1981 gs750 (gs750e) Retro Racer project

[LarsKroghStea]

Be aware that the modern inverted forks are typically shorter than the oem resulting in less ground clearance.
On top of that the triples have less offset resulting in more trail made worse if you keep the 19" wheels.

Thanks for the heads up I did some research before I bought the forks, and adjusted the original forks to the same hight to test ground clearance. I have the 4 into 1 Kerker exhaust where all the pipes are centered, so the engine will scrape the ground before the exhaust. I've also moved the footpegs up a few centimeters. The gsxr triples have the same offset as the gs (about 25mm iirc) and although the axles are mounted differently, they aren't that far apart vertically. As you can se from the image below (which is an internet image of the exact model bike I had before) you dont need much ground clearance as these forks have much less travel. They achieve this by having bigger valves and stiffer springs. This makes them much better at absorbing small sharp bumps and at the same time they dive much less when braking hard or turning at high speeds. I've bought a used Gopro with a tube-clamp, and I'm filming different angles to see if somthing is close to rubbing or scraping. I've used it to check clearance between front wheel and the exhaust and when I have the seat in place I will use it to check iff the rear wheel is close to rubbing against the inner fender (under the airfilter). But I apreciate the concern!

2020-05-08_10-43-51 by Lars Krogh-Stea, on Flickr

[80GS1000]

Great project here. Subscribed to your thread.

Something to consider that might save you a little time and money: K6-K7 (2006-2007) forks are anodized black, sprung from the factory with 1.0 kg/mm springs (suitable for the weight of a GS), relatively cheap, relatively long compared to other modern USD forks, and the stock triple clamp has offset good for creating enough but not too much trail as long as you address the rear end of the bike as well. If I could do it over again, I'd probably use these forks on my bike instead of the K4 GSXR 1000 forks on it now.

Which segues into, if you go the USD fork route, you'll need to take care of the rear end of the bike as well to get your geometry numbers right (rake, trail, swingarm angle, shock(s) angle etc etc).

Merely installing a USD front end on a GS in my experience without taking into account all the geometry numbers and the rear suspension will make it handle *worse*, not better, because the offset of the triples is set to create enough trail on the steep rake angle that modern sportbikes run. On a GS without rear end modifications it will create too much trail and cause the bike to be hesitant to turn.

If you do get it right though, man, what a difference from stock!! Super flickable, light handling, stable, and monster brakes compared to stock. Not to mention if you run the GSXR 17" wheel you can run any sticky sportbike tire you want.

[LarsKroghStea]

Great project here. Subscribed to your thread. <img src="images/smilies/smile.png" border="0" alt="" title="Smile" smilieid="1" class="inlineimg"><br>
<br>
Something to consider that might save you a little time and money: K6-K7 (2006-2007) forks are anodized black, sprung from the factory with 1.0 kg/mm springs (suitable for the weight of a GS), relatively cheap, relatively long compared to other modern USD forks, and the stock triple clamp has offset good for creating enough but not too much trail as long as you address the rear end of the bike as well. If I could do it over again, I'd probably use these forks on my bike instead of the K4 GSXR 1000 forks on it now.<br>
<br>
Which segues into, if you go the USD fork route, you'll need to take care of the rear end of the bike as well to get your geometry numbers right (rake, trail, swingarm angle, shock(s) angle etc etc). <br>
<br>
Merely installing a USD front end on a GS in my experience without taking into account all the geometry numbers and the rear suspension will make it handle *worse*, not better, because the offset of the triples is set to create enough trail on the steep rake angle that modern sportbikes run. On a GS without rear end modifications it will create too much trail and cause the bike to be hesitant to turn. <br>
<br>
If you do get it right though, man, what a difference from stock!! Super flickable, light handling, stable, and monster brakes compared to stock. Not to mention if you run the GSXR 17" wheel you can run any sticky sportbike tire you want.

