1981 gs750 (gs750e) Retro Racer project

[LarsKroghStea]

Sorry to hear about your speedo mishap. Glad you made it home safely, though. That's all that really matters.

Why not just buy a new/used speedo assembly?

Because I think it will be easier to fit spacers for the new brake rotors if I don't have to take the speedo assembly into consideration. And I already have the parts (the magnetic bolt)

 

[93Bandit]

Because I think it will be easier to fit spacers for the new brake rotors if I don't have to take the speedo assembly into consideration. And I already have the parts (the magnetic bolt)

Gotcha. Carry on!

 

[LarsKroghStea]

Well.. I've been drilling some holes and the "corner"-holes on the nuts are a pain to drill. The bolts are OK, I drill them at a 90 degree angle, but the nuts.. I've given up on them. Mostly because I dont have any 2mm drill bits left :/ 3mm drill bits hold well, and I'll use 2,5mm drill bits for the rest of the perpendicular holes (it's a national holiday today in Norway, and I can't get new drillbits). I think the problem is that the bits snag on the hardened surface when you push through on the other side. I've heard that drill bits for drilling circuitboards are cheap and work great for this. Maybe I'll try them later.The bolts for the brake discs will be OK when they are drilled and wired. I'll buy titanium bolts for the brake conversion later.


For the sprocket bolts I've ordered these Pro-bolt pre-drilled aluminum nuts with steel core. They are very popular in MX competition and should hold up well. They also make them in titanium, but these were in store, and I don't want to wait.
lspn10dbk_7 by Lars Krogh-Stea, on Flickr

 

[LarsKroghStea]

Perseverance has turned frustration into joy once more I googled around on how to drill bolts and got some tips on drill-speed and pressure on drillbit. I've read before that small drillbits require high speed, but this guy said to use slow speed, max 1100 rpms. Geared mine to the lowest, wich is 550 rpm. He also said to oil the bit every 30 seconds :0 That told me I was way too impatient! I can now drill the bolts by listening to the sound the drillbit makes. If it makes a kind of crackling sound, the bit is either about to pack full and overheat or about to brake through the hardened surface on the other side. If the first is true, I back the bit out and clean it and reapply oil. If the bit is about to break through I'm very careful with how much pressure I apply. This was a bit tideous, but I didn't breake anymore drillbits. Took me about 1.5 hours to drill all the bolts for the front and rear discs. I also painted the center of the rear brake rotor. Pictures included as usual:


Rear rotor, painted and drilled bolts torqued down, The one odd bolt is the magnetic one. Should have drilled it last, as it made the drillbit magnetic and a pain to clean.
IMG_20200521_222827 by Lars Krogh-Stea, on Flickr


This is how it was before:
IMG_20200520_210733 by Lars Krogh-Stea, on Flickr


This is how the safetywiring looks now. I went with connecting two in pairs instead of three. I watched a youtube clip of an older guy wiring a propeller for a small plane, and tried to copy what he did. He had several very good tips on how to twist the wires to keep them tightly in place.
IMG_20200521_180148 by Lars Krogh-Stea, on Flickr

 

[LarsKroghStea]

I'm sure you're pretty tired of me posting pictures of safetywire by now, but I recieved the new nuts today. Must have been an older version as they were NOT drilled Anyhow, as they do look much nicer than the ones I that are on now, I drilled them myself and put on a couple of nuts to see the results. Pretty satisfied
2020-05-25_11-12-34 by Lars Krogh-Stea, on Flickr


I also put together the GSXR fork to get a bether view of how it all fits together, and I must say I'm sceptic as to how I'm supposed to fit a spoked wheel between those calipers? The angle is little off on the picture where I measure. The more correct measurement is about 47mm (1.85inches). As they are radially mounted I could get some 320mm discs but that would only move them 1cm towards the narrower part of the wheel.
2020-05-22_09-15-19 by Lars Krogh-Stea, on Flickr


2020-05-22_09-15-57 by Lars Krogh-Stea, on Flickr


2020-05-22_09-15-43 by Lars Krogh-Stea, on Flickr


I checked the Cognitomoto wheel and they have laced it with all spokes on the inside. Mine are the usual inside/outside. If that's how it's done I'll need another set of spokes to get all of them in the correct angle, and then I might as well get a wider front rim/tire. then it starts to get expensive.. If I have to do the front wheel all over again, I will have to wait for the winter to do the fork swap :/
cognitomotowheel by Lars Krogh-Stea, on Flickr

 

[LarsKroghStea]

I have mounted the hoodlatches for the "quick-release" seat:


IMG_20200530_210232 by Lars Krogh-Stea, on Flickr


Cut the bolts to length, drilled and tapped them. Then I mounted them on flat piece of steel:
IMG_20200530_210210 by Lars Krogh-Stea, on Flickr


IMG_20200530_210059 by Lars Krogh-Stea, on Flickr


I will cut this plate to size and screw it on to the frame of the rear seat:
IMG_20200530_210104 by Lars Krogh-Stea, on Flickr


This is how it looks underneath the rear fender when the seat is locked in place:
IMG_20200530_205935 by Lars Krogh-Stea, on Flickr

 

[Guest]

This look like a proper build. Wil follow this thread.

