1981 gs750 (gs750e) Retro Racer project

[LarsKroghStea]

Made a new drawing of the front wiring. The current setup is a bit messy, and with the new fork/headlight, I just as well redo it all. Too many wires together makes the loom stiff, and I'm grouping the wires by maximum five.I'm also running the wires inside the clip-ons this time, including the front brake-switch.

Wiring front by Lars Krogh-Stea, on Flickr

[LarsKroghStea]

Like that look.
You can get a 120/70 on there? Excellent!

Yes, the plan is for a120/70 Pirelli, same as the rear.

[steve murdoch]

Like that look.
You can get a 120/70 on there? Excellent!

[LarsKroghStea]

And here's a couple of pictures of the wheel/GSXR rotors and the custom Cognito Moto steering stops I bought with bonus points from buying the wheel. The unboxing experience really is premium and the wheel comes with soft cloth "showercaps" on both sides for protection. The bolts and steerings stop come in separate small cloth bags that are tied to the spokes so it wont get lost ine the styrofoam pellets.


Cognito Moto Gsxr Wheel by Lars Krogh-Stea, on Flickr


Cognito Moto Gsxr Wheel by Lars Krogh-Stea, on Flickr


Cognito Moto Gsxr Wheel by Lars Krogh-Stea, on Flickr


Cognito Moto Gsxr Wheel by Lars Krogh-Stea, on Flickr

[LarsKroghStea]

Well, it isn't christmas without a gift from me (and sometimes my wife) to my motorcycle This year it was the nicest gift since the brake master/clutch. Most of the stuff I get for my bike is either pretty cheap to buy, or they're cheap because I make them myself. And to start, the bike itself was as cheap as they come. The most expensive parts so far are the m-unit and the Accossato brake master and clutch levers. Second is the Motogadget Mini and the rear wheel. The Cognito Moto front wheel that I bought this year probably costs the same as all of the others added together. It certainly cost more than the bike itself.


On the other hand, insurance for the GS750 is next to nothing and compared to the insurance on the Kawasaki Zx7-R I drove when I got my license, the Cognito Moto wheel costs about two years worth of insurance. Better to give the money to myself Just realised that what you have been witnessing in the last few sentences is me justifying to myself the process of bying a ridiculously expensive part for my bike


Anyway..I've been looking for cheaper ways to convert to GSXR front fork with spoked wheel. There are other companies that make conversion hubs for motorcycles and most are even more expensive than Cognito Moto. Most of the time they are also far from being as complete a package. I thought of milling one myself, but there's not much room for errors and as T8erbug (thegsresources) said, aluminum blocks that size are so expensive that it doesn't make sense when you factor in errors and do-overs. Another option was a Harley front wheel or buying just the hub from CM and sourcing the other parts myself. I know Harley wheel is only 1mm or so narrower. Used Harley parts aren't as cheap in Norway as they are in the US. Adding to this is the price of special spokes. The cost of other options vary wildly, but the CM spokes are really high quality and looks great.


The last option was to make new front axle to convert my original front wheel. This would be a good option if I didn't want to use a radial fork. The radially mounted calipers sits closer together than older brake setups. On the original GS fork there's already little room between the calipers (depending on wheel size, some use GS1000 calipers with the tapered back to get the wheel to fit).


Long story short; I could probably, with a substantial amount of work/time spent making contacts/sourcing parts/money spent on parts that I didn't need/money spent on materials for adapters and so on, make a front conversion wheel for something between 50 to 75% of the price of the CM wheel.


The way the situation is now, that time is better used working overtime at my regular job and using the extra money to buy a preassembled CM wheel. It's certainly a satisfying feeling to have made something like this (I've alredy made the rear wheel from parts ). On the other hand, you can't take away the feeling when you unpack such an awesome product as the CM wheel.


Conclusion/TLDR: You can not go wrong with buying the Cognito Moto front wheel. The quality is awesome and if you factor in your own hourly rate, I don't think you can make it cheaper (unless you are a professional).

[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

[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]

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]

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]

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]

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]

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

[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.

[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/motorcycle-front-end/planner0/planner_002.html#bXSHIFT0bYSHIFT0aFWD26.1aRWD26.5aFNR28.0aWB59.8aFL30.5aFTO1.1aFAO1.2aRT-0.0bFWD25.6bRWD26.3bCHOP0.0bFL28.3bFTO1.2bFAO0.0bRT-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!

[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/motorcycle-front-end/planner0/planner_002.html#bXSHIFT0bYSHIFT0aFWD26.1aRWD26.5aFNR28.0aWB59.8aFL30.5aFTO1.1aFAO1.2aRT-0.0bFWD25.6bRWD26.3bCHOP0.0bFL28.3bFTO1.2bFAO0.0bRT-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