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Discussion Starter #1
Hi, I'm building a custom seat for my café racer project and have a few questions. I am going with a flat piece of stainless sheetmetal for the pan. I will put foam on it, wrap the leather around to the underneath and hold in place with rivets.

question: I want the seat to ramp up slightly towards the tank and have a nice snug fit against the tank. Would you recommend I extend the pan up vertically by cutting/welding a piece of sheetmetal that rises up vertically against the tank? or do I just cut the foam the way I want and upholster over it? The seat foam near the tank will be around 2.5" high and I'm worried that when upholstered it will deform that section somewhat.

the foam in this pic is temporary, just for getting an idea of the shape I want.

seat.jpg
 

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If you are going to use leather and want a proper fit you'll need to do some cutting and sewing. Make the cover to fit the foam.
 

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Hi, I'm building a custom seat for my café racer project and have a few questions. I am going with a flat piece of stainless sheetmetal for the pan. I will put foam on it, wrap the leather around to the underneath and hold in place with rivets.

question: I want the seat to ramp up slightly towards the tank and have a nice snug fit against the tank. Would you recommend I extend the pan up vertically by cutting/welding a piece of sheetmetal that rises up vertically against the tank? or do I just cut the foam the way I want and upholster over it? The seat foam near the tank will be around 2.5" high and I'm worried that when upholstered it will deform that section somewhat.

the foam in this pic is temporary, just for getting an idea of the shape I want.

View attachment 97333

Hard to tell from the photo, but it looks like a DIY mono shock conversion. If so, how well it is welded and braced should dictate what you make the seat pan out of. It might also dictate the shape.... I'm thinking something that resembles a jock?..... and yes I'm an asshole.
 

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Agreed that getting the seat cover to fit properly is critical, which will likely involve some sewing. Instead of rivets, which can easily cause puckering, I'd suggest using contact cement to stick the leather to the underside of the seat pan. This will allow the cover to be easily be re-positioned to allow micro adjustments. You will need to put multiple coats on the leather to saturate the edge of the leather so that it will stick.

As for extending the seat pan, it will depend on the geometry of the gap. I am guessing that the gap is where the 2" Styrofoam is placed, which might require the support of extending the pan, easily done if you glue or rivet a bit more stainless sheet in place.

Have you figured out what foam you want to use? I have had trouble finding foam firm enough for a motorcycle seat, and have gone to using under carpet padding (very firm, but can be layered, cut and shaped to build a foundation), and then a layer of 1" latex foam to increase the cushy-ness. I use spray adhesive to tack it all together.

Just some thoughts.
 

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use closed cell foam for the seat padding. it's weeatherproof and it's what most modern motorcycle seats are made of:

https://www.grainger.com/product/5GCF1?gclid=EAIaIQobChMIzJLMvaDX4QIVkMDACh1acAYxEAQYASABEgITdPD_BwE&cm_mmc=PPC:+Google+PLA&ef_id=EAIaIQobChMIzJLMvaDX4QIVkMDACh1acAYxEAQYASABEgITdPD_BwE:G:s&s_kwcid=AL!2966!3!281698275981!!!g!471318999323!

if you don't want that, go to joann fabrics and tell them you want an outdoor upholstery foam and they will probably show you all the high density seat foam they have.
https://www.joann.com/sewing/foam-and-fiber/foam/

carpet pads come in two varieties: prime foam and frothed foam. Prime foam is less dense than upholstery foam and tends to move around a lot, frothed foam is as dense as upholstery foam and is pretty much the same thing although made slightly differently.

as far as wrapping around the tank at the back, there is no wrong way to do it. The oem's in the 70's usually just left this upholstered without a seat pan, but if you look at any 1970's honda there is always a wear mark in the paint from where the seat wore it away while moving around. Starting in the 1980's the OEMs began to make pans that had a little front support so the seat didn't do this as much. aftermarket seat makers like corbin are the same way - I have an old corbin for my beemer that wraps around the back of the tank without a supporting pan (it's just foam and leather), but the corbin I had made a couple years back for my 1979 cb750F has a bit of the pan come up to wrap around the back of the tank.