 

[LarsKroghStea]

Did a test run with a GoPro mounted to the swingarm to check how the shocks work after rebuild. These are vintage Koni 7610 with the preload set to the lowest out of 3 (twist sleeve) and rebound set to 2 out of 4 (twist dial on top). They're filled with 5W fork oil. The short videoclip is from passing over a speed bump at moderate speed. You can also spot one of the hood-locks. The seat stays well in place.

 

[LarsKroghStea]

As the Covid situation drags out, and I'm busy working overtime at the hospital, I made the decision to buy the Cognitomoto front wheel. This wheel costs more than I paid for the whole bike, but there's no way I'm gonna have time to do all the nessesary modifications to my old front front wheel. This will be my xmas gift to myself, a reward for working all the long hours:

photo_2020-11-27_23-48-38 by Lars Krogh-Stea, on Flickr

 

[steve murdoch]

That wheel is beautiful.

 

[LarsKroghStea]

I've done some preparations ahead of the fork swap. Put the before and after measurements in to a free calculator i found online:

https://sebwiers.github.io/motorcyc...bFTO1.2bFAO0.0bRT-0.0bNO0.0bNU0.0bRO0.0bRU0.0

The graphic presentation looks like this:
forkswap_2 by Lars Krogh-Stea, on Flickr

As you can see, the fork is a bit shorter (2") and the wheels aren't as tall. The rake changes from 28 to 26,4° and the trail changes from 4,3" to 5". Also, the wheelbase is shortened by 2", part because of going away from leading axle and part from the framedrop resulting from the shorter fork. The triple offset i almost identical, but the offset is still cut in half because the leading axle adds another 1,2" (30mm). I've kept the rim diameter equal to stock, but I've changed tyres to 150/70-18 on the rear (4.25" rim, ran this tyre this summer and I'm loving it) and 120/70-19 in the front (3.5" Cognito-moto wheel being built as we speak). Ran with 100/90 on spoked 79' GS-wheel this summer. I think I've avoided spooky turn-in problems by not swapping in a 17" front wheel while lengthening the trail, but I would love if anyone could look over the numbers and tell me if I'm running into any surprises
The measurements are not exact, but pretty close unless I've done any measuring or conversion errors (I'm taking measurements in mm).

forkswap_1 by Lars Krogh-Stea, on Flickr

 

[John Kat]

I've done some preparations ahead of the fork swap. Put the before and after measurements in to a free calculator i found online:

https://sebwiers.github.io/motorcyc...bFTO1.2bFAO0.0bRT-0.0bNO0.0bNU0.0bRO0.0bRU0.0

The graphic presentation looks like this:
forkswap_2 by Lars Krogh-Stea, on Flickr

As you can see, the fork is a bit shorter (2") and the wheels aren't as tall. The rake changes from 28 to 26,4° and the trail changes from 4,3" to 5". Also, the wheelbase is shortened by 2", part because of going away from leading axle and part from the framedrop resulting from the shorter fork. The triple offset i almost identical, but the offset is still cut in half because the leading axle adds another 1,2" (30mm). I've kept the rim diameter equal to stock, but I've changed tyres to 150/70-18 on the rear (4.25" rim, ran this tyre this summer and I'm loving it) and 120/70-19 in the front (3.5" Cognito-moto wheel being built as we speak). Ran with 100/90 on spoked 79' GS-wheel this summer. I think I've avoided spooky turn-in problems by not swapping in a 17" front wheel while lengthening the trail, but I would love if anyone could look over the numbers and tell me if I'm running into any surprises
The measurements are not exact, but pretty close unless I've done any measuring or conversion errors (I'm taking measurements in mm).

forkswap_1 by Lars Krogh-Stea, on Flickr

Hi Lars,

I've looked into this some time ago:
Basic maths for fork conversions (thegsresources.com)
Basically lowering the front end by 1" will decrease the rake by 1 degree.
This corresponds to your numbers.
As you are not changing the diameter of the front wheel, the trail ( with the reduced rake) now only depends on the offset of the triples.
Decreasing the offset increases the trail and your numbers seem right but I would use my formula to check the numbers.
In my view the optimal numbers are 25? of rake and 100 mm of trail like most modern bikes.
It's difficult to get to these values on our bikes specially if you keep the 19" front wheel.
The bonus is that the bike will be very stable but slow to turn in...
Congrats on the project!

