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

[Frame Maker]

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

[Seizer]

Subscribed. Looking forward to seeing where this goes.

[matty]

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]

Originally Posted by Seizer

Subscribed. Looking forward to seeing where this goes.

Originally Posted by matty

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.

[budman]

Hell of a start to the first thread posted here.

[bikewanker]

Originally Posted by budman

Hell of a start to the first thread posted here.

Definitely agree. Fuels my daydreaming.

[Maddevill]

This looks like fun

Mad

[Lonster]

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.

[flipstyledsm22]

Subbed!

Amazing work and a great passionate guy that loves 2 wheels! Wish you the best man!

[Frame Maker]

Originally Posted by flipstyledsm22

Subbed!

Amazing work and a great passionate guy that loves 2 wheels! Wish you the best man!

Thanks

Originally Posted by Lonster

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.

PM replied to

[auntiebling]

sweeeet
julian which CAD program are you doing the geometry with? tony foale perhaps?

[SFMCjohn]

Originally Posted by Frame Maker

AK-1 race_2 by andbike, on Flickr

Stay tuned for more coming soon

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.

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 ...

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

[kuksul08]

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!

[DannoXYZ]

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

[Frame Maker]

Originally Posted by kuksul08

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.

Originally Posted by DannoXYZ

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

Thanks. I'm also anxious to see how it rides on the track