RZ350 re-frame... classic 2-stroke gets modern upgrade

Frame Maker

Well-known member
Part 1, the inspiration and beginning design process:

This story begins back about 20 years ago when I built a small batch of road racing frames that were powered by EX500 engines. These bikes, dubbed the AK-1s, were very successful in AFM 500cc Twins racing and won numerous races including a championship in 2002. Shortly after the success of the AK-1s began to get some public recognition (a 4-page feature in RoadRacing World magazine helped) I began getting inquires from a few folks asking if I could build similar frames for an RZ350. The answer was always an enthusiastic "yes", but the conversations always seamed to fizzle out.

Over the past 10 years or so I continued to have similar conversations, only with the added discussion about how thanks to the use of aftermarket Banshee parts, the little RZ350 could now be made into fire breathing monsters easily making 80, 90, 100hp or more. Again, the answer was still an enthusiastic "yes". And again the conversations always fizzled out.

Fast forward to 2018 and I finally said to myself "fuck it, I'm going to build one for myself and if anyone else decides they want a copy, then so be it". Game on! Let the design process begin.

I would use the AK-1s as my benchmark for the basic geometry... 54" wheelbase, 21 degree rake, use the longest possible swingarm and move the engine as far forward as possible. This combination (at least with the AK-1s) resulted in a bike with amazing handling. I like to use the word "telepathic". I'll never forget the first test ride at ButtonWillow Raceway getting up to speed and hitting the first set of esses and the bike seamed to know what to do before I could provide the input. It was an amazing experience that I hoped to replicate with the RZ350 frames.

I've spent most of my career (I work as a consumer product designer/engineer) commuting to San Francisco or locations in the south bay so I spend a lot of time on trains. I always have a sketch book with me, so the beginnings of the RZ frame project began as little "ideation" sketches. Eventually these would be referenced as the design process migrated into CAD. This would be the start of the RZ350 frames.

Prototype AK-1 frame...

AK1.p4 by andbike, on Flickr

AK-1s at Laguna Seca for AMA/World Superbike. Both bikes, ridden by Tom Dorsey and Shawn Reilly, competed in AMA ProThunder Class. Our goal was to not come in last place... which we achieved.

DSCN0090 by andbike, on Flickr

Tom Dorsey at Sears Point doing what the AK-1s do best... get around corners fast!

AK-1 race_2 by andbike, on Flickr

Early ideation sketches for the RZ frame project...


20190303_182704 by andbike, on Flickr

20190303_182636 by andbike, on Flickr

20190303_181020 by andbike, on Flickr

20190303_181127 by andbike, on Flickr

I would also come across this bike here on BARF in some very old "for sale" posts. It has also served as inspiration for the RZ frame project. Some of you may recognize the bike, or even know the owner...

DSC_0368 by andbike, on Flickr

And then the CAD process begins. I start with the very basic components in place first and connect from there...

20190311 image 1.05 rz350 frame assembly 2019 by andbike, on Flickr

One of the goals with the new frames is to allow as much possible space for expansion chamber routing under the chassis. My good friend Brian Turfrey has always recommended TZR250 3XV swingarms for RZ project bikes as the TZR has the shock and linkage located fairly high leaving optimal space for fatty pipes. So I decided to use a TZR swingarm, or at least replicate the geometry...

20190311 image 1.1 rz350 frame assembly 2019 by andbike, on Flickr

CAD is starting to look like a motorcycle...

201905019 image 1.1 rz350 frame assembly 2019 by andbike, on Flickr

201906011image 1.5 rz350 frame assembly 2019 by andbike, on Flickr

201906016mage 1.0 rz350 frame assembly 2019 by andbike, on Flickr

Stay tuned for more coming soon:ride
 

matty

Well-known member
:thumbup
I'm really looking forward to seeing the progress on this.

And I remember that red frame, I had it as a screen saver for a minute as I wrestled with whether to buy it or not. I didn't:(
 

Frame Maker

Well-known member
Subscribed. Looking forward to seeing where this goes.

:thumbup

:thumbup
I'm really looking forward to seeing the progress on this.

And I remember that red frame, I had it as a screen saver for a minute as I wrestled with whether to buy it or not. I didn't:(

Thanks!

The SRX600 (red framed bike) never did sell. I've been in contact with the owner off and on several times since I originally saw the bike about two years ago. The owner still lives in Fremont, but the bike has been at a relative's house in Arizona so I haven't had a chance to see it in person. I'd still like to see it if it ever comes back to the bay area. Its a great looking bike for sure.
 

Lonster

GaMMa RaNGeR
I saw your frame on KPKE's post, and suggested you make a thread about it. THANK YOU for considering it. I am looking forward to everything you post about it. Also, PM sent.
 
