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Discussion Starter · #1 ·
Hi All,

I have read and enjoyed the site for some time, and now I'm going to become active and post. Some history about me - for most of my life I've been a car guy, typically appreciating classic stuff. I've completed ground up builds on a number of early 911's, Datsun 240Z, and even an early RX7. I am not so into stock restoration, and prefer to take something older and make it 'go fast and handle good'. I've also spent quite a lot of time on the racetrack, including a memorable weekend punting around Bathurst here in Australia in an early Porsche I built myself. Over the years I've taught myself to do most things, including fab, engine builds, fibreglass, and paint. My sheet metal skills are very limited though! and I don't own a sewing machine so trimming I typically farm out. I have mates who are into bikes, although mostly Harleys. While I'd still like to build an old school Harley one day, the cafe racer mentaility more suits my overall ethos. Hence here I am undertaking my first bike ground up build.

The unlikely subject is a 1982 Yamaha XS400 DOHC. Also called the Seca in some markets. The positive - a relatively ligthweight 45hp twin cam motor. Cheap and unloved. The negative - probably everything else. The plan is to lighten it up, add much improved more modern suspension, brakes, and rubber, alter geometry to suit, and end up with a narrow, agile, moderately powered thing that also looks good. Yes, this will be a lot of work :)

Project start.

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Earlysport
XS400 Project, CB750 future project, Ducati 996R
 

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Discussion Starter · #2 ·
You will note the forks are a little bent, which means they have to go. The PO has also cut off the rear of the frame - true cafe racer style already complete! Not. So frame mods are in order. The factory wheels are also too skinny for my liking, so a wheel swap is also order of the day.

Tear down.

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Now for the fun stuff, the rear of the frame is going to need fabrication, so the factory remains have to come off. Note the DOHC uses the engine as a stressed member, hence there's not much frame left at this stage..

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And with the frame stripped bare I figured a good time to check if my planned tank would fit - a Yamaha TX650 version. The TX tank required some gentle widening in the tunnel to drop down neatly..

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Earlysport
XS400 Project, CB750 future project, Ducati 996R
 

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There are far better bikes to pour your enthusiasm into, GS450 and GSX400 twins spring to mind. GSX400 twins are very fast for what they are: like a heavy 125 with the power of a SR500. The GS450 is more of a commuter/tourer.

Those forks are not so bent that a pro could not straighten them in about ten minutes, don't throw them away, someone might want them one day. It's easy to sand minor rust off fork tubes too. I doubt you could squeeze fat modern tires into the swingarm and clear it and the chain. Big tires won't make this bike ride any better.

This series of Yamaha twins have a strange convoluted airbox that "sucks" differently to each carb. Yamaha may have compensated for this (from a dim memory) by fitting different jets (and maybe even needles) to each carb. CV carbs are very hard to get running right in any case without the OEM airbox. If the carbs are Hitachis it might be very hard to get jets, etc.

I rode one of these bikes once and it had the most comfortable seat and riding position of anything apart from maybe a GS850, all it needed was power, handling and brakes.

Bear in mind, bikes are totally different to cars. Bad cars leave you stranded on the road, but bad bikes kill. They need a different strategy entirely.
 

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Witworth is essentially right. I diverge from his thinking a little in that I like larger tires on a smaller bike since I am a big guy and larger tires slow the handling down to where I like it. I am not a fan of twitchy handling, but others are. He is also correct that the swingarm will probably not handle a much bigger tire. I would look at a 120-18 rear and a 100 - 18 front. If you can find the complete front end off an 84 or 85 FJ600, that would give you 36mm forks dual disks and a wm-3 18" front wheel. You could also run the matching rear wheel but it has a disk brake so you would need a caliper and master cylinder. If you used rearsets from an FZ or FZR 600 the whole mess would bolt up pretty easily, if you can get therear wheel to fit in the swingarm and get the chain to line up. You could look at the aluminum swingarm from an IT 465 that has the same configuration as the Seca. It will take a wider wheel and tire, but you may still have alignment issues. The RZ350 and SRX600 have the same wheel pattern as the FJ and I think the rear is slightly narrower but will still take a 120 tire well.

Witworth is also correct about the carbs and the Hitachis are the worst of the lot. I wouldn't even mess with them before I replaced them with a set of 30 or 32mm VM mikunis.