 

[Buffalo Bill]

Just a suggestion to make your project more complex, vintage racers cut and weld the head tube on at a steeper angle/rake like modern sport bikes.
However, I don't think I'd do that unless I was seriously competing on track.
On my country roads I find the standard rake of my GS1100G with it's 19" wheel gives a steady trustworthy direction into and through all the difficult turns. I've raised the rear and lowered the front.
The steeper rake and smaller 17" wheel on my 87 CBR600, kinda turns too quick and I'm not trusting it as much. I see a lower speed on the speedometer through the same turns.

 

[LarsKroghStea]

Thanks for the info Your posts brings up something I'm curous about. Longer trail means more straight line stability, but at the same time people report "too quick" turn in. My guess is that with too much trail, the wheel will react when the bike leans over by wanting to flip over, into the turn. This makes the bike unpredictable. The whel wants to stay straight until it suddenly wants to turn too quickly. My feeling (and it's just really a semieducated guess) is that this is most pronounced with a smaller diameter wheel. Does anyone have any facts to support/contradict this? Will also read the thread you referenced

 

[John Kat]

Thanks for the info Your posts brings up something I'm curous about. Longer trail means more straight line stability, but at the same time people report "too quick" turn in. My guess is that with too much trail, the wheel will react when the bike leans over by wanting to flip over, into the turn. This makes the bike unpredictable. The whel wants to stay straight until it suddenly wants to turn too quickly. My feeling (and it's just really a semieducated guess) is that this is most pronounced with a smaller diameter wheel. Does anyone have any facts to support/contradict this? Will also read the thread you referenced

You might also be interested in this thread I wrote ?
A modern fork upgrade for 17,18 and 19" wheels - Page 4 (thegsresources.com)

 

[LarsKroghStea]

You might also be interested in this thread I wrote ?
A modern fork upgrade for 17,18 and 19" wheels - Page 4 (thegsresources.com)

I've read your posts, and lots of others.. I've found out what they're talking about, when they say bikes with too much trail is dangerous in the corners; if the steering is too slow, the front wheel will not follow a natural line around a bend in the road. It will tend to follow a wider arch. If you're expecting it to steer quicker, you will lean into the curve and the bike could fall over because there is not enough force acting perpendicualr to the curve to counter your lean. The result will be a lowside crash.

Not that I'm a pro rider, but I think this would be a bigger problem for inexperienced riders. I've encountered similar effects when entering curves at 100mph with a passenger on the bike. The bike is heavier than normal and wants to "stand up", resulting in a wider arch. Inexperinced drivers will often react by turning more into the curve, resulting in the bike standing up even more and you risk driving off the road on the outside of the curve. Or crashing with cars in the opposite lane. This is countered (opposite to what inexperienced drivers may think) with reacting quickly and turning the front wheel towards the center line of the bike, like you wanted to run off into the woods. This will momentarinly tip the bike over enough that you can lean into the curve and the bike will follow through.

I've never had bikes with very large trail numbers myself, but at a time, early in my riding career, when I was used to my Kawazaki ZX7r which had very short suspension travel I borrowed a Kawasaki Zephyr with very soft, long stroke forks. When I took the offramp near home at the same speed and lean angle that I was used to, I nearly fell into the turn. The trail is about the same 99 vs 102mm, but the suspension reacts diffently.

My belief is therefore that many accidents linked to forkswaps are as much about riders doing what they're used to with a bike that handles totally different instead of taking the time to learn the bike as if it was a totally different bike.

I'll go ahead with the swap, knowing that I'll end up with 1-2 cm more trail. I'll go easy with the riding until I know how it handles, and if I'm not comfortable with it, I'll make the ride hight taller at the rear. If that's not enough I'll make or buy triples with more offset. I don't think the bike will be dangerous or unpredictable, I just think it will have a lower tolerance for high speed in sharp corners and that has to be taken into consideration when riding

 

[John Kat]

For someone used to riding bikes, there's no danger in more trail, it's just a question of getting used to the different feeling.
BMW with their R nine T for example has 108 mm and 27.8? of trail and rake.
I would say that reducing the trail makes the bike more fun to ride in the twisties.

 

[LarsKroghStea]

For someone used to riding bikes, there's no danger in more trail, it's just a question of getting used to the different feeling.
BMW with their R nine T for example has 108 mm and 27.8? of trail and rake.
I would say that reducing the trail makes the bike more fun to ride in the twisties.

