1983 Honda 750 Nighthawk

[chickenStripCharlie]

I have shaved calipers for fit on several occasions, and one thing I am watching for is heat transfer. In my case, i currently have an AWD Chevy Astro van which has a 1/2 ton rear end and brakes made for 16" rims. I wanted 15" rims for better offroad tire options, so I shaved a mm or two off the top. So far so good, but something to watch. I imagine boiling brake fluid is going to make your brake really, really soft or non-existent.

But think of the savings on unsprung weight

Considering that mass is the #1 factor for absorbing heat why do bike rotors continue to be drilled out so much?

In today's post asbestos world, brake pads arent really prone to gas buildup like before. If you look at performance cars, the holes are tiny, and in the really high end cars, the holes are cast into the rotor, not drilled into them to reduce cracking.

So why are the bike rotors still being drilled and drilled so much! Most rotors are all holes. Even my ATV where the rotors arent even visible! Is it for looks?

Sure, the holes increase surface area, but I doubt that the increase in surface area provide a cooling effect that offsets the heat absorption loss from the lower mass after it's been drilled out

Am I out to lunch or is it all looks?

 

[sav0r]

Bikes don’t generate that much braking force so they don’t generate that much braking heat. What heat they do make they rely on transferring to the hub and wheel, where it is then dissipated.

Cars have way less braking cooling due to their shrouded rotors, they are generally much high mass, and generally brake at much harder rates. All of that means they have way high brake, upright, wheel assembly temperatures after heavy use. To cool them they rely on pumping air through the rotors, or forcing it through via ducts, and in many cases a combination of both.

If you look into this I think you’ll find stuff under ”thermal conductivity” and “thermal transient analysis”, but it’s been a long time since I paid this stuff much attention.

 

[chickenStripCharlie]

Bikes don’t generate that much braking force so they don’t generate that much braking heat. What heat they do make they rely on transferring to the hub and wheel, where it is then dissipated.

Cars have way less braking cooling due to their shrouded rotors, they are generally much high mass, and generally brake at much harder rates. All of that means they have way high brake, upright, wheel assembly temperatures after heavy use. To cool them they rely on pumping air through the rotors, or forcing it through via ducts, and in many cases a combination of both.

If you look into this I think you’ll find stuff under ”thermal conductivity” and “thermal transient analysis”, but it’s been a long time since I paid this stuff much attention.

Yeah, interesting point! The bike rotors are quite a bit more exposed than the concealed car rotors.

The part about transferring the heat to the hub and wheel doesnt sound healthy for the wheel bearings at all. Lol.

It's ok. It was more of a theoretical thought than any concern for me. My bike is not ever going to be ridden as hard as the components on it are built for. I'm not a spirited rider.

(Yes, that means the USD fork is for looks!)

 

[chickenStripCharlie]

A lady was selling some carpet underlay for $20 so I picked up the roll ..... um, it turned out to be a little bit bigger than I thought.

I had to ask her for a knife to cut it in half so I can fit it into my car. Embarassing. Shouldve just taken my truck to pick it up. Lol

Its 3/8" or 10mm or something like that. Never worked with this before. But for $20 I'll give it a go.

 

[Maritime]

wow, you can go into the seat padding business

 

[Jimbonaut]

You could pad your entire garage with that. Walls, ceiling, floor - the lot. Your very own asylum, with tools.

 

[NoRiders]

Excellent, rattlecan/spray glue layers to get the thickness/form right, then shave it with an electric carving knife and finally smooth it using an electric sander with 60grit paper. You could skin it with 3mm yoga mat as it's closed cell and waterproof whereas I think the underlay won't be.

 

[chickenStripCharlie]

The adapters have arrived .... long time coming from when the forks first went on.
It took me a long time to figure everything out including to find a machinist (thanks @sav0r ) mostly because i don't know how to put what i want into a suitable CAD file. My local machinist could do this on a manual lathe, no problem, but he was a bit too expensive for this job.

Much more complicated design than just sticking with the GSXR wheel, but i like the 19/17 stance, and besides, at 6'5" i already make most bikes look small. Add a small front wheel and it just somehow looks much worse!