One thing I would suggest is forget the "stainless steel" pan and make it out of fiberglass directly on the bike. You'll get a better fit and if you want the seat pan to wrap around the back of the tank, it's way easier than bending, weldign and shaping sheet metal (esp stainless sheet which is a bitch and a half to work with).

Here is a tutorial:


and yes, I am an asshole also because I am pretty sure your mono-shock conversion is a death trap - mostly because for every 1 I see that takes real engineering into consideration there are 100 that are just slapped together using pictures off google image search as a "guide". But prove me wrong - post some pics of it up.
 

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I would make the entire pan from fibreglass, use closed cell foam and a jell pad and barb strips to hold the cover. If your cover is sewn to fit the seat there shouldn't be a need to adjust it. Make your cover, steam it before you put it on (a bit of water in a steel bucket drop in oven heated rocks), which will give it some stretch, put it on and it will contract to tighten things up. Any small creases or puckers I work out with water in a spray bottle, my fingers and a warm smooth river rock.

Personally I wouldn't use leather for a seat.
 

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Discussion Starter #8
Thanks for all the responses and good info! I think I follow everything for the seat. I plan to shape the foam myself then get it upholstered by someone (that knows what they are doing) so they will definitely be able to sew it in the required shape. Yes, the area I was talking about is up near the tank. I like the idea of contact cement vs rivets. I may add a small pan upright extension at the front, just to help with shaping and give the upholsterer something to stick to.

What do you use if not leather? Some type of synthetic?

I bought foam already. I read this article (linked) and went with their recommendations of Sunmate brand foam, 1" of extra firm on bottom and 1" of medium for top layer. I got it already, seems decent, like a stiff memory foam.
https://www.sunmatecushions.com/pages/motorcycle-seat-retrofitting-with-impact-absorbing-foam-applications

I like the epoxy fiberglass idea. For this I like the asthetics of seeing the whole frame rail.
 

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Discussion Starter #9
As far as the monoshock goes, your point is well taken (now that I understand what you mean).

Firstly, my calling this a café racer is a bit of a stretch, it is more of a custom/frankenbike with a vintage tank. The frame, which started out with a monoshock, is now highly custom. The swingarm was a monoshock as well. I spent considerable time (in solidworks) designing the linkage actuation so it pivots correctly and with the appropriate amount of spring load on the shock and cnc'd the various components. I didn't run a finite element model on the upper frame where I moved the mount, but used similar construction as the original (and ran it by a stress engineer). I will post some pics!
 

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Discussion Starter #11
This whole project is still a bit of a work in progress. Motor is a GS500, frame started out as a steel dirtbike frame that I widened and lengthened, front end and swingarm are from a ninja zx6.

Here is some of the intermediate modeling steps
model.jpg

here is the suspension linkage/rotation scheme
suspension.JPG
 

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Discussion Starter #12
Here are some pics showing the monoshock details. I still have some finish welding to do and to remake a few linkage parts (currently have some placeholders ones in there).

The mount at the top is pretty close to where it was originally, I moved it back about an inch.
 

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AutoCAD? Damn I need to learn me how to use that. My shit is generally scratched up on some scrap of graph paper with crayons LOL

It resembles the rendering so good on you for that
 

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well I am impressed - a lot more thought went into it than the average trash heap monoshock conversion.

That being said, I get the feeling while you grasp complex engineering concepts, some basic motorcycle practices are unfamiliar. I think you would really benefit from tony foale's book: https://motochassis.com/

dirtbikes are designed for rough terrain and often aren't required to lean over all that far to turn. If you ever watch arena cross you'll see all the turns are banked. To that end generally what works for them in frame design doesn't work for road racing motorcycles all that well. even still most follow the tenant that you seem to have deviated from - the best handling motorcycles are the ones with the strongest connection between the headstock and the swingarm pivot. This is why most modern sport bikes have "wishbone frames" be it a trellis design like ducati or cast aluminum like a zx10. the stronger and more direct you can make that connection the better your bike will ride.

here are some notes:

- I don't think your swingarm pivot is robust enough, I think when cornering hard the front wheel and the rear wheel will try to come out of alignment.
- I don't think there is enough reinforcement between the rear frame upright and the headstock and again I think the wheels will try to move out of alignment under hard cornering.
- I don't think that front down tube is strong enough, and if you made the engine a stressed member (more than it already is) it might be unnecessary, but that requires fixing the head to the backbone.
 