I agree, and I'm grateful for the oppurtunity to have this conversation. I was afraid I was missing some important element to the eqation. I've seen some youtube pro-racing instructional videos and read several articles the last couple of days, and my (and others) conclusion is that much of what you read on the internet about the dangers of too much trail is hearsay stories told by people that don't really know what they are talking about. In one YT-video they went so far as to post questions to several forums about swapping forks. There was a big crowd of keyboard-experts warning about the "dangers of changing geometry". When they were asked followup questions, about 5% of the people warning people not to swap forks knew what they were talking about.

I will go forwad with the swap and do both the calculations and the psysical measurements. I took pictures of the color coding on the Koni shocks before I painted them, and I will maybe order new springs for them. I have the 4-step preload on the highest setting and they seem pretty soft. I haven't measured sag yet, but if sag is way off, I will either order new springs or new more modern (and maybe longer) shocks with better preload adjustment. The Koni shocks are serviceable, and I have recently serviced them. I also think they match the bikes look, so if I can keep them i will.

There are still a lot of stuff to do before the bike is ready, so I will save the fine adjustments for later. It's important to never get too satisfied with the bike. It's a hobby after all, and I don't want the journy to end

 

[John Kat]

I agree, and I'm grateful for the oppurtunity to have this conversation. I was afraid I was missing some important element to the eqation. I've seen some youtube pro-racing instructional videos and read several articles the last couple of days, and my (and others) conclusion is that much of what you read on the internet about the dangers of too much trail is hearsay stories told by people that don't really know what they are talking about. In one YT-video they went so far as to post questions to several forums about swapping forks. There was a big crowd of keyboard-experts warning about the "dangers of changing geometry". When they were asked followup questions, about 5% of the people warning people not to swap forks knew what they were talking about.

I will go forwad with the swap and do both the calculations and the psysical measurements. I took pictures of the color coding on the Koni shocks before I painted them, and I will maybe order new springs for them. I have the 4-step preload on the highest setting and they seem pretty soft. I haven't measured sag yet, but if sag is way off, I will either order new springs or new more modern (and maybe longer) shocks with better preload adjustment. The Koni shocks are serviceable, and I have recently serviced them. I also think they match the bikes look, so if I can keep them i will.

There are still a lot of stuff to do before the bike is ready, so I will save the fine adjustments for later. It's important to never get too satisfied with the bike. It's a hobby after all, and I don't want the journy to end

The best book I read on the subject is :
Motorcycle Dynamics by Vittore Cossalter.
It's very technical but covers any question you might have about the dynamics of a motorbike.
There's a chapter in particular that focuses on the role of the "normal trail" that aligns the front wheel with the forward velocity while the vertical load on the tire tends to cause the wheel to fall into the curve.
This is very noticeable when you put the bike on the side stand.
During normal riding, the rider must thus compensate for the action of these two forces to maintain the equilibrium.
It's all on page 34 of the second edition of the book!
BTW it's quite possible to get the magic 25?/100 mm numbers with a 19" front wheel as demonstrated by Ducati with their latest Multistrada V4 S quoted at 24.5?/102.5 mm.
Last but not least, let's not forget that the reason trail bikes use a 19" front wheel is because when you hit a rock while riding off road there is a sudden big decrease of trail with the consequences we know...

 

[LarsKroghStea]

The best book I read on the subject is :
Motorcycle Dynamics by Vittore Cossalter.
It's very technical but covers any question you might have about the dynamics of a motorbike.
There's a chapter in particular that focuses on the role of the "normal trail" that aligns the front wheel with the forward velocity while the vertical load on the tire tends to cause the wheel to fall into the curve.
This is very noticeable when you put the bike on the side stand.
During normal riding, the rider must thus compensate for the action of these two forces to maintain the equilibrium.
It's all on page 34 of the second edition of the book!
BTW it's quite possible to get the magic 25?/100 mm numbers with a 19" front wheel as demonstrated by Ducati with their latest Multistrada V4 S quoted at 24.5?/102.5 mm.
Last but not least, let's not forget that the reason trail bikes use a 19" front wheel is because when you hit a rock while riding off road there is a sudden big decrease of trail with the consequences we know...

Got a hold of the PDF and read the first chapter. Pretty math heavy, but good indepth explanations of how a motorcycle behaves. One doesn't have to follow all the calculations to understand the hows and whys. I see the next chapter is about motorcycle tires, and I expect to get som good answers there too, about stuff that's hard to get to the bottom of by just reading articles online. Absolutely a great book