Since you have removed the rear subframe, I would get on e-bay and look at the variety of available bolt on subframes from other yamahas and find one you like. I would look at either the YZ 125 or I like the FZ600 since they are tubular steel, easy to modify and not too heavy. They just bolt on at 4 points.
 

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I can't bag on the 400 twin cammer. I was only able to ride a new XS400R Seca only once, for about 40 miles, but I really enjoyed it. It was a huge step up from Yamaha's previous SOHC XS400, and nowhere near as bland as Honda's CB400T. Motorcyclist Magazine went as far as dubbing it a "born again RD400." But it was a product of its era, and the forks, dampers and carburetion are all atrocious by today's standards. I thought (unlike most folks, it turns out) that the creased shapes of the engine and the sculpted bodywork were very attractive. Especially remarkable since it used the same general design language as the horrifically ugly 650 Turbo and 550 Vision. I remember it as being fun to rev and having very comfortable ergonomics. But perhaps it just reminds me of being a younger man.

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The Seca 550 wheels will probably bolt up better than most, but the Seca rear is not much if any wider than the 400 wheel and the 550 front is narrow, too. I do have a set of them off my race bike, if anybody wants them. I put CBR600F wheels on my race bike Seca 550. I, too, like the styling of the Seca 400 tank and seat.
 

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Discussion Starter · #8 ·
Thanks for the feedback guys - an aspect to this build I do like is that I can find almost no builds in internet land for this model bike, so breaking some new ground is somewhat appealing.

The engine has Mikuni CV carbs. 34mm I think. I have considered replacing them with a pair of flat/oval slide carbs - maybe from a two stroke dirt bike? I definitely want to remove the airbox - in fact I'd like to run velocity stacks, but not sure I want to sacrifice real world usability for the tiny performance gain. At the end of the day I'm not chasing power for this bike - I'd rather pursue light weight and feel. I have a 996R for when the power bug bites. If the XS can end up as something that is fun to really wring it's neck at moderate speed then that will be mission accomplished.

And yes, I did some playing around with the stock swingarm and while it looked like it would take a large tyre the shape prevents it. It's also incredibly heavy.. so in a stroke of good fortune I picked up an alternative that I think is more suitable.

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The 05 GSXR swingarm and linkage weighs approximately half what the factory square steel XS piece does. And kindly came with the rear wheel which is also way way lighter than the factory XS wheel and tire. A 180 tire is definitely overkill I know, but I guess this is one area where I want to make the aesthetic a priority. Hopefully the performance downside of running too big a tire is not much. The challenge was how to fit the swingarm, which is a good 20mm wider than factory. I looked at narrowing the swingarm but this would require machining to be farmed out and also wouldn't solve the problem of chain offset and clearance for the much wider tire. I decided the alternative was to widen the frame. This would also allow a variable amount of motor offset to get the chainline correct (with no doubt an offset sprocket also required).

Mockup below after splitting the frame, ensuring everything would centre up correctly. The plan is to use the stock GSXR swingarm pivot bolt in the factory XS frame mounts, but opened up for the bigger diameter bolt.

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After mockup the frame was tacked up with temporary braces and then welded. The lower tube originally had the centre stand mount brackets. I am determined to run the GSXR linkage based monoshock also, replicating the factory setup on a GSXR. hence the lower tube has been cleaned up and mounts welded in place for the lower dogbone from the rocker linkage. Finding the correct geometry diagram for the GSXR rocker linkage was a challenge and I ended up measuring a real bike. The custom rear section of the frame will have to incorporate the shock mount.

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I intend to further re-inforce the lower tube by fitting an internal sleeve of thick walled tube. Of course with such a rear end in place the front suspension will definitely have to match, fortunately this is the easy part of the build..



Earlysport
XS400 Project, CB750 future project, Ducati 996R
 

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While you have the welder out you might want to think about an out-rigger bearing to support the wider front sprocket you are going to need to line up the chain with that rear wheel.

I'm guessing you're probably going to need to space it out 30-40mm.
 

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I wonder if you should fit the rear wheel from an SV 650. It is narrower and will fit a 160 rear tire, enable you to line up the chain easier and not require as many collateral fixes. Also, if you are modifying the frame, I would suggest you move the swingarm pivot as far forward as possible to shorten the wheelbase as much as you can.
 