Thanks! I'm sure you are right about the K6-K7 forks. My bike though, is not only about making it handle the best, it's about building a bike that looks the way I envisioned it looking when I started the build. The K6 fork is 12mm longer as you say, and has stiffer springs. I believe the offset is the same. When I bought my forks, I couldn't find any K6-K7 forks with chrome forklegs, only copper or smoke. And I didn't want that for my bike. I could have had smoke, but the ones I found were for later bikes and were well over twice the price. I have access to a CNC router and can make my own triples if I need to, but as you say, I think I can get the geometry I want by taking care of the rear. In an earlier post I got some great tips on how to make shock risers. I'm sticking with the 19" front wheel though, but as you say, that's not the best choice for handling, but it gives the bike the look I'm after. I appreciate the feedback from you guys, and I will surely need them when I start puttting the new suspension setup together. But I will have to make compromises to take care of the looks I don't think I'll mind the softer fork springs. I weigh about 78kg (170lbs) and the bike is about 45kg (100lbs) lighter than stock. Please keep following the build, and keep the feedback coming. I think I'm pretty good with the research, math and geometry, but compared to you guys I definitely lack experience. I'm including a photoshop picture I made shortly after I bought the bike that shows what I'm after. I didn't know much about cafe racers at the time, but I was googling a lot It has the classic tires that I love the look of. They had to go, as I'm not willing to ride an unsafe bike just for the looks. I think the tyres I have now is a great compromise. I went with darker colors and the blue stripe, as I think brushed metal is cool in pictures, but from a distance it just looks plain. The base for this picture is a random Honda CB, so the kickstand is of course not there, and I dont mind. I ditched the idea of using pod-filters pretty early, as I wanted near stock performance and reliability. I have the Kerker exhaust (shortened) and GS1000 carburettors with different jetting, but everything else is stock engine wise. I also found out during the build that I wanted upgraded fork/brakes but I don't want that to compromise the original plan too much. <br><br>I don't usually write this much in a post, but I know from my days modifying VW bugs that modifications like changing wheel/suspension setups (I had BF Goodrich 265/50-15 on the rear, mounted on widened original steel rims) sparks a lot of input both from purists and the perfomance specialists. That's why I wanted to explain that wandering too far from the original concept is not an option, even if it would give the bike better handling. As long as the bike is safe (1st priority) and handles to my standard, I'm satisfied. Again, thanks for your advice Here's the picture:


The Noise_bike_mad_skills_zpsvjqtjnqk by Lars Krogh-Stea, on Flickr

[John Kat]

Great project here. Subscribed to your thread.

Something to consider that might save you a little time and money: K6-K7 (2006-2007) forks are anodized black, sprung from the factory with 1.0 kg/mm springs (suitable for the weight of a GS), relatively cheap, relatively long compared to other modern USD forks, and the stock triple clamp has offset good for creating enough but not too much trail as long as you address the rear end of the bike as well. If I could do it over again, I'd probably use these forks on my bike instead of the K4 GSXR 1000 forks on it now.

Which segues into, if you go the USD fork route, you'll need to take care of the rear end of the bike as well to get your geometry numbers right (rake, trail, swingarm angle, shock(s) angle etc etc).

Merely installing a USD front end on a GS in my experience without taking into account all the geometry numbers and the rear suspension will make it handle *worse*, not better, because the offset of the triples is set to create enough trail on the steep rake angle that modern sportbikes run. On a GS without rear end modifications it will create too much trail and cause the bike to be hesitant to turn.

If you do get it right though, man, what a difference from stock!! Super flickable, light handling, stable, and monster brakes compared to stock. Not to mention if you run the GSXR 17" wheel you can run any sticky sportbike tire you want.

I can't agree more!
My best combo so far has been with R6 first gen front end complete with 17" front wheel and mono shock GSXR 2nd gen rear suspension.

[LarsKroghStea]

I can't agree more!
My best combo so far has been with R6 first gen front end complete with 17" front wheel and mono shock GSXR 2nd gen rear suspension.

That's a nice setup Many people do the R1 monoblock caliper swap on GSXRs, and when I did the calculations, they are a better fit for my Accossato brake lever than the Tokicos that I have now.
I bought the Accossato to mach the original caliper, and that was an impressive upgrade As I have radially mounted calipers this should be an easy swap and will buy some if there's too much travel on the brake lever with the Tokicos.

This will surely be an exciting part of the build. The geometry and weight the bike has now fits me well, and it is so stable that I can easily drift the bike on gravel with the clipons mounted as they are now. I can also brake it really hard without loosing stability, and flick it easily from side to side in normal and above normal highway speed. My main consern is that the front wheel won't be able to react quickly enough to uneven surfaces because of the slow flowing valves in the front forks. That may not be an issue on track or in warmer climates, but in Norway, where frost hives create sudden changes in level of the asphalt, this is an important consideration. This is of course not a problem on the newer main roads, but a common problem on some ofthe fun B-roads and in the twisties :/ A bit softer springs also helps with this.

