Project: Zephyr 550 w/ZX6R Front End Conversion

Some of you have followed my progress on this, and know that there is another thread here with some details about it, but another relevant thread was deleted in a forum update/cleanup. And so I thought that I would post about where it's currently sitting, as I do have some exciting news. 

So the goal has always been the same, fitting a set of upside down forks to my Zephyr 550. I could argue that it's about performance, because I will be gaining three-way adjustable modern cartridge forks, and four-piston radial-mount brake calipers. Technically, an improvement. But I'll be honest and tell you that the main motivation has always been that it looks cool. I got into motorcycling when the modern version of a cafe racer resto-mod was all the rage, and upside down forks were a must. At one point I decided it was too much hassle and not worth it considering my all-original bike was in great shape. Then I got rear-ended and had to rebuild the bike on a salvage title. Figured since it was never gonna be a fully clean title bike anymore, might as well do what I want with it.

It's been a challenge, and I have wanted to give up multiple times. But it became a thing of finishing what I started, and doing it because I set down this path. I had a goal, and along the way other goals have surfaced, but seeing something through, seeing it finished, is one of the things that keeps us going. I'm here now and it is still not done, but a significant portion of the project has been invested in, and will see the light of day soon enough.

While many of these kinds of conversions have been undertaken by others, I took a slightly different, and undoubtedly more difficult path. My version was to create it such that it looks factory, and not hardly custom at all. Keen eyes will no doubt recognize that the forks and brake calipers didn't exist in 1990 when my bike was new. But if I could retain as many original components as possible, including the front wheel, brake rotors, fender, headlight, gauges, turn signals, etc., maybe I could pull it off. The issues... were numerous...

There were few off-the-shelf components that would make this a simple plug-and-play situation. I have sourced a custom axle from Spec Engineering in Japan via Webike, made custom rotor spacers via SendCutSend, determined that bearings, axle spacer, and speedometer drive from a ZX-600, and rotor bolts from a Suzuki would work, had a fully-custom left side axle nut made, and rebuilt the '03 ZX6R forks with (again, fully custom made) damper rod extensions. Here are some photos in probably non-chronological order.



You can see in some of the pictures that I have 3D-printed prototypes of custom triple trees. The biggest part of this project has been designing, prototyping, redesigning, prototyping, and ultimately finalizing a design for custom triple trees. This was done because again, there was no off-the-shelf option. I wanted to maintain the exact steering geometry and wheelbase of the bike. In order to do that I came up with my own design for triple trees, modeled after a hybrid of the stock parts, sized to fit the larger forks, with some design elements pulled from the donor front end's lower triple. The upper triple includes a 10mm drop, which serves to lengthen the forks, and allow clearance of the fork caps under the stock handlebars. 

 

The big news is that the triple trees are officially bought and paid for, and being made by none other than Cognito Moto, arguably the industry's leading shop and experts on custom triple trees. Currently I am just waiting. There is still a lot to do. I'll need to measure for and order custom brake lines, and design and fabricate a mount for the gauges/headlight/etc., as well as sort out whatever I'm gonna do for the front fender. But with fingers crossed I'm hoping maybe it'll be done sometime this year... and then I get to install a big bore kit. 

As a fellow "Sunk Cost Enthusiast" let me say nice work and bravo for sticking through till the end. If it was easy everyone would do it.

A question was asked in another thread  about speedometer drive gear ratios and tooth counts, and it has plummeted me deep down a rabbit hole.

While acquiring parts for this front end conversion project, I had been encouraged at one point to try and beef up the axle as much as I could. The stock Zephyr 550 axle is 15mm, and that's pretty skinny for an upside down fork setup. Basically, the axle is providing a lot of the rigidity in the front end, so bigger is better. There was sadly no way of using a stock 25mm ZX6R axle because the bearings and everything else would never fit my front wheel. I finally settled on a 20mm axle, which is very often used on upside-down fork street bikes (the 2020 Z900RS Cafe for example), and I found an axle made by Spec Engineering in Japan which has the correct 32mm axle clamp diameter that fits the ZX6R fork lowers, and steps down to a 20mm shaft. It's a beautifully machined part.



Stepping up the axle diameter meant different bearings, different right-side axle spacer, different center collar (the spacer that sits inside the hub between the two bearings), and a different speedo drive gear. I found that all of those items could be taken from a '98 ZX600. Other than some 1mm shims behind the bearings (McMaster-Carr part #90214A548), and the fact that the center collar is out of production (I had one custom made, but could've saved money getting one from a Z900RS and just having it cut shorter to 73mm), it was easy to get all of them through Webike or Ebay.

However, as I learned this morning, not all speedo drives are created equal. And by that I mean they can have different tooth counts on the internal gear*. So I've spent the last few hours doing research. First, the stock ZR550 Zephyr gear has 24 teeth, and the ZX600 gear I had intended to use has 23 teeth. Now, that's only a difference of one tooth, and some people might not be bothered by it, but in case you hadn't noticed, I am trying to do this thing as meticulously as possible, despite my limited budget and sanity. Thus I am looking for a speedo drive with the elusive feature set of a 20mm axle, 24-tooth gear, and designed to fit over a 42mm OD bearing. As of yet, I have come up empty, and I would love any help that anyone here can offer. Are there any models of Kawasaki that offer that particular combination? 

MegaZip has been a great resource for me to cross-reference parts specifications. I entered the part number of my 24-tooth stock gear (41060-1025), and it shows a number of models that use that gear . But I think I've tried all of them, and there are none that have a 20mm axle. Some appear to be 17mm, but most are 15mm.