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I wonder if you should fit the rear wheel from an SV 650. It is narrower and will fit a 160 rear tire, enable you to line up the chain easier and not require as many collateral fixes. Also, if you are modifying the frame, I would suggest you move the swingarm pivot as far forward as possible to shorten the wheelbase as much as you can.
First Gen SV650 wheels, forks and brakes stuff is cheap, works well and is plentiful on flEbay. Plenty out there.
 

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Discussion Starter · #14 ·
Getting the chainline correct was some challenge. A lot of mocking up with aluminium stock clamped to sprockets etc. By using an aftermarket sprocket I can get the chain running closer to the tire and gain just under 5mm, which provides just enough frame clearance. I'm going with a 525 chain but in hindsight a 520 would have been better. Offsetting the engine some in the widened frame gained me some more, to the point where I need an offset sprocket of 11mm. Of course no such thing is available for this model XS so had to have a local machinist make one up from two sprockets.

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Next step an All Balls pair of steering races to fit the matching GSXR forks to the front end. I'm spacing the forks up some to get me a reasonable riding position with the clip-ons and the geometry I'm looking for. Geometry will be fine tuned with adjustable dogbones in the rear suspension linkage, rear pre-load, and fork position in the trees.

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The ugly ignition key had to go, so has been cut off. The top triple tree will be then re-worked to be nice and smooth. A hidden ignition switch is the plan, along with a complete re-wire to suit the factory GSXR hand controls.

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And lastly the first mockup with new front and rear ends in place. The rear end adds just under 2" to the factory wheelbase - which I think is OK. The front end setup again is considerably lighter than stock, even though so much beefier in appearance! And yes, I think I'll be overbraked, but I'm not sure there is such a thing.. The piece of wood clamped in place in the pic is to start getting the angles right for the rear subframe construction and actual tank mounting angle etc. Still a lot of playing around here to do.

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So far I'm pleased with how it's shaping up - and am reasonably confident that I'm on the right track? Getting the GSXR gear at the right price is what has really sent this project in the direction it's going. The comparison between 1982 SX standard equipment (forks, rear swingarm, wheels) and the 05-06 GSXR parts has been astonishing particularly in terms of weight. All of the GSXR gear is way lighter. While I haven't been weighing each part to see how much I'm saving I'm looking forward to putting the bike on scales when complete. The bike had a 169kg dry weight stock. I'm pretty confident I'll be at least 10kgs down on that, maybe more..

Earlysport
Yamaha XS400 DOHC Project, Future CB750 Project, Ducati 996R
 

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You need to plan well ahead with your seat subframe and shock mount.

You need space for even small K&N air filters near the shock mount. Those carbs will never run correctly gasping to the open air. I may be wrong, but I still think the carbs are Hitachis: which are absolute horrors for jets, parts and tuning.

The shock mount needs to feed all the forces into the rear main frame tube loops, and not just to a "café racer style subframe" . If you are getting frame design hints from sites like Tripebuns, think again, and look at genuine old race bikes. Bike chassis design is not a black art, and there are now thousands of well made race frame images on the web to copy and gaze at. The ones that look "awesome" are usually the worst made, and are best avoided in emulating them.

If fitting thick wall tubing on bike frames made them stronger, the factories would do it that way. Welding in a stronger tube can often mean you are just moving the stress "next door" for a weak piece of plate to have deal with.

I bet the most successful webmasters ride bikes like stock Ducatis, and not the things that they show on their websites.

You've got skin in the plot now, lets all see how it ends.
 

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Seat and subframe are of little consequence, compared to the significance of replacing a direct mounted top shock with a linkage rear suspension, that is one radical ambitious modification you have there.
The subframe is little more then something to perch your butt on and hold up the taillight.

Bigger wheels, suspension and brakes will result in greater forces being transferred to that delicate frame and steering head which in this case are now woefully under-rated. You might have a difficult time differentiating suspension travel from frame flex unless you similarly beef up the frame in which case the best you will end up with is an under-powered GSXR.
…outside of the design challenge, I'm not sure I follow the logic of building something like that around a mild tuned 400cc twin.

But please do carry on, it's fun to watch :I
 

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That photo is obviously a stray promotional poster for the unreleased movie franchise "Harley-Davidson and the Marlboro Man, The Gixxer Chronicles" . It was subtitled "No Luck 'n' TRUCK! 'n' *%cK!" .

PS: A little schadenfreude does indeed lighten up a dull day fine. Dinosaur Jnr knows all about feeling the pain.
 
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