project
T350 69 & 71
Honda 85 CB650SC & 86 CB700SC
09 Triumph Bonneville SE
User avatar
Suzsmokeyallan
Moto GP
Posts: 4326
Joined: Fri Oct 24, 2008 9:11 am
Location: Mostly Barbados, sometimes Florida and western Canada
Contact:

Re: GS forks on a GT750

Post by Suzsmokeyallan »

I can see your point about using what you already have, and with what you invested so far it makes sense to pursue it to see if you can make it work.
Keep in mind your costing here though, if you are having to throw $150.00 or more at them, its time for a rethinking of the idea.
I think the PO got caught up in the facts the forks were of a bigger diameter and close in their dimensions so 'its got to work better' but then found out after there was way more needed to make it work and just gave up and lived with it as it was.
That 35 psi they considered the upper limit is way too high, I would not recommend any fork to carry more than 10 psi per leg, a fact that still holds forth today now its known those pressures will blow seals
You have to remember back in those days forks for stock road bikes were still following rules of 'lets try this' and see what happens, basically in layman's terms, they were guessing.

History lesson time: :lol:

Its kind of ironic too as the motocross bikes of this same time period were using 41 to 43mm conventional forks and had adjustable compression and some type of rebound dampening adjustments. The road bikes were way behind in anything closely resembling such a fork and it showed.
Technology did not help either, because by the time of the late 80s, they had sorted out the conventional forks in the dirt to levels where the factory race teams were using them with minor mods; they then flipped them over and went right back to the drawing board for years.
The inverted forks brought a visual 'wow factor' to the game, but for years they were plagued by stiction, leaking seals, running dry in the uppers and grinding the bushings and cases, all because they could not get the internal lubrication in the uppers correct.
Ok thats enough of that, let us know how you make out.
Two strokes, its just that simple.

69 Suz U70
69 Suz T500
72 Suz GT750 cafe
74 Suz TS250
74 Suz GTXVR project
75 Suz RE5
75 Suz GT750
76 Suz TS400
76 Suz GT750
81 Suz GSX1100
86 Suz RG500x2
88 Hon CR500
93 Hon CBR900RR
98 Suz GSF1200x3
15 Kaw Ninja H2
Post Reply