[John Kat]

I fully understand your logic: you find a model café racer and then try to duplicate it.
Some time ago, I fell in love with this bike:

https://www.custom-wolf.de/handmade-bikes/mocca-racer/

I bought the GSXR 1100 engine brackets including the Benelli swingarm ( https://www.custom-wolf-stuff.de/deu...ekit-gsx-gsxr/ ) but didn't actually install the latter.
For another project maybe?

[LarsKroghStea]

I fully understand your logic: you find a model café racer and then try to duplicate it.
Some time ago, I fell in love with this bike:

https://www.custom-wolf.de/handmade-bikes/mocca-racer/

I bought the GSXR 1100 engine brackets including the Benelli swingarm ( https://www.custom-wolf-stuff.de/deu...ekit-gsx-gsxr/ ) but didn't actually install the latter.
For another project maybe?

That's a cool project But you're not quite right about the process. I made a cafe racer in photoshop that didn't look like any other cafe racer that I'd seen. I got a lot of flame on cafe racer forums because it looked too heavy, they didn't like the dented tank, they didn't think I could get the Honda tank to fit the wide frame and they didn't think I'd get the straight cafe racer line through the tank and seat. This is easy to do with the straight frame on the 70s Honda CB og Yamaha XS. So this is nothing like copying another bike. This is hours of testing and failing in photoshop to get something that I hadn't seen before and then making it in real life

[LarsKroghStea]

So, here's a picture that shows how tucked the Kerker exhaust is to preserve lean angle.

49877386078_6ddd2e10b2_o by Lars Krogh-Stea, on Flickr

[LarsKroghStea]

Testing the jig for drilling nuts and bolts:
IMG_20200510_111013 by Lars Krogh-Stea, on Flickr


Here's a front brake rotor bolt drilled and ready for safetywire:
IMG_20200510_114258 by Lars Krogh-Stea, on Flickr

[LarsKroghStea]

Small update; the Fireblade rotors came today. They look good, but dirty. Will give them a good cleaning, and paint the gold centers black.

2020-05-16_11-03-40 by Lars Krogh-Stea, on Flickr

[bobtail84]

Are the discs for the legendary salty_monk brake conversion?

[LarsKroghStea]

I'm not exactly proud to show you the images I'm about to post.. Normally I'm very concerned about safety. When I reassembled my last bike, I followed the manual to the letter and I had no worries with running it at 140mph+. Remember I hastily bolted the bike together to ride it home from my father? When I was almost home, some noise from the front wheel made me stop and check. I couldn't see anything and assumed it was the bearing. Drove it home and parked it. Became aware of the shocks, fixed them and today I was going to fix the front wheel. And the speedometer, as it stopped working shortly before I arrived at home.


It seems the two problems are related. The rotorbolts that became loose have grinded the speedogear to bits :/


IMG_20200517_112631 by Lars Krogh-Stea, on Flickr


IMG_20200517_112644 by Lars Krogh-Stea, on Flickr


IMG_20200517_115207 by Lars Krogh-Stea, on Flickr


Pretty sure this speedogear is not going to work again..
As the new plan is to use the original front wheel with the Gixxer forks, the speedo analog/digital converter is now a forgotten, but fun, sidetrack. The new plan is to use a magnetic bolt on the rear brake rotor.

[LarsKroghStea]

Are the discs for the legendary salty_monk brake conversion?

No, it's for a little more modern setup; a 2005 GSXR USD fork with radially mounted calipers. I have a set of original Nissin calipers, but will swap them with R1 monoblock calipers if the ratio is off with the Accossato master. It will look pretty much like this:

Cognito-Moto-Custom-Honda-CB550-Café-Racer_8 by Lars Krogh-Stea, on Flickr

But mine are almost the same rotors (mine are from a 2002 Fireblade 900 RR and have the same PCD.

[LarsKroghStea]

On the positive side I've been practicing my safety wiring skills. I don't quite know why I started drilling the corners of the bolts. They are pretty small, and hard to drill in a way that looks nice. Adding to that it looks better when drilled straight through. I redrilled the middle bolt and will drill that way on the remaining bolts. I also had to put a shim under the bolts because of the threadless part, that's why they're not positioned perfectly. I don't have the bolts for the new rotors and wil probably buy predrilled bolts for them.


2020-05-17_08-01-03 by Lars Krogh-Stea, on Flickr

[93Bandit]

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?