In a manic state of desperation I attempted to disassemble a second ZX600 speedo drive that I have (yes, I have been buying two of almost every part for this project, so that I will have spares). Admittedly I destroyed it in the process (yes, this all happened since this morning). But on the plus side I do feel like I know how to do it properly next time. The idea is that maybe I can remove the 24-tooth gear from the stock drive, and install it in place of the 23-tooth gear in the new drive. Problem solved? We can wait and see, but my previous measurements of the speedo drives in question seem to indicate that they are going to have different inside diameters. 

Interestingly, every speedo drive that I have looked at has had a 9-tooth worm gear.

All of this leads me to perhaps what is the actual important question at hand here. What would happen if I just use the 23-tooth ZX600 speedo drive as is? Yes, all of these tiny details are important to me, and I absolutely will lose sleep over them. But, I'm also realistic and know that I can't always get what I want. So what exactly are the results of using a speedo drive that is one less tooth than my gauge is calibrated for? I understand that the ratio has to do with tire size/diameter and the distance traveled in one full revolution of the wheel. Therefore presumably a different tooth count will also effect how my mileage is input to the odometer. But I don't understand whether that translates to an up or a down. In other words, is my speedometer going to read like as though I'm going faster or slower, and is my odometer going to clock more miles or fewer, if I'm riding at the same real-world speed?

One thing I noted in my research was that the ZXR250, a JDM market bike that notably has the same front tire size as my bike, plus *I think* has a 20mm axle, uses a speedo drive with a 23-tooth gear (effectively meaning it's the same speedo drive as the ZX600 drive I already have). If they used a 23-tooth gear on a different bike with the same tire size as my bike, than any difference in the way MPH is calculated must be happening at the speedometer gauge itself. Or maybe... there's no difference at all? Or not enough to matter? Or... maybe there's a part in the gauge I can swap out to... oh god... I shudder at the thought...

 

*I assumed that they were all "outputting" the same "speed" because places like 4into1  sell these aftermarket speedometers that come with a 2240:60 (2,240 rotations = 60mph) ratio, that they say is universal for most Japanese motorcycle brands. Perhaps there is some amount of variance that is just accepted? The world may never know.

Time to do some maths (wheel circumference x rotation to give distance travelled for a specific number of rotations. Also you could open your horizons and look outside of the Kawasaki model range (my build has parts from all of the big 4)
Also the difference between 23 and 24 is only 4% and speedo's generally can be up to 10% optimistic across the automotive world

All of the Kawasakis I have owned had speedometers that read 8-10% fast. Going from 24 to 23 on the drive gear should result in a reduction of indicated speed, likely making it more accurate. This could turn out to be a win for the detail obsessed among us.

I was intrigued so I have come home and since Mrs Wookie is shopping I have done the maths (based on your 2240:60 ratio) and both 9:23 and 9:24 rations for the speedo drive bevel. This is what I found
To be spot on at 60mph the gear would need to 22.588888 teeth !!!
23 teeth at a distance travelled of 60 miles based on wheel circumference of 1.90984 meters - the speedo would read 61.14mph
24 teeth at a distance travelled of 60 miles based on wheel circumference of 1.90984 meters - the speedo would read 63.79mph
So as Texas said, running the 23 tooth will actually make you more accurate than stock - stick a fork in it and call it done

There is something to be said for the digital world, I am using a ZX6R set-up on my build and I have bought a unit from the Netherlands that plugs between the speedo and the speed sensor on the gearbox sprocket. I then need to punch the sprocket and tyre size into a phone App and it will generate a code to programme the unit to re-calibrate accordingly. If I change the sprocket ratio's or tyre size I can simply generate a new code and re-programme 

When working your mats, did you factor tire diameter?

wheel revolutions per mile x speedo gearbox ratio = rev count in the speedometer. so you kind of have to use the tire size to figure out the actual speedometer revs.  I came up with 2277 for the 93 zx-7r from that other topic. but that number can vary depending upon the actual tire measurements. it's probably rare for a speedometer to actually spin at 2240 revs per mile.

Wookie58 wrote:

So as Texas said, running the 23 tooth will actually make you more accurate than stock - stick a fork in it and call it done 
 

Well obviously this is fantastic news! Thank you Wookie for doing the math. I had pondered it a bit and answered my own question when I realized that 23 teeth would turn a 9-tooth worm gear less than a 24 teeth would, all other things being equal. It was then mostly a question of how much, and this confirms that the stock setup was so “optimistic” that reducing the gear would actually bring it closer to reality. 

Appreciate all the replies and input everyone, well done! 

Stereordinary wrote:

Wookie58 wrote:

So as Texas said, running the 23 tooth will actually make you more accurate than stock - stick a fork in it and call it done 


 

Well obviously this is fantastic news! Thank you Wookie for doing the math. I had pondered it a bit and answered my own question when I realized that 23 teeth would turn a 9-tooth worm gear less than a 24 teeth would, all other things being equal. It was then mostly a question of how much, and this confirms that the stock setup was so “optimistic” that reducing the gear would actually bring it closer to reality. 

Appreciate all the replies and input everyone, well done! 

The stock set up is only 5% over which is normal (manufacturers always go this way to avoid litigation claims from convicted speeders) the drive is shared with 2 pages of other models with the same axle size (on CMNSL) hence why it has the 24. It's not cost effective to make all bespoke parts for every model

TexasKZ wrote:

When working your mats, did you factor tire diameter?

I did indeed, for anybody struggling to sleep  below is how I came to the results I did 

Good to see you went through the extra effort for a large diameter front axle.