|GT750 - Transmissions|
Before we delve too deeply into the GT750 transmission and what can be done to improve it, let's take a moment to consider how gearboxes work.
The transmission consists of two shafts, three selector forks on two rails and one shift drum or cam. Individual gears either slide from side to side on splines on the shafts or they cannot move from side to side but can rotate freely on the shaft
The way that gear selection works is the gear change lever is moved up or down and that rotates the long gear shift shaft which in turn acts through a ratchet assembly to rotate the shift drum. The shift drum contains three grooves which are effectively a cam and as the drum rotates, it cause one or other shift forks to move from side to side on their rails.
As the sliding gears move sideways, dogs on the gear engage with dogs on one of the rotating gears so that gear is now locked to the shaft.
In the case of a GT750, the input shaft from the clutch, first gear (closest to the clutch) is actually machined into the shaft. Next is 4th which rotates freely but cannot move left to right. Next is 3rd gear and that is free to slide laterally on the splines. Next one over is 5th which, like 4th can rotate but not move side to side. Last gear on the right is 2nd and is pressed onto the shaft and cannot move in any direction relative to the shaft.
The output shaft has the opposite layout. Starting at the clutch side, the first gear is 1st gear and it rotates on the shaft. Then it's 4th which is splined, 3rd which can rotate, 5th on splines and finally 2nd which can rotate.
How it Operates
When first gear is engaged, 4th gear on the output shaft slides over so that it's dogs engage in slots milled into first gear. That effectively locks first gear to the output shaft and power is transmitted to the sprocket and via the chain to the rear wheel. What allows that to work is that 4th gear on the input shaft is free to rotate on the shaft. That's why some gears rotate on the shaft, some are fixed and some are splined but can move side to side.
For second gear, as the shift cam rotates it pulls 4th gear back out of engagement and at the same time, moves 5th gear dogs into engagement with second gear which is closest to the sprocket, and so doing locks second onto the output shaft.
Moving to 3rd gear pulls 5th gear dogs out of second gear and simultaneously slides that sliding 4th gear into engagement with 3rd to lock it onto the output shaft. The matching input shaft gear is splined so it remains locked to the input shaft.
4th and 5th are engaged by moving third gear on the input shaft to engage with 4th or 5th on that shaft.
All the dogs tend to get rounded off over time, but the dogs on 3rd gear (output) suffer the most wear on a GT750 because they mismatch the dogs on 4th gear with which they engage. On a GS750, the dogs which engage with first gear were redesigned and on teh GSX750 8 valve motor and GS1100E onwards, the other set of dogs on 4th gear were also redesigned.
Suzuki addressed long term wear issues and not surprisingly, when they fixed one issue, the next one became apparent. They changed dog designs and materials and made some gears slightly thicker. And that's why a transmission from say a GS1100E looks so much like a GT750 transmission. Because it is basically the same box but steadily improved.
The GT750 transmission is pretty much bulletproof but the ratios are far too wide for one that has been ported. That's not so much an indictment of the Suzuki design team, but it's a reminder that back in 1970, that's how bikes were made.
Before, we explore options and improvements, let's look first at what gear sets Suzuki designers settled on for the GT750 and how they changed over the years.
The J (72), K (73), L(74) and M (75) up to GT750-66220 share the same gears and ratios. At that point, fifth gear was changed for slightly lower gear which brought 4th and 5th closer.
Near the very end of the A (76) production, 2nd and 3rd gears were strengthened according to a Suzuki bulletin and they changed the ratios to even out the gaps.
So the last GT750 gear ratios were much better than the early ones. When Suzuki introduced the 4 stroke GS750, they took the opportunity to use the GT gearbox and made one more small change to the gearing and it was a great change to make. They raised first gear and made the gap between first and second smaller. And that is why people have been trying to fit GS750 gearboxes in GT750s.
With the 1100cc 16 valve monster motor, they raised first gear a little more, which is great for a motor with tons of low end torque or for race motors, but not a great match for the average street rider with a heavy stock GT750. In addition, several of the gears were made wider and shafts grew a little longer to accommodate the wider gears. It should be possible to machine the shafts and crankcases to allow an 1100 transmission to be fitted to a race GT750.
A modern 4 stroke has an oil pump to lubricate the cranks and cams etc, and it is normal practice to bleed off a some of that oil supply to lubricate the gear shafts. many modern two strokes also have small oil pumps to make sure that the shafts and gears are always lubricated. But back in the early seventies, that just wasn't the way that things got done.
Instead of adding another oil pump to the already complex machine, Suzuki designed a series of light thin sheet metal stampings to collect oil and to allow it to run down through the shafts using gravity. the most complex of these ducts is the feed to the input shaft. Oil is collected just above the large gear on the clutch. It picks up copies amounts of oil as it rotates and some of that oil is caught by a small duct above the clutch. Oil then passes down into the small round outer cover outside the clutch and passes through a small oil seal into the clutch pull rod which sits in another oil seal in the end of the input shaft.
Complex but ingenious. Of course if we start thinking about using gears from a different motor, such as a GS750 or 6 speed GSXR, we need to keep that in mind.
GS750 4 stroke
Ok, so we discovered that the gear ratios in a GS were better than the GT750, so do they fit? What's involved in making it work? Check this page for more details of the GS750 transmission with lots on input from Suzuki Dave Bonigut from the Sundial and other on line forums.