Greyborg, High Power DIY Electric Bike Frame

January 1, 2014

We’ve written articles before on mild-assist electric bikes before, but…this…is NOT one of those articles. And before anyone posts a comment to make sure to say that this is an electric motorcycle with pedals…we know!

The story of the Grayborg frame starts in 2001 with an engineer named Zvonimir (ES member HAL9000V2.0), who lives near Zagreb, in Croatia. This beautiful country is on the coast of the Adriatic Sea, between Italy and Greece. Next to Zvominirs home is a steep hill that is over 1,000 meters high. For fun and exercise, he used to pedal his mountain bike to the top, and then ride down very fast to the bottom.

Doing this takes about 4 hours, and he enjoyed doing it several times a week, but after his son was born he didn’t have enough spare time to do this as often as he wanted. As an engineer, he decided (of course) that he must learn everything about electric bikes, and then design the ultimate hill-climber to speed up the times when he does this.

Zvonimir has been experienced with bicycles and motorcycles since he was a young man, and he knew what he liked when it comes to the geometry on a full-suspension frame. He quickly put together some components on a mountain bike as a test mule for experiments (the Grayborg Warp G). This E-bike also had a series/parallel switch that allowed it run at 36V around town (for maximum range) and then switch to 72V for hill-climbing. However, it was immediately clear that even the strongest existing  down-hill (DH) bicycle frames were not going to be strong enough for what he wanted.


The original Grayborg G prototype.

The original Grayborg G prototype. The custom large-diameter disc brakes were made because…right from the beginning, this was made for shooting down steep Croatian mountains very fast.


Zvonimir began bending and welding Cromoly steel tubing to make a custom frame of his own design, taking into consideration the best ideas from the most respected DH frames, and also his many years as an experienced rider. The result was the Greyborg Warp production frame. As a side-note, he changed a letter in the name of the frame, so it was now the Greyborg Warp (with an “e” instead of an “a”). He showed it to his friends on the Croatian electric vehicle forum, ( in Croatian, if you visit there, use Google chrome so the website will be translated into English).



This is a first-gen Greyborg (notice the bottom Bracket is not concentric with the swingarm pivot). It was designed around the readily available “Old school” Sealed Lead Acid (SLA) batteries. Modern lithium batteries would be smaller and lighter for the same Watt Hours (WH), but this pic shows the high volume of battery that this frame can carry.


They encouraged him to build more frames and sell them, so he got busy making the first run of 30 Greyborg Warp frames in 2009. A friend named Vladimir (ES member Accountant) bought one of these frames, and was so impressed with it, he asked Zvonimir if he would like to form a company with him to sell them world-wide, and to also to develop new products. This was the beginning of the company Zelena Vozila (“Green Vehicles” in Croatian). At this time, the frames were designed around the Crystalyte 5305 motor using a 72V / 40A controller.


The Crystalyte 5305

The Crystalyte 5404/5405/5406. Until the Cromotor, this was the most powerful E-bike hubmotor. The narrower width of the “X5” family of hubmotors allows them to still fit seven speeds.


From that point on, growth has been steady. The first prototype frames had used 12V sealed-lead-acid (SLA) batteries, Crystalyte 5-series motors and high-amp Kelly controllers. The triangulated rear swingarm was set-up to accept a shock absorber between 145mm-185mm in length. Feedback from the first round of builders was vital in deciding on useful improvements to the frames, so that other optional motors, controllers, and batteries will be easier to fit in the next “V2” version.



48 Headway 3.7V cells made from the safe LiFePO4 chemistry at 10-Ah each = 72V / 20-Ah. These are part number 38120 (38120 = 38mm diameter /120mm long), Notice on this next version (V2), the BB and the swingarm pivot are concentric.


In 2012, ES member “zombiess” (from Ventura, California) bought a Greyborg frame. He was so happy with the performance, he travelled to Croatia to find out more information, and he came home as the North American dealer for Greyborg frames.

To date, there have been over 200 Greyborg Warp frames sold, but for the next part of this story, we must move on to the next big stage in their growth, which resulted from the most powerful hub-motor in production today. They developed it to satisfy their customers requests….the Cromotor . The powerful Cromotor has been a big success, with over 500 units sold worldwide.



The popular and well-known 9C is on the left (of course!), and the first-gen Cromotor used a very similar stator, but is twice as wide to allow the most amps possible, while still fitting in a bicycle frame.


The Cromotor was made for the Greybog Warp frame. And while Greyborgs have been built up using a variety of motors, the Cromotor has also been installed in a variety of frames. The Greyborg Warp swingarm was improved to allow the Cromotor to fit better, and the V2 swingarm pivot was also redesigned to incorporate a concentric Bottom-Bracket, a sophisticated feature which eliminates chain-growth issues.