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I wouldn’t be inclined to ride it. With that front end, decent tires and assuming that engine puts out what it is supposed to, it’ll be a wild ride if you push it. That frame will flex a fair amount with the stock engine in it. The bracing for the top shock mount looks thick, but the question now becomes what is the wall thickness of the tube it is welded to? That doesn’t necessarily pose a problem because a good welder can deal with different thickness in material. However... how good are your welding skills assuming you did the welding. Judging by the extent of the grinding, I would say you need more practice and maybe a better welder. Judging welds in a photo is sometimes redundant, but it sure looks like you took a lot of material off that tube in the last photo..just under the rear of the tank. I know with welding tubing in situations like that you can overlap a weld and end up with material you want to remove so it’ll look better, but shouldn’t be a need to go to town like that. Good you changed the countershaft location from the drawing. How is that shock mount tied into the backbone of the frame? Did you re-drill that hole in the linkage to alter the ride height? Any idea what the trail is when everything is sitting where it’s supposed to?
 

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Discussion Starter #17
Geeto67
ebook bought and downloaded! I will start break into it tonight, thanks for the recommendation, looks like a classic! Good comments and notes which I will take under consideration. I think I could beef up the swingarm pivot. The other two would be more of an undertaking with this starting point. I will start with looking through the book.

Cyorg
I am defiantly not an expert welder, and yes, I did a LOT of grinding, which tends to look suspicious. I did multiple passes on a few of the sections and then ground them down to look smooth. The top shock mount is welded directly to the main upper tube, not sure on thickness for that tube. I believe the drilled hole you are referring to it a temp piece I made to adjust the ride height when I was working out the linkage (it is a place holder). I don't know what the trail (and rake) are, I will measure/estimate.

Frame rigidity/flex concerns seem to be a recurring theme here so that is something for me to think about.
 

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You guys need to read up a little more about dirt bikes. A GS500 and a DR650 produce virtually the same horsepower and the DR is geared for more torque. If you guys don't think dirt bikes, or even more so, dual sports don't do much leaning than please let me never get behind you on trails or twisties. Dirt bikes lean, especially in supermoto gear, you're just in a different riding position.

 

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You guys need to read up a little more about dirt bikes. A GS500 and a DR650 produce virtually the same horsepower and the DR is geared for more torque. If you guys don't think dirt bikes, or even more so, dual sports don't do much leaning than please let me never get behind you on trails or twisties. Dirt bikes lean, especially in supermoto gear, you're just in a different riding position.]
Not sure who “you guys” are and not sure how you know it’s a DR650 frame unless my reading comprehension has gotten worse, which is a possibility or he mentioned it on another thread. Don’t most dirt bikes have all kinds of interesting flex built in? You tell me, I don’t really have any desire to read up on them. Let’s assume yes. Combine that with a (presumably) first attempt to make major modifications to a frame. USD forks, a swingarm that isn’t likely go to flex behind that engine, fat tires etc etc. and my guess would be that for a street bike... or whatever he uses it for, it will flex more and not in a desirable way.

OP, did you plot out the rake and trail before you started.? If not, you may be in for a surprise. Even if you did plot it out, the change in ride height will have changed that. You may want to make further changes or swap out the triple, to get more trail. Might want to add a steering damper, but better to have them set up so they don’t shake to start with.
 

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OP. I’m operating with a really sketchy internet connection. I type shit and it disappears and punts me out. I have some suggestions for measuring rake and trail, so you can see what your changes do to them in real time. I’ll post it when I get back to civilization.
 
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