AK-1 race_2 by andbike, on Flickr

Stay tuned for more coming soon:ride

one thing I love so much about your projects is seeing them at work in the crucible of a dirtbag challenge or a race track ... so very cool. :thumbup

appreciate the hand drawn sketches, too, fun part of the moto design art we don’t always get to see?

guess that SRX frame is bump-start only? there’s commitment for you ... :gsxrgrl

hope to see some pics of your EX500 ADV bike from dirtbags gone by ... :afm199
 

kuksul08

Suh Dude
Really cool work Julian. I love tinkering and making parts, but a whole bike is another level of time and complexity.

How much math goes into picking your geometry? I took a single track vehicle design class in college, and the influence of just 1 degree or 2mm of offset can have is startling. Do you have the basics figured out from other bikes/experience and stick to what works?

So awesome you're using the RZ engine too!
 

Frame Maker

Well-known member
Really cool work Julian. I love tinkering and making parts, but a whole bike is another level of time and complexity.

How much math goes into picking your geometry? I took a single track vehicle design class in college, and the influence of just 1 degree or 2mm of offset can have is startling. Do you have the basics figured out from other bikes/experience and stick to what works?

So awesome you're using the RZ engine too!


Great question with no single answer... when I first started building custom motorcycles 30 years ago I looked first at existing bike geometry. It was easy to open a magazine and get specs for rake, trail, wheel base, etc. What the magazines didn't tell you were things like swingarm angle, axle rate (effective spring rate at the axle), and how much progression the suspension geometry was creating. This was long before CAD software was available at reasonable costs to common folks, so I figure out ways to find these answers on my own. I did a lot of reading in Tony Foale's book so I had a good idea of the fundamental principles, but no data base to use as a starting point. I was especially interested in understanding axle rates and progression curves being used in modern sport bikes of the time.

What I came up with was a very complex equation that I wrote in Excel. When I found out that Excel could understand complex trigonometry I was set. I spent a few months writing the equations and when I was done I had a sheet where I could enter all of the dimension for both 2-point and 3-point linkage designs, add the actual spring rate at the shock... and the little spread sheet program would plot the effective axle rate as a bar graph along with actual numbers as a percentage of the spring rate.

Having this tool, I then found a few modern sport bikes and began to measure and record my findings. I noticed a few trends along the way... Going back to the mid-80 to early mono-shock bikes, these bikes had very aggressive rising rates. As the designs migrated into the 90s, the rising rate curves became flatter (less aggressive). I also noticed that smaller bikes had stiffer rear suspension compared to the front suspension compared to heavier bikes. Of course heavier bikes had overall higher spring rates, but as a percentage front to rear, smaller bikes were stiffer at the rear. This made sense because the rider's weight (biased toward the rear) made up a greater portion of the overall gross vehicle weight. Anyway, I used this information as design reference for my first big project.

Over the years I've designed enough bikes to have numbers that I like and tend to do less of the calculating, although I still look closely at suspension linkage to see what my rising rate curves are doing.

All of that said, some of my bikes (think Dirtbag projects with odd front-ends) I tend to consider these as experiments and I spend less time analyzing the geometry and just kind of build and see what happens.

this is amazing! Looking forward to seeing bike on track.

Thanks. I'm also anxious to see how it rides on the track:thumbup
 
Last edited:

Frame Maker

Well-known member
Two additional comments about chassis design that I have come to understand...

First, no matter how good the chassis is designed, individual riders will want change things to fit their riding style. When Shawn and Tom were racing the AK-1s I had the bikes initially set-up identically. As we began sorting the bikes and getting the details dialed in, each rider had their own tire preferences as well as gearing preferences making each bike highly competitive for each rider. The following year I let Zoran take one of the bikes for the season. Even though Shawn and Tom won many races and nearly a championship, Zoran thought the bike was too soft for his riding style and he immediately replaced springs to make the bike stiffer at front and rear. He just had a different preference.

The other thing I recall is our second race weekend happened to be the first time each rider had their own dedicated bike to race. It was a Sears Point event and both riders did very well getting podium finishes in 500 Twins. Needless to say, they were very happy with how the bikes worked. Towards the end of the day on Sunday we began to notice some mysterious bright dust appearing on the rear / left side of the bike. After closer examination we determined that in the rush to assemble the bikes Friday night before the race weekend, the rear wheel on one (maybe both) bike was WAY out of alignment. The wheel was point as far to the right as possible without rubbing on the swingarm. This resulted in the chain wearing the sprocket badly on one side thus creating the bright dust. But the odd thing was that neither rider had noticed anything odd with the way the bikes handled. If it was noticeable, they just adapted and rode the bikes the way they were.

