GS 750ES fork oil, tires, etc...

[Guest]

Just a few related (mostly) questions.....

I'm getting ready to do my fork seals on the 750 Kat (pop up) and am wondering about fluid level. How far down from the top? Basically the same as an 83 GS 750 E/ES, etc... I have a new quart of Bel-Ray 15 wt and new seals, I'm itchin to go!

Brakes: Why are many smaller pistons better than single big uns? The only thing I can think is that they distribute pressure more evenly across the pad. Fluid heating is probably less on the single pots, as there is less surface area to transmit heat. CC displacement is probably about the same. Multiple smaller pots must be better because that's all new modern bikes come with these days....

Tires: Anyone had any luck with Kenda? I saw some in the Chapparral (and Dennis Kirk) catalog for ridiculously low prices, H rated for 130 MPH, and load rating C. I've seen Kenda tires on bicycles for many years and they appear to be of good quality. Yeah, I know, bicycles and motorcycles are vastly different animals. What, by the way, is load range rating and where can I get more info?

Thanks so much for the help..

[Mark whiz]

I run my '85 GS700 (same forks) fork oil 6" from the top of the fork with the fork compressed. I believe the factory setting is just a tad different, but I do this based on an old magazine article I have saved and it has served me well.

You answered your own question about the brake calipers - you're dead on with your assessment.

Couldn't tell ya about the Kendas, I'm getting ready to go for the Chen sengs myself.

[Griffin]

My old GS550ES (which I sold last year) had Kendas on it when I got it. They were a few years old, so I can't tell you how they were brand new, but they had plenty of tread left. The front wheel was really squirrelly over rain grooves in pavement. The rear slid all over the place while flat out corner charging. I tired differing the inflation pressures, but the bike didn't ever handle better.

I replaced them with Avon Super Venoms, and the difference in handling (and braking) was pretty remarkable.

[Steve]

Brakes: Why are many smaller pistons better than single big uns? The only thing I can think is that they distribute pressure more evenly across the pad.

Not only do they distribute pressure more evenly, they do it on an effectively larger diameter of the rotor, so braking leverage is better.

Assume for a moment that you have 12" diameter disks and 2" diameter brake pistons. The center of the piston is 1" in from the edge of the disk, reducing the effective diameter to 10". Replace the 2" piston with two 1" pistons. Since their centers are only 1/2" in from the edge, they are now squeezing on an effective 11" diameter. Squeezing farther away from the axle is better leverage, so you don't have to squeeze as hard. If you don't have to squeeze as hard, you don't build up as much heat. If you don't build up as much heat, brakes last longer.

Now go back and replace all those numbers with what you really have, but you get the idea. 8-[

.

[Smokinapankake]

Thanks for the replies. Mark, is that with or without the springs in?

Steve, I understand the illustration with larger rotors vs. smaller. Thanks for making it a little more clear for me. Assuming the multiple pots allow for a larger diameter rotor, which makes sense as the caliper body can be smaller and more linear, conforming closer to the diameter of the wheel.

I have Metz Laser/Metronic now, just looking at replacements in the future. I doubt I'll go with the Kenda as I can get some BT 45's in my size at a reasonable price. Also looked at the Cheng Shins, I've heard some good reports about them.

Any info re: Load Range Ratings?

[Griffin]

Would it be helpful if I scanned my GS700ES Owner's Manual and emailed it to you?

[p_s]

Assume for a moment that you have 12" diameter disks and 2" diameter brake pistons. The center of the piston is 1" in from the edge of the disk, reducing the effective diameter to 10". Replace the 2" piston with two 1" pistons. Since their centers are only 1/2" in from the edge, they are now squeezing on an effective 11" diameter. Squeezing farther away from the axle is better leverage, so you don't have to squeeze as hard. If you don't have to squeeze as hard, you don't build up as much heat. If you don't build up as much heat, brakes last longer.

I don't think there is a heat difference--all the heat comes because you are turning kinetic energy into heat using friction. Plus sportsbike rotors are getting smaller, not bigger, as time goes by. Less rotational mass and easier turning. I think multiple pistons arose to keep the diameter of the rotors down and possibly to spread the force on the pads in two places which might keep the pads from twisting.

I haven't really read up on this, and I could be completely off-base, but that is my take.

[Smokinapankake]

Griffin,

Thanks so much for the offer, but I have a service manual on order from the UK, specific to the Katana. It just hasn't arrived yet.

PS,

I disagree with your theory of rotors getting smaller. I haven't measured, but look at a rotor on a Hayabusa with a 17" front wheel and the rotor is almost as large as the wheel. They keep the weight down by a combination of things: Aluminum carriers and drilled holes. The drilled holes also aid in water shedding and heat dissipation. I, too, am no brake expert but that's how I see it.

[p_s]

I disagree with your theory of rotors getting smaller. I haven't measured, but look at a rotor on a Hayabusa with a 17" front wheel and the rotor is almost as large as the wheel.

The Hayabusa isn't a proper sportsbike. It is long and heavy and has slow steering so it can be stable at 200 mph. Just so we're on the same page, if two rotors weigh the same but one has a bigger diameter, it has a greater moment of intertia, which the motorcycle press calls "rotational mass", and it makes turning harder and acceleration slower. And in addition to that, if it has a bigger diameter it will have to be heavier, because it will have more area. But by the same token it should have better heat dissipation and more capacity to absorb heat.

I was reading about radial brakes earlier and they were talking about the rotors getting smaller as time goes by--if I remember it was because the calipers could take the greater force at a smaller distance from the axle without using expensive materials. Unfortunately I can't find any proper numbers online, though. Anyways, enough armchair physics--it's easy to miss one important detail and draw the wrong conclusion entirely.

[Mark whiz]

smokinapankake,

That oil level is measured with the forks compressed and the springs out. If you compress the forks with the springs still in, they will stick out of the top of the fork tube by about a foot - very hard the measure the oil level that way.

I've run the Metzlers several times on my bikes and have always liked them - they're just a tad pricey.

I really don't think they worry too much about load ratings on bike tires, since you're not going to get too much more on them than 2 people anyway. Speed rating is the big thing for them - like we're going to be getting these models up over 130mph in stock form. :shock: Suzuki has recommendations for air pressures based on 1 or 2 riders and following that is probably your biggest worry.

[Smokinapankake]

Thanks Mark, I never even thought of the springs poking out the top. My bad.

PS, You're probably right, I might have missed something along the way. Just trying to clear it up in my mind...