GT750 Drag Bike

[Old School]

Cool vid, It have been nice to hear the bike better. To tell more what is was doing. In the first video.

[water cooled]

Dave, I slowed the video down to see what was going on and unfortunately, the audio goes away. I'll post the normal speed tomorrow. If you watch the slow-mo, you can see the bars hit then pitch the bike back down. It bounced 3 times before settling down. The rear slick is new and I was trying to find the right pressure. I started at 8.4, spun it and then lowered it to 7.6 for the next pass. I raised the bar one full turn on the eye bolt to compensate. When I post the vid normal speed you can hear the motor and see the bouncing better. It was the first time I started to get the front wheel up but I wasn't ready for it and moved around way too much. I also dont have fork stops installed so as the bike rises, the forks telescope out with the wheel which also shift some load forward and off the back wheel. Anyway, thats my initial analysis.

[tz375]

Is that a Bob Prior technique that he learned from taming 'Roos?

That was a heck of a launch for sure. You did well to stay on and headed in the right direction. And I was contemplating building a killer motor for the Stock Street class... WTF was I smoking/drinking/dreaming that night? I think I'll keep mine stock.

[Suzukidave]

Kevin , did you ever explore the idea of lowering the rear to set the swing arm in neg droop ?

[water cooled]

SuzDave, yes, I can drop the back end slightly but not the front without machining the forks down.

I compiled the video clips from this weekend with some from last year. Unless my boys come with me, its not easy to capture footage at the races so I dont have some of the quicker passes made. Hopefully next year, they will come and record some my 9.99 second pass. Anyway, here is the video. Let me know what you think:

http://www.youtube.com/watch?v=5Q7LIiD_KKY&feature=plcp

[Old School]

The swingarm angle itself plays a very small role if any in a soild suspended bike. If he were using shocks with actual movement, now thats a whole different matter.

Kevin as you already know, there is most likely some benefit to lowering the entire bike.

The single biggest factor in launching anything with a clutch, is clutch management. Over the years I have seen way too many racers not get the most from a good combo because the clutch was not correct.

[tz375]

I think that Suzukidave was thinking in terms of lowering the bike at the rear rather than changing suspension (!) geometry. He may also have been thinking about the squat effect of applying more force to the rear through more effective torque transfer.

Old School Dave - you mention clutch management and it looks like many racers just seem to dump the clutch and let the tire and suspension work out the kinks. The Late Bill Calcutt said that he dumped the clutch and let tire slip/grip act as the progressive slip arrangement. That didn't sound consistent with what I hear from racers about avoiding tire spin, so what gives?

I come from a road race background and we feather the clutch to keep revs pretty much constant at peak torque until the clutch is all the way home and it's time for another gear.

What the most effective technique for drag racing?

[water cooled]

Dave, I have to keep 2" min between the pavement and the bottom of the center pipe. I can go down some but converting the center pipe to a sidewinder would be the way to go, then smash the bike down and cut the forks or change the top triple clamp so the handle bar wont be directly above the fork tubes. Then, I could drop the both ends. The plan for now is to beat the index of 10.00 for this sub-class and then change frames to a chromoly setup I have waiting in the wings. Then, chase the all-out record for a GT-750 single engine... which was set by Robin Miller and Dick Fortino back in 1977 at 9.39 sec. Im still blown away that these guys achieved that so long ago! That would be impressive today given advancement in ignition and pipe designs.

I have a stock clutch with ALTO Red Eagle friction plates and heavier GS springs. One extra friction and steel plate to meet the correct stack height but theres not much more to do in the adjustment. Ive set the cable so rpms rise as the clutch lever goes out from the bar but the launch RPM is programmed into the MSD. Once I solve the traction issue with the rear tire pressure and bar height, I will start raising the launch RPM incrementally until I hit the next limit. Peak torque occurs at 7555 rpm but I doubt the bike will launch that high.

What were you thinking regarding the clutch setup? Im not sure what more could be done (short of straight cut gears or going to a slider).

[Old School]

Richard Dumping the clutch does work on occassion with some condititons, and as you posted it is not very consistant. In a perfect world, you would not want the tire to slip at all. The clutch to be set perfectly to allow the engine to apply the power down without bogging or tire spin at the quickest rate possible.

The most effective way depends on how much traction is available, Slick or street tire and track conditions. In most cases with a slick and bars, I would think that "dumping the clutch" method with the clutch doing all the work would be preferred over wheel spin.

