Restricted Growth project update - March

Islabikes Restricted Growth Image

 

Here is an overdue update on our Restricted Growth (RG) project. Since we last communicated a lot has happened.

A quick recap: over the years we have been approached by people with restricted growth in the hope our children’s bikes were suitable for them. Though our components are, the frames are not. Two years ago, we built the first RG prototype frames to test our sizing assumptions. We’ve been slow to develop the bikes further, although we have had plenty to keep us busy…

Nearly a year has passed since the first lockdown – the one where traffic was so light that cycling at 5pm on Friday was like 9am on Sunday. Two weeks before the start of “lockdown 1.0”, all our office staff were already working from home, whilst others were building bikes all over, from garages to garden sheds.

The bike industry experienced a boom and bikes became as scarce as hen’s teeth. Lead times for key components went from 30 days to 600 (yes, one and a half years!) and containers of PPE clogged Felixstowe port, meaning ships running to tight schedules were diverted to Rotterdam.

Throw in a few COVID cases and a management buyout, and 12 months passes fast.

Excuses aside, we made some progress last year; two people tested our mk2 prototype frames and, after a period of real-world riding, we sent the frames to Germany for fatigue tests – the only UK biketest lab having closed last year.

Step-through frames are notoriously challenging to design without making them obscenely heavy, and in this case the lightweight tubing we tested, folded like a paper napkin. We have since looked to the Spitfire and motorsport for something with more grit.

Fast forward to this week and we started building two frames; one has a down tube with a wall thickness of 2.64mm and the other 2.03mm – we are hoping the thinner and therefore lighter tubing makes the cut! For context, lightweight diamond-shaped frames have down tubes with walls barely thicker than a business card.

Our finite element analysis model (FEA) suggests the frames will pass, but FEA on welded steel bicycle frames is not well established (we can tell you all about aluminium frames), so there is no substitute for real-world testing.

 

 

We use these plots to identify the stress distribution within the frame and how close certain areas are to failure. The red highlights are the areas that we have focussed on strengthening.

The left image shows the former material for the downtube and the right shows that used in our new testing prototypes.

Our next steps are to complete two new test frames and send to Germany, for testing to be completed before the end of April. Depending on the results we may follow up with further testing before making the first bikes available. Our aim is to have availability this summer; we hope you will forgive us for our vague timelines.

We will send monthly updates, even if it is just a short paragraph to say we haven’t made any progress!

Please feel free to share this newsletter with anyone you think might be interested in the project.