Front Rotors... Does size really matter?

Greetings All!

A question for the sages:

A recently revived ST1100 ( 1994 non ABS ) has begun to display wear on front rotors, wearing away a thin film of rust from it's previous season(s) of rest. The PO tells me he got it from someone who had 'dropped' it, this is obvious from some of the cracks and chips in tupperware up front. What is NOT obvious to me is why the L front rotor would look like this:



Have measured both rotors, thickness is nearly identical and within spec, but diameters are L~=11.63 and R~=12.43. What should the L diameter be? As you can see from image the pads seem to be expecting a rotor with larger radius. Did PO (or PPO) install wrong diameter rotor? How else could the caliper and rotor be so far out of registration? Looking at the pads on L side we can see the pads have worn to conform with existing rotor, this has been an issue for quite some time! No wonder I can't seem to pull off a stoppie... Bike is only using 75% of the surface area available.

Ordering parts to deal with the failing fork seals, so will have front end apart. Will replace springs ( 45K on bike ), bushings and seals so dealing with a new rotor would be a snap at this point, if indeed this is the issue.

Thank you for your time!

ma Che

Yeah I'd say it's the wrong rotor. Is the other one as discolored as that? It looks like the rotor is discolored from excessive heat. The larger the diameter of the rotor (with calipers properly matched) the greater the stopping power due to increased leverage or mechanical advantage.

That smaller rotor loses stopping power because of both it's smaller size and the pads not fully contacting the the rotor. Less surface area as part of the braking process means less braking. How's the other rotor holding up? It's doing most of the braking so wear might be a consideration.

Yes the size matters as in equal brake force ! I have never seen a bike with dual rotors of different sizes. Maybe someone machined the outer lip off when it was worn badly would be my guess !

MIKE

Silver Bullet said:

Yes the size matters as in equal brake force ! I have never seen a bike with dual rotors of different sizes. Maybe someone machined the outer lip off when it was worn badly would be my guess !

MIKE

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That's the first time I've ever seen that too. If the outer lip was machined off that wouldn't explain the missing pad contact area in the smaller radius area of the rotor. It looks like a ST rotor, but of a smaller diameter than the correct one. I don't have my bike or service manual handy here at work, but the 12.43" diameter sounds like the correct one, so this one would be too small.

It's been machined down. In your pic the braking area has two rows of venting holes' probably as a fix from the first owner's spill- a quick check on my own and I have three rows of holes. Bet the right side has three rows. I bet all the brake pads have worked over time

the_gr8t_waldo said:

It's been machined down. In your pic the braking area has two rows of venting holes

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It has 3 rows. Look at the pad wear on the rotor. One set of holes touch at the color change inside and one outside. Then the third is in the pad contact area. A picture of the other side would be interesting. I am assuming the 1100 and 1300 work the same way in that the pads should be contacting the entire disk. This is very evident that it is not.

Looks like you might have a rotor from a different model Honda.
The 1994 Honda CB500 used a pair of 296mm front rotors that look like the ST1100 rotors.
Here's a Wiki link for the CB500.

I would guess it is from an ABS bike. From what I understand, they have the same bolt mount pattern but don't interchange (diameter as seen here). I have been wrong before.

I'm sure Uncle Phil could make a comment or 2 on this. He has done some wheel swapping and has probably dealt with this.

I would suggest picking up another rotor of the correct size off eBay.

Anna'sDad said:

Looks like you might have a rotor from a different model Honda.
The 1994 Honda CB500 used a pair of 296mm front rotors that look like the ST1100 rotors.
Here's a Wiki link for the CB500.

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Nice find, I think that explains where it came from.

For reference here's a picture of a ST1100 non-ABS rotor for size comparison, its 315mm diameter.

Thanks to all! I believe we have found the answer. Occam's Rotor! Have spent time pondering anti dive linkage/travel, caliper, etc All along it was simply the wrong rotor. Can anyone recommend a domestic source for the correct rotor? Here in Columbus I might be able to get one from Iron Pony, currently priced at $225. Since the right rotor is still 4.95mm thick, there seems to be plenty of life left. Would it be bad form to just replace the incorrectly sized rotor or does one always replace both? Thanks again!!

ma che said:

Would it be bad form to just replace the incorrectly sized rotor or does one always replace both?

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I'd replace both at a time... the different wear/grooves will cause different friction coefficients...

it took a lot of looking, but it does appear that there's three sets of holes in the pic- Kevin you were right. I did a lil researching on line and it looks like the 1100 had the same disc for both the ABS and non-ABS wheels, up to the model year '95. for the model years '96 and after, they introduced a different disc for the ABS equipped bikes. I'd be inclinded to have the right dressed down a lil bit to restore the braking surface to flat again. if you get the left from e-bay, both.

One rotor is from an ABSII (the smaller one) and one is from a standard if I was guessing. The bolt patterns are the same and will fit any of the ST1100 wheels (I've test that personally).

No need to replace a good rotor with a new rotor. Just get one more correct rotor.

It was the wrong rotor and I think Uncle Phi is correct. The smaller disc is 296mm the same size as the the ST1100's front discs and it sure looks like the same style carrier.

If the right (STBD side) rotor is within spec don't replace it. If the pads are in good shape so are you. While replacing the bad rotor you might want to see if the calipers need any service and cleaning.

Your stoppies should improve significantly dare I say dramatically. LOL

And the ABSII rotor will not go up far enough into a standard caliper to make much contact with the brake pads. ;-) So when you replace it, you will probably be amazed at the better braking. I'm surprised the bike does not pull very badly to one side when you hit the front binders.

Uncle Phil said:

I'm surprised the bike does not pull very badly to one side when you hit the front binders.

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Some smaller bikes have just a single disk on one side of the front wheel, and nothing on the other side, and it doesn't create any axle turning force during braking.

In my experience, Iron Pony sells a lot of merchandise at standard retail - I've found their prices to be no bargain. Having said that, I've spent well over a thousand $$ there in the last year because their selection is unparalleled. They do have sales, and if one comes up for parts, go for it. For something as simple as a rotor, (you don't need to try it on, the color doesn't matter, and it is identified by a part number - no ambiguity) I would suggest you shop around - google the part number (Honda xxx-xxxxx) and see what comes up.

I am glad they are a bit more than two hours away - it saves me money.

Did you look at eBay?

http://www.ebay.com/itm/98-HONDA-ST1100-ST-1100-ROTOR-FRONT-LEFT-BRAKE-DISC-5858-/371171732929?pt=Motorcycles_Parts_Accessories&hash=item566b8f2dc1&vxp=mtr

http://www.ebay.com/itm/1991-2003-Honda-ST1100-left-front-brake-rotor/161459964942?_trksid=p2047675.c100005.m1851&_trkparms=aid=222007&algo=SIC.MBE&ao=1&asc=27538&meid=e13fe909547a463cb72fafcb282fc8f8&pid=100005&prg=11353&rk=4&rkt=6&sd=141156884204&rt=nc

dwalby said:

Some smaller bikes have just a single disk on one side of the front wheel, and nothing on the other side, and it doesn't create any axle turning force during braking.

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But those bike are designed that way ...