Honda RS1000 - CB400F Mini Replica

Rick4004

New Member
I've just started a new project that I hope doesn't drag out too long, and doesn't take up too much time. Most of the parts used will be just stuff that I already have sitting around from past projects.
My inspiration - Honda RS1000 Racer Circa 1979.
These go by various names, RCB900, RCB1000, RS1000. These bikes were built from 1976-81 in various configurations, in fact, being hand built, it's doubt-full that any two were exactly the same. If you do a google search you will find lots of different seats/fairings/tanks used over the years. The last ones built in 80-81 even had mono-shock rear ends.
To me, the iconic signature features of these bikes is the solo seat, long tank, half fairing, and the curved muffler.
Anyway, as a teenager, these were my favorite racebikes, and when I was 17 I bought a 1980 DOHC CB750F that was the child of these racebikes.
I've got enough CB400F rolling chassis sitting around that I decided my next 400F should be an mini endurance racer.
Attached is a photo of the inspiration, and a quick CAD model of my 400F mini replica.
 

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Rick4004

New Member
Re: Honda RS1000 Mini Replica

I wanted to laydown the shocks to get that period twin shock race look. Step one was to measure the physical dimensions of the RS1000 and input those parameters into Tony Foale's suspension software. I have a 1/12 Tamiya scale model of the RS1000 and used that in conjunction with photos I was able to gather the dimensions, and estimate a spring rate. This gave me the spring force at the rear wheel and the rising rate curve.
I was then able to transfer that data to locate the shock mountings on the 400F. The spring force on the 400 is a little less than the 1000, but the 400 should be lighter, so I think it's a good starting point.
 

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Rick4004

New Member
Re: Honda RS1000 Mini Replica

I bought these generic chinese shocks off of ebay. 340mm long. My research indicated that these come with either 7mm or 8mm diameter springs on them, depending who the seller is. I chose the seller based on the fact that it was specifically stated in the description that the springs were 8mm. Of course, when they arrived, they were 7mm. I think the spring will be too light and I'll be searching for heavier springs.
I cut off the old shock mounts from the frame, cleaned up the tubes, and used some 3d printed marking guides that I built off the CAD data to position the holes for the new shock mounts.
 

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Re: Honda RS1000 Mini Replica

Tamiya make a model of the RS1000, you might find it helpful for measurements etc.

CBX550 engine would give you a twin cam engine cheaply

Sent from my SM-T560 using Tapatalk
 

Rick4004

New Member
Re: Honda RS1000 Mini Replica

julain.allard66 - Yes, I have the Tamiya model, I took some measurements off of it when inputting the suspension parameters. The CBX550 would make for a cool project, they were fast for their day. But, they are also pretty rare, and parts aren't easy to find. As I stated, this is a cheap build using existing parts, and I have lots of 400-4 stuff sitting around.
 

Ichiban Moto

Bad Ass Cafe' Style
Re: Honda RS1000 Mini Replica

Chris has things for those shocks and this tool:

http://chrislivengood.net/wp/advanced-spring-rate-estimator-new/

ps. anyone know Chris' username here?
 

Rick4004

New Member
Re: Honda RS1000 Mini Replica

I wanted to disassemble the shocks, check them for assembly issues, and rebuild them with fresh oil. But first, I needed a spring compressor that fit. I have an S&W compressor, but it wouldn't work on these shocks. So, out came some flatbar, nuts,bolts, and threaded rod. Worked like a charm.
 

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Rick4004

New Member
Re: Honda RS1000 Mini Replica

ICHIBAN - I emailed Chris and he was very helpful. I was going to just purchase the shocks from him already rebuilt, but the shipping from the USA to Canada was $65USD. I ended up buying them direct from China for $85 US shipped to Canada, so I figured I'd might as well just rebuild them myself.
 

Rick4004

New Member
Re: Honda RS1000 Mini Replica

I wanted a better swingarm for this project, something longer, wider, and stronger. I had a CBR600F1 sitting here with all the parts that I need to make it work. Only problem is that it is 20mm too wide to fit in the frame.
Cutting out the 20mm is the easy part. Getting everything lined back up and keeping it straight during welding will be the harder part.
 

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Rick4004

New Member
Re: Honda RS1000 Mini Replica

1fasgsxr - Just look at the photo, there isn't 10mm of material available to remove. Plus, by taking it out of the middle I leave all the bearing surfaces undisturbed, which means stock Honda bearings/seals can be used. Just need a shorter bearing spacer in the middle.

As an aside, the CBR300R uses the exact same bearings as this 1988 CBR swingarm.
 

Rick4004

New Member
Re: Honda RS1000 Mini Replica

Well, looky what showed up at the door this morning. Wheels from a Suzuki GS500. I've always used wheels from the CBR600F1 when getting rid of the spoked wheels on the CB400F. The front CBR wheel is good for a 110 wide tire, and the rear is good for a 130 or 140 tire. But, they are getting harder and harder to find with good brake disks, so the nice sets that I used to find for less than $100 a set are now creeping up to $200 - $300 a set.
I thought about trying CBR250/300 wheels, but they are still pretty expensive too. Then I thought about the GS500. The stock tires are 110 front and 130 rear, and unlike the CBR that was only made for 3 years, the GS soldiered on basically unchanged from the early nineties into the mid 20-teens, so there are tons of wheel out there and they are cheap. I paid $90 USD for this set from a 1994 model. The shipping to Canada was more than the wheels.
After a quick check it looks like the rear axle for the GS will mate up with the CBR-F1 swingarm just fine. The GS front axle is a meek 15mm diameter, while the CBR-F4 forks that I have sitting here are base on a 20mm axle, so I'll have to see if I can do some bearing switcharoo to use an F4 axle, or make some sleeves to use the GS axle.
 

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Rick4004

New Member
Re: Honda RS1000 Mini Replica

I made up a mandrel to fit into the swingarm pivot tube, and the mandrel expands as you tighten it down to grip and hold the tube. Then just some machine clamps on the arm.
I can now machine out the center section width that I need, and I end up with perfectly square surfaces for welding everything back together. (In theory anyway).
 

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Rick4004

New Member
Re: Honda RS1000 Mini Replica

So much work, so little accomplished. I made up a dual expansion mandrel to fit into the swingarm pivot tube to align the two halves before/during welding.
Took about 3 hours of machining, then 15 minutes welding. I think I'll add some half moon sleeves on the outside of the pivot tube, but it's good enough for now to proceed with mock-up.
 

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Rick4004

New Member
Jumping around a bit already. I'm waiting for some tooling to continue with the swingarm so I turned my attention to the tail section for a bit.

1. The pile of parts are 3/4" MDF cut up to make a buck for the tail section.
2. Dry fit all the parts to make sure they all go together.
3. Everything glued together, and all the edges and radii "faired" to each other.
 

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Rick4004

New Member
I started with the top panel of the tail section. Started forming the edges around a piece of 3" diameter tube. Got it roughed into shape. Will have to refine the edges as the other panels are mated up.
 

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