With a street tire and no bars, there are a couple schools of thought. Both still not wanting wheel spin. First is holding the bike at redline or a desired high rpm and "hanging onto the clutch", (letting it out in a slower controlled motion). Second is similar but done starting at a lower rpm and getting the clutch completely out sooner. The key with both is to make everything one fluid smooth motion.

Kevin I understand the gound clearance issue. One thing I am going to look into is, using a set of 1 inch bar spacers and running flatter bars. This will allow me the slide the tubes up higher. I already have a 18 inch PM Chicane with a low profile tire to help lower the front. I like a "no bar" bike to be low in the front.

It is hard for me (or anyone) to give solid advice about the clutch without actually seeing and hearing it in person. It would be my guess that with the limited power of GT, you would want a "controlled clutch slip". A slider would be nice, short of that a lockup with light springs.

On your setup, I would try to lighten the clutch "grab" as much as I could without getting into top end slipage. What I am suggesting is a more stock setup (stock clutches, stock number, springs) to get some controlled slipage on lauch. Again this being said without knowing how it acts firsthand.

[Suzukidave]

I think that Suzukidave may have been thinking about the squat effect of applying more force to the rear through more effective torque transfer.

It was also noticed years ago with the stock GT frames that they would load up like a big spring ( noticed in cornering at the time ) and then release , load and release resulting in speed cornering wobbles . This gave the GT's the name Flexy Fliers back in the day .. this can also happen with a hard start , at the launch the frame is loaded in the first second with the clutch release . With the swing arm in positive droop the chain is trying to pull the rear wheel to the engine acting to lift the rear . With negative droop the rear would want to squat plating the tire and pushing the front down at the same time .

[water cooled]

SuzDave, Id not heard that explanation before but it makes perfect sense. I knew about the Flexi-Flier part and Ive looked at where the roadrace guys would brace the frame. I had always thought that the GT frame twisted but your explanation is excellent about how the chain pulls the rear wheel to either side of the pivot point. I need to think more about this.

I did get an explanation from two or three of the two-stroke racers all agreeing that wheelie bar height might have been slightly high which loaded and unloaded the tire. Like anything, there is debate about Goodyear vs Mickey Thompson but I think I am really going to like the change to the 7" MT rear wheel.

Dave (from Anderson), I have been thinking a great deal since yesterday about the clutch setup and some of the ideas you put out there. Regarding the slider, Kevin Bookheimer of MTC spent a bit of time with me to determine a solution for the GT750 clutch. The ZX-12 forged basket and hat assembly seemed to be the closest fit for the GT. With a minor mod to the basket height and dremel work on the inside of the lower case on a bolt hole, the MTC would fit. However, there were other factors such as working around the helical clutch gear on the GT where a straight cut setup would be preferred but still workable. The final solution would require a mod to the RH clutch cover which included hogging out some of the aluminum to clear the slider hat but still provide bearing support through the cover. It all seemed doable but MTC was getting loaded up with work and it was hard to keep moving forward.

[Old School]

Dave is correct with the explanation on how the suspension reacts on a bike with shocks. However on a rigid bike (one with struts like Kevin's) This dynamic is taken out of the picture because the suspension no longer has any movement.

Kevin If your wheelie bar was set too high, the bike would rock back onto it with enough force to lift (or unload) the rear tire causing it to slip.

Debate or not reguarding which tire is best, I have always gone back to known proven ideas or things that work. Not to be disrespectful in any way (I love these old GT's too), but with only 150hp on tap I just can't believe that any good slick wouldn't be up to the job if everything is correct. My reasoning to say this is, I see too many "street bikes" with street tires and no wheelie bars running down in the 7's. They are able to do this with good clutch management.

[tz375]

Time for straight cut primary gears. Put the new clutch gear on your new clutch drum and you are good to go. They are available from Nova and Oily Tom IIRC.

[Suzukidave]

Time for straight cut primary gears. Put the new clutch gear on your new clutch drum and you are good to go. They are available from Nova and Oily Tom IIRC.

And maybe the GS transmission ? If only i had my metal working machines i left in Florida i would machine you up a set

[Suzukidave]

Dave is correct with the explanation on how the suspension reacts on a bike with shocks. However on a rigid bike (one with struts like Kevin's) This dynamic is taken out of the picture because the suspension no longer has any movement.
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My thought is with the GT's flexable frame , the frame itself is acting as suspension . With the wheelie bars and struts attached to the rear of the frame , at launch the frame is " coiling up"just like a spring . i think it was SmokeyAllan that saw this first hand when he was fixing the bent neck on a 72 frame , with a pipe through the steering head he was surprised how easy it was to move the neck around .