GT500 twin disc upgrade

[Willsy69]

Hi
I’m looking to upgrade my front brake on my GT500 front end to twin disc
I have the 750 fork bottoms and additional caliper.
Would the GT500 master cylinder work to do I need a GT750 one?
Anyone else done this?
Any advice appreciated
Thanks
Colin

[joolstacho]

I haven't done this, but it stands to reason that a twin disc / twin calliper setup will have more fluid volume to move, so I'd say the master cylinder would need to be correspondingly larger.

[Willsy69]

Thanks joolstacho
My thinking was that as the pads are almost touching the disc there is very little fluid movement. Then again it might not exert as much pressure.
I’ve sourced a GT750 master cylinder on eBay so I’ll have a look at the piston size when I rebuild it to see what the difference is
Cheers
Colin

[joolstacho]

Yes mate, my experience was with CX Hondas - going from single disc to twin.
Agreed, not much volume moves, nevertheless, the volume moved by the master cylinder AND the amount of movement of the actual lever needs to match the slaves volume and piston movement.
Good that you can get the 750 master cyl, can't go wrong with that eh?

[jabcb]

The ratio of the surface area of the master cylinder piston to the sum of the caliper piston surface areas is important.
You are doubling the total caliper piston surface area, so a change in master cylinder is good to consider.

Modern brakes have much better performance & brake feel because of better selection of that ratio and much improved designs.

The GT750 dual disk setup is a good improvement. The GT750 master cylinder piston diameter is larger than the GT500’s.

The GT750 forks have better damping. You can improve that with mods if you want.
Switching fork springs & setting the sag also helps the ride a good bit.
There is some good info on this — let us know if you are considering this.

Additional improvements can be had by switching to GS-series brakes & forks that were designed after Suzuki switched to 4 strokes.

Also recommend switching to modern stainless steel brake lines. They are available with a black outer sleeve so they look pretty much the same.

[Willsy69]

Hi jabcb
I’m just using the GT500 damper assembly. I only got the fork bottoms with no internals. Everything seems to fit but I haven’t done a full assembly yet.
I’d be interested in any improvements that can be made to the front suspension if you have any info or links
Cheers
Colin

[jabcb]

Back in the day Cycle World magazine had some articles about upgrading suspension.
This covers the GT triples: https://magazine.cycleworld.com/article ... me-part-ii
This covers the GT500: https://magazine.cycleworld.com/article ... uki-gt500a

Improved damping + better spring rate selection + properly set sag makes a big improvement.

I have two GT750s. The red bike has stock suspension. The gray bike has GS-series forks + properly set sag.
The ride is much improved & the forks don’t bottom like the stock forks.

It is possible to get modern ride quality + the vintage look.
The GT-series forks don’t use top out springs. Suzuki started using top out springs on some early GS-series bikes.
Early GT- & GS-series bikes do not use upper & lower fork slider bushes, which reduces static friction.
There a few Suzuki GS models that have forks which fit the GT500 triple tree and have top out springs + both bushes.
Adding RaceTech emulators & setting them up correctly give impressive results.

Articles on suspension improvements can be quite confusing.
Pre- top out springs articles tend to discuss using softer springs + more preload.
With- top out spring articles tend to discuss using stiffer springs + less preload.

[markush]

There a few Suzuki GS models that have forks which fit the GT500 triple tree and have top out springs + both bushes.

Right!

Here is an example of this:
http://www.suzuki2strokes.com/forum/vie ... =7&t=11651

The smaller 14 mm master cylinder (from GT250-550) is used here for double discs and works perfectly as long as Steelflex lines are used everywhere.

[Alan H]

Interesting read re. the forks. Many years ago, Frank Whiteway & Eddie Crooks told me that they used 20w50 oil in the forks of their racing bikes to stiffen the damping, and I've always followed their advice.

I've twin front discs on my 550A (and one on the back!) and originally used a 1200 Bandit master cylinder, but changed it for a pair of cheapo chinese master cylinders with 14mm pistons when I made the clutch into hydraulically operated too.

[jabcb]

Went feature crazy on my 73 T500 cafe build.
GT500 triple tree
GS550 inner fork tubes & GS750 outer fork tubes with top out springs & both fork bushes
Racetech emulators & springs with preload set for my weight
2.15x18 rim
dual-piston caliper from a Suzuki VX800
Nissin master cylinder for a Suzuki GSX + ASV levers, bore selected to get desired brake feel
320mm EBC brake rotor
twin-piston caliper adapter bracket for GS forks
SS brake line
Acewell speedo/tach

cafe-T500-2629 by jabcb, on Flickr

[Willsy69]

Thanks for all of the info
Would these be any better than the stock springs.
https://www.wemoto.com/bike/suzuki/gt-1 ... th-germany
Cheers

[jabcb]

Probably an improvement. They don’t list the spring rate.
Setting the sag will make a big difference.

[markush]

Yes, probably, maybe, possibly.
Without knowing the spring rate of the original and aftermarket springs, it's a case of trial and error.
In general, the GT Wirth springs are harder than the originals.
I don't think they put much thought into how to achieve a reasonable GT fork setting. e.g. They recommend using their SAE10 fork oil when using their springs. https://www.wirth-federn.de/produkt/wir ... 1972-1976/
This is basically nonsense, as the GT forks damping require at least SAE20, or better SAE30 oil.
Since the GT fork has quite soft damping, it is rather unfavorable to simply install harder springs, as the damping can only be adjusted to a limited extent by the choice of oil and, to a very small extent, by the filling level.

If the original springs are unusable or lost, etc., the (harder) accessory springs are an alternative.
But I would rather get some cheap used original springs and choose from them some with the same length and spring rate.
But for the price of the (accessory) springs you can get a good used GS(X) fork, it's definitely better than a GT fork and almost plug and play.

[jabcb]

The Cycle World approach for the GT series forks is softer springs with more preload. This is for forks without top out springs.
Given markush’s observation about Wirth’s springs, I would go with a different solution.

Spacers are used to get the correct length & preload. You set the sag by adjusting the spacer length. This deals with spring length variations & sets the preload.

GS series forks that fit the GT500 triple tree may longer than the GT forks. (The 4-stoke motors are taller & narrower — the heads have the valves & there are no transfer ports. Suzuki also stopped putting so much stuff on the end of the cranks.)

That’s why I used GS550 inner fork tubes. The GS550 forks are shorter than the GS750 forks. That wasn’t enough to match the GT500 fork length. Switching to an 18” rim helped. Used longer top out springs to shorten the forks a bit more.