The most recent V3 version of the Grayborg Warp frame  has several upgrades that are a response to Zelena Vozila’s communication with their passionate customers. A completely new swingarm was designed that has three specific features. First, the swingarm is slightly longer to reduce the bikes tendency to wheelie when using max power, such as the several 125V set-ups. Second, it allows a much longer shock absorber to be used, so a unit between 240mm-260mm is now recommended. Third, the swingarm was beefed up to allow the removal of the triangulation, and this helps a variety of custom builder options, such as using a Gates belt, instead of a chain.

Zvonmimir is now 44 years old, and for the last 3 years he has been working full-time as the  head of mechanical engineering at Rimac Automobili (link Rimac Automobili is the developer of the “Concept One” Electric car, which is a vehicle that we will all be hearing a lot about very soon (link”


2009, the first production Greyborg batch.



Computer Aided Design (CAD) allows every part to be precisely tested for size, shape, and placement before a single piece of metal is cut. Useful changes can be rapidly incorporated if needed.



This heavy-duty jig was made so all of the frame pieces can be welded while being firmly held in precise alignment.



Precisely cutting and bending the Chromoly steel tubing, ten at a time.



Water-jetting and laser-cutting can accurately and rapidly produce complex shapes. They are part of a family of processes called Computer Aided Manufacturing (CAM).



The background in their shop may not look fancy, but this machine shows that they are as serious as any manufacturer anywhere. This is a 5-axis notching machine…that they designed themselves!


A first-gen Grayborg frame welded up.

A first-gen V1 Greyborg frame welded up.


Here's an early first-gen Grayborg on top of the mountain that started it all, near Zvonimir's house.

Here’s an early first-gen Grayborg on top of the mountain that started it all, near Zvonimir’s house.


The first version (V1) of the production Greyborg Warp frame was immediately popular. Then, in response to many customer requests, Zelena Vozila worked with a major Chinese electric hubmotor manufacturer to develop the Cromotor. Previously, the most common motor used was the Crystalyte 5304/5305/5306’s. The new Cromotor is not only more powerful, it is also lighter.


The new Cromotor is so wide, it can only fit a single-speed freewheel on the right side, so Zelena Vozila made sure the high-quality Schlumpf internally-geared two-speed crankset is an option

The new Cromotor is so wide, it can only fit a single-speed freewheel on the right side (instead of the previous 7-speeds), so Zelena Vozila made sure the high-quality Schlumpf internally-geared two-speed crankset would work as an option.


Here's a close-up of the concentric BB, and how the two halves of the swingarm are now connected, so the bearing shells slide-on.

Here’s a close-up of the V2 concentric BB. It shows how the two halves of the swingarm are now connected, so the bearing shells slide-onto the sides of the Bottom-Bracket.


The white ABS canopy is made from automobile-grade materials that are UV resistant, and are heated and then vacuum-formed into their final shape. The frames are powder-coated (much tougher than paint). The canopy inner width is 110mm, and the swingarm accepts up to a 19-inch motorcycle tire. The wheelbase is 54-inches.



Here is a V2 frame (the swingarm pivot is concentric with the Bottom-Bracket), along with the ABS canopy sections.



Customers asked for a dual front disc brake system based on lightweight bicycle components, so Zvonimir designed and produced them. Also notice this V2 frame uses a clean and quiet Gates belt-system instead of a chain.



Here is a CAD rendering of the latest swingarm they have designed. It lengthens the wheelbase to reduce wheelies, it creates a longer suspension stroke with a bigger shock absorber, and it also allows a belt-drive as an option.


Here's a close-up of the clamping drop-outs on the new swingarm.

Here’s a close-up of the clamping drop-outs on the new swingarm. Although the Cromotor is perfect for the Grayborg Warp frame, some builders have used the Crystalyte HT/HS series, and “The Crown” motor, also from Crystalyte.



Have you been wondering what are the maximum watt-hours that this frame can hold? Here is the recommended 24-FET controller, with 18 bricks of 6S / 6-Ah LiPo. This frame has been run at everything between 48V to 125V, but at the recommended 18S / 72V the pack shown would provide…a whopping 36-Ah! 72V X 36-Ah = 2,600-WH


By focusing on using a rear hubmotor, the entire frame is available to hold a large controller and the maximum amount of battery (which is a restricting limit for most common frames). However, I have found one Greyborg owner who has decided to build up a light-weight mid-drive using an Astro motor, ES member “comradegerry”:


The Greyborg is a DIY frame, and this builder put together a mid-drive.

The Greyborg is a DIY frame, and this builder put together an Astro-motor powered mid-drive.



With over 200 Greyborg Warp frames sold worldwide, this is only the second mid-drive build that I’ve found. A GNG motor from ES member “comradegerry”


Written by Ron/Spinningmagnets, January 2014

Grew up in Los Angeles California, US Navy submarine mechanic from 1977-81/SanDiego. Hydraulic mechanic in the 1980's/Los Angeles. Heavy equipment operator in the 1990's/traveled to various locations. Dump truck driver in the 2000's/SW Utah. Currently a water plant operator since 2010/NW Kansas


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