Hi Gang,
First of all, I'm happy to have found this forum full of like-minded individuals. I have been doing a bunch of google searching and somehow a lot of my searches end up with me reading threads on here. I hope I can contribute just a little bit to this pool of 2-stroke knowledge!
I wanted to share some work that I am doing on my '77 RD400. It's been a bike I have kept mostly stock since I've always had other bikes to fill in for track days or commuting. As of a couple of years ago, this is my only bike and I'm finally ready to put a little bit of work into making it a sportier street bike.
This is as it sits now before any significant modifications:
rd400.png
I'll spare the full details of how I ended up deciding to do a 2018 R3 front end (non-usd) swap, but the main reasons were cost and upgrade parts availability. With all the people racing these things there are a few cartridge kits to choose from. For braking I will be using an FZR600R (3HE) front brake rotor, keeping the stock front wheel, and using the R3 brake caliper. My constraint here was that I didn't want to have to use any brake adaptor brackets which meant that the stock calipers drove all the brake dimensions. Also since I'm keeping the R3 triples, a stock R3 wheel should bolt on in the future if I want to try 17" tires.
I am pairing this front end upgrade with SR400/500 Ohlins rear shocks (part #YA218). The SR500 seems to be pretty close to the mass properties of the RD400, and they offer compression and rebound adjustment so this should more than cover my sporty street riding goals.
On the right side, the lower spring perch slightly interferes with the brake caliper mount. My solution to this is to use a slightly shorter brake tension bar so the caliper rotates a little towards the rear of the motorcycle.
Here is a quick test-fit of the rear shocks:
rearshocks.png
I also went through and measured the loaded and unloaded weights of the motorcycle with full fuel tank. This might be useful if anyone needs a real-world reference point for their mostly stock RD400. My motorcycle does have shorter bars but otherwise it is stock (for now!)
Masses are all in kilograms, I'll let you plug it into your AI tool of choice to convert to your preferred units:
Unloaded Mass with full fuel and oil:
F: 76 kg
R: 94.5 kg
Total: 170.5 kg
Mass with rider:
F: 102 kg
R: 136 kg
Total: 238 kg
I may be checking masses after the upgrades to see if I somehow managed to reduce any weight with the modifications, but I'm not too optimistic.
Next update I should have the rear suspension put back together!