Where to cut!

Hey guys,

I am an absolute noooob. Getting deeper into this project and looking to throw a brat style seat on my 1978 Honda cb750f

Here is my bike, the images underneath is what the direction I would like to go although I believe those bikes are 550's. Any idea where to cut?

Here is the link to what I think I need?

http://www.dimecitycycles.com/dcc-originals-diy-custom-rear-tail-hoop-for-large-cc-motorcycles.html

Neither of those two bikes have a frame hoop welded on. Also, both probably have tire clearance issues. That hoop on the back of the stock frame is about how much tire clearance you need at full suspension travel. Best method to check has been said many times on here, take a shock off and ratchet strap the swingarm to the frame until the shock bottoms out. Build 1" of clearance from there and you're golden.

Don't trust anything with Firestones

That's a horrible bike to want to copy. That is all.

+1. Cool looking bike, but some things to consider:

Most of the alterations on this bike from the stock bike are a substantial downgrade in performance. I'm all for making your bike custom and having a cool looking bike is a big part of motorcycling for most people. But if you build your bike into something that's only cool to be seen with but sucks to drive to the place you get to be seen, expect a lot of criticism. Probably takes quite the edge off of looking cool.

Lowering the bike like that renders the suspension nearly useless compared to stock, and the reduced ground clearance limits lean angle enough to be potentially dangerous. I am not saying the bike is unrideable or even dangerous, but it is a huge downgrade from stock. I doubt those tires are as horrendous as everyone seems to think, (never ridden a bike with them, so can't offer an opinion), but they are much too large for the bike, which is unquestionably detrimental. They are also heavy, putting weight in the worst place possible reducing the unsprung weight ratio which negatively impacts whats left of the suspension. Motorcycles are already afflicted with a bad USW ratio - making it worse has a noticeable impact even with a great suspension. Lightening the rest of the bike which is normally not a bad idea compounds this problem. There are plenty of other aspects to the redesign in your example bike to warn against, but this should be enough!

Think about keeping your bike fun and safe to drive. Most of the look you want can be had by customizing the styling - leave the suspension and running gear alone unless you know what you are doing from an engineering perspective. You will enjoy riding your bike as much as looking at it.

As far as that rear cross brace goes, its importance as a structural element is far less important than it seems generally thought. It does far more to hold the fender and seat than offer much chassis rigidity. That said, there is not much reason to remove it as you need the space for suspension travel, and you still need to mount a fender and seat. You can't cut it out and put the seat in its place because the tire will hit it, so you may as well leave it and design the seat around it. If you go down this road anyway, at least make sure your suspension is what limits your wheel travel. Hit a bump in a turn that compresses the tire into the seat pan and you will be on your ass, and wish you took the good advice offered in this forum.

like he said
and what you need to do is get this brat nonsense out of your head brat bikes are built by idiots that don't understand how a motorcycle functions and they probably dont care neather cause they just gonna look at it on a pictchur on there fone
anyway you have already made it more difficult to ride safely confidendtlty and comfortably with those hideosus STUPID drag bars ...put some superbike bend bars on the goddam thing or 2-3" rise mx bend and put the goddam stock seat back onnit all you are going to end up doing with the brat disease is ruin a decent bike and crash the fuck out of it and maybe hurt innocent people
just remember drag bars are for trannies and stance rimes with dance

What he means is drag bars are for going straight. Just straight.

Alright guys thanks for the responses.
Let me restart, take those bikes out of the equation.

I do not want to lower my bike. I was just noting that I like the seat.
What I need to know is how to put a seat that is not stock on my bike. Preferably one that has a flat look to it

Oh and as far as the bars go, I am going to put Renthals on there soon.

you cant really put a flat seat on there we explained that to you already put the stock seat on for now and concentrate on what matters that is making it perform better good tires fresh brake pads fork brace and good rear shocks
all those bikes with flat seats look really stupid you just like them because you don't know any better

The first thing to do is to see what you have to work with. Compress the rear suspension like CALfeRacer suggested. You might have a little bit more room to work with, but keep in mind that that 1" extra clearance is the minimum. Hit a really hard bump and the suspension will need pretty much all of it - most being taken up by the rubber bump stops on the shocks which wont be compressed much compressed with a strap. Do not eliminate the fender especially behind the engine. Bikes look great without fenders, but are a disaster to use - though you can slim them down and shorten them quite a bit and still keep a lot of functionality. Remember, the people that design the aesthetics of the bike originally hate how fenders look too, but still have to make something that works.

Once you mock up the functional parts, you can see what you have to work with. You may be able to reduce the cross member height or eliminate it altogether to get some real estate for the seat, but work out the engineering for the fender first. You can decide whether or not you want to add a loop to the frame and whether it will support the seat, the fender, the tail light etc, or not.

You will have to decide for yourself what to build your seat out of. I like fiberglass myself, but lots of folks like steel or aluminum or start with the stock pan if it is steel. I won't dignify using a skateboard as a possibility. The problem you face is getting that straight flat look when you have to get over the hump required for tire clearance. You can consider making a pan to follow the chassis shape and straightening it out on top with the upholstery. You won't be sitting back where the hump will be, so you could make your foam thick enough in front of the hump and super thin at the hump to make a flat line front to back. Probably suck for a passenger, but no more than a typical no padding brat seat.

Building a seat is one of those fabrication jobs that is really a lot more work to do well than it seems and takes a variety of skills. Many old bikes need to have access to underneath the seat, and retaining this capability can add a lot of fiddling - one of the reasons starting with the stock pan is attractive. Take your time thinking through how you will make everything and how everything will need to perform. Mock things up and don't be afraid to spend time being thorough. Don't underestimate the importance of a proper seat height and the foam needed to be comfortable. Those skinny seats look cool, but most people won't want to ride very far on one.

here is an example of a period correct aftermarket cb750 seat, back in the day seat makers knew the wheel went up and down on bumps, nowadays not so much :'( :-\ :-[

Where to cut!

Definitely don't use a skateboard. I did it on my first bike because I thought it looked cool, and didn't know any better. I'm building a new seat now that will allow full wheel travel and be comfortable. If you don't want a passenger the idea of a thick padded seat with a tiny bit of foam and cover to go over the rear crossmember and fender is a solid idea.

and remember brat rhymes with gnat and they have a tiny,tiny brain..... more like a wide spot in the nervous cord than a brain, really

xb33bsa said:

here is an example of a period correct aftermarket cb750 seat, back in the day seat makers knew the wheel went up and down on bumps, nowadays not so much :'( :-\ :-[

Click to expand...

Hey, that seat is pretty nice.