Don't tell me the reverse comstar with 320mm rotors doesn't look absolutely badass!

 

[chickenStripCharlie]

 

[chickenStripCharlie]

As far as i could find online, this is the first reported case of GSXR front, reverse comstars and conventional mount calipers. (To anyone reading this in the future, stick with the newer forks with radial mount calipers. They are easy to space out with just some round spacers that you can even buy on eBay. These require more work!)

inside set of holes matches the comstar hub.
outsdie counter sunk are the GSXR rotor bolts.
The third set of half-holes is just for a bit of weight reduction.

The adapters should work well. I seem to have measured the thickness well, as the rotors sit centered in the calipers and wheel spins freely.

I did, however, discover that one of the rotors is warped a little bit. This will likely have to be swapped out. F***n eBay. "guaranteed straight" ...... guaranteed straight my azz!
I should have known better than to buy rotors that some dildo had spray painted yellow .... That should've been my first clue.

I'll deal with that once the bike is ready to hit the road.

The way the surface was milled was a little weird. I did have them put on a lathe and resurfaced the mounting surfaces again just to make sure everything was square

 

[chickenStripCharlie]

Got the seat foam glued up. I dont have access to an electric knife and the second hand stores around me are only good for used clothes it seems.

Gonna be a hell of a mess but I'm going to try it with a coarse flapdisc on the grinder sometime this week or weekend.

 

[cbrianroll]

Flappers arent too bad, but kinda grabby...I may have bad technique...

 

[Jimbonaut]

Shaping up nicely Charlie. If you can get hold of an electric carving knife I think that's your best bet (from what I've heard)

 

[pidjones]

Ebay is your friend there. Otherwise, they can be bought new pretty cheap.

 

[Maritime]

Check walmart. I got one for like 19.99 also an analog serraded bread knife will work as well. Just need to be careful. Shaping up well.

 

[sav0r]

A belt sander can be surprisingly good on seat foam, so I bet that flapper wheel will work great too.

 

[chickenStripCharlie]

So i'm calling this swap done now that everything on the front end is buttoned up and brake lines are on ...... For anyone researching in the future, i've included my diagram of the profile of the spacer i used.
The rotors need to be spaced out 27mm away from the Honda Comstar hub to line up properly in the calipers. This recerse comstar wheel is from a 1982 CB750c and the hub width is 80mm.

So 80 + 27 +27 = 134mm. That's the rotor mounting surface to rotor mounting surface distance.

The rest of the dimensions are not hard to come by on your own by measuring the wheel and rotor themselves.

The Honda wheel has a PCD of 78mm
The GSXR rotors of this year have a PCD of 91mm

This page has good info on various GSXR fork/rotor/hub dimensions:
http://twfracing.com/vbforums/forum.../1645-what-rotors-fit-k6-gsxr-600-fornt-wheel

I will try and change the title of this thread to make it easier to search for in the future.

 

[CarbsAndCylinders]

That looks pretty sharp.

 

[chickenStripCharlie]

so i made a big mistake, which i did not catch over the course of several weeks ..... *shakes head*

I measured seat/tank clearance while tank was sitting on the frame at the back. I forgot to place the rubber mount UNDER the tank! d'oh!!!!!!!!!!

Now i need to figure out a vibration-free mount that doesn't raise the back of the tank much.
Thinking some thinner rubber between frame & tank, and a low profile bolt on top and ditch the stock rubber mount, as much as i'd like to use that.

 

[chickenStripCharlie]

Update: bike has finally hit the road. The forks work awesome!!

I think too many people are worried that the shorter forks and steeper angle will make the bike turn in "too quick" with a shorter trail.

The reality is that the smaller offset increases the trail more than enough. Having kept the 19" it increases it even more and I find the bike really nice and stable at highway speed.

It seems to turn in corners just fine for me. Zero regrets.

One issue: tank clearance. The smaller offset and wider forks does bring the tubes much closer to the tank and the steering stops need to be extended to a point where tight turns are certainly limited.