Because of these experiences, I'm convinced that good motorcycle design is more art than science.... and the areas that are science, aren't an exact science.
 

Lonster

GaMMa RaNGeR
Great info. There is a common thought that there are 2 types of rider/racers: The first is someone who needs to have their bike setup perfectly to be competitive or win a race. If not perfect, they fight the bike and finish poorly. The second is the rider/racer who can adapt to any short comings, and ride the bike to it's maximum, despite this. It sounds like the two gents who rode your bikes are of the second type.
 

Frame Maker

Well-known member
Part 2: the 2019 detour

Screenshot-from-2018-04-24-03-22-44 by andbike, on Flickr

My original plan was to utilize the 2019 Dirtbag Challenge as my motivation to build the first prototype bike. I also had an idea for a crazy dual-shock girder front-end that I also wanted to build. So the plan was to build the prototype for 2019 DBC, then add the crazy front-end to the prototype bike for DBC 2020... but that didn't go as planed.

DBC 2019 came many months earlier than anyone expected. Although Dirtbag has no official date and varies year to year, it has traditionally been fall or early winter months toward the end of the year. 2019 DBC was announced early summer catching many people (myself included) off guard. I normally like to have my Dirtbag builds designed and planned well in advance of the "GO". Wanting to build a ground up prototype, I was not prepared for the early announcement.

I quickly came up with an alternative plan... for 2019 I'd build the crazy front-end and install it on an existing RZ350 frame. Then hold off on the prototype frame till 2020. To make the build schedule worse, I had an out of state dual sport ride planned for early summer, so that came right at the beginning of the build window and would take a week out of my build schedule. Oh, and I still had CAD work to do to finish up the front-end design before I could begin building. Then to top off an already tight schedule, I'd be using a frame that hadn't been titled and I'd be building a bike from spare parts... so this meant extra DMV, CHP, and brake and lamp inspections to deal with. Okay, game on!



my dual sport ride exploring the back roads of northern Vermont with three good friends. I rode a rented BMW GS700. We rode some very technical class 4 (basically jeep/ATV trails) and I was still able to return the GS without a scratch, although extremely dirty :teeth

20190719_104857 by andbike, on Flickr

20190720_124931 by andbike, on Flickr

20190720_104530 by andbike, on Flickr



CAD work for the girder fork...

20190704 RZ350 frame assembly 2019.1 by andbike, on Flickr

20190704 RZ350 frame assembly 2019.2 by andbike, on Flickr



And the donor RZ350. The frame was an ex race bike that had been cut in a few places, so no restoration value. It was gifted to me by BARF's very own KPKE. The pipes were also a BARF donation... thank Butch! Engine is from a Banshee and found on CL for $500. The DRZ/supermoto wheels and GSXR600 forks (would end up on the 2020 build) were inherited from a good friend who had passed away a year earlier. FUCK CANCER:thumbdown

20190629_084606 by andbike, on Flickr

Back from Vermont and with CAD work completed the construction begins, starting with lots of small turned parts...

20190705_193536 by andbike, on Flickr



Laser cutting is outsourced to sendcutsend.com. They have a great service making it very easy to upload files. Prices are excellent and parts arrive at your door a few days later...

20190727_102558 by andbike, on Flickr


CNC parts are from a secret Chinese vendor... and are dirt cheap! I often wonder how they can even afford the material at the prices they offer. The parts pictured were around $800 total. I love these guys (actually a gal that I deal with) :party

20190727_103447 by andbike, on Flickr

From here its lots of tube bending, cutting, notching, fitting, welding, and stuff coming together...

20190727_180633 by andbike, on Flickr

20190728_092855 by andbike, on Flickr

20190728_153706 by andbike, on Flickr

20190728_170512 by andbike, on Flickr

20190810_134138 by andbike, on Flickr

20190810_153715 by andbike, on Flickr



With the crazy front-end on the bike, I'd need to do something odd at the rear to balance the aesthetic...

20190811_104525 by andbike, on Flickr

To be continued...
 
Last edited:

DannoXYZ

Well-known member
I can't quite visualise it, but it appears from angles of upper and lower arms, that wheel-axle is moved rearward when fork is compressed upwards?
 

Frame Maker

Well-known member
I can't quite visualise it, but it appears from angles of upper and lower arms, that wheel-axle is moved rearward when fork is compressed upwards?

In that photo the top linkage hasn't been adjusted to final length so its a bit exaggerated... but yes, a "J" shaped travel path is common for girders. The nature of a girder you'll never get a true linear path. Always an arc of some sort. Even though that may seam odd, it really doesn't do anything weird when riding. I have two bikes with girder forks and both are a blast to ride.
 
Top