Ask the Geek: Fork Oil Height and Stone-Age Tires

Your Abnormal Guide To Things Abnormal

Fork Oil Height And Scooter Brakes
If I do not want to increase front preload, but do want to reduce the chance of bottoming the fork, I can increase the level of fork oil. Is there a safe limit of air gap at the top of a fork with springs in and at full compression? I have a Honda with linked brakes. It's getting time to replace all the brake lines with stainless steel hoses. The front caliper is a Nissin 3-piston sliding unit, with the center piston actuated in tandem with the rear single-piston caliper (it's a NSS250 Reflex). If I would run two lines from the front brake master to the normal front connection and the center piston connection, all front braking would come from one master cylinder. I would then run a single line to the rear. The reason for doing this would be to increase front braking, simplify the plumbing, reduce the cost of converting to braided steel, make the scooter more consistent in braking operation with the two other motorcycles I ride (none with linked brakes). My concerns are if the front brake cylinder will require much more travel to feed three pistons instead of two, or if the braided lines will offset this somewhat. The rear brake feels weak, so I don't think running it by itself off the original dual circuit will make it too powerful.
Tom Mix
LaCrosse, WI

Citing a limit for fork oil height and air gap is tricky because there are so many variables involved. The danger of a too-high level is that the resulting air gap inside the fork is too small, in turn raising pressure so high when the fork is compressed that the seal could give out. Generally the level is measured with the spring removed and the fork compressed, because it's impossible to measure with the spring installed and fully compressed (as you'd have to do in order to find out how much oil the spring itself displaces). The service manual for your bike will specify a standard oil level with the fork compressed and spring removed and may list a minimum as well. Start at the standard oil level and use a zip tie on a fork leg to see if your suspension is bottoming. If that's the case, add oil in 10mm increments until the zip tie shows a few mm of travel remaining. If more than 20 or 30mm additional oil doesn't fix things, it's time to consider some stiffer fork springs.

Your Reflex with linked brakes has a lot going on in that department, including the linked setup, a delay valve, parking brake and ABS on some models, with all the associated hoses and connections. If you activate all three pistons in the front caliper as opposed to just two, the increase in piston surface area is about 30 percent (the middle piston is small compared with the outer two pistons). You can try the stock master cylinder, but as you say the travel may increase too much with the extra piston to move. You'd have to enlarge the master cylinder piston area by an equivalent amount to the piston area increase for similar feel. About the only way to determine the size of the stock unit is to look for the diameter stamped on the bottom of it; you'll want a replacement 2 or 3mm larger in diameter for a 30 percent increase in area. You should be able to make these changes on a non-ABS model without any problems, but an ABS model is better left alone unless you're comfortable with disconnecting the entire system as well.

Stone-age tires
I picked up a clean and original 1984 Interceptor 500. Going into it I wasn't expecting much, then I saw the unit and it was complete, original, rust- and damage-free. I did a compression check and everything is on spec with the service manual. No oil burning or anything. I walked away having spent a mere $250 on it! I'm pretty thrilled, I always wanted one when they came out, but being a teen I didn't have the cash to make it happen. I think it will be fun to have now and ride on Sundays and some vintage races, which brings me to my question: The bike still has its original tires which are cracked beyond belief (but that's what 26 years can do). I did some searching for tire availability with its sizes (100/90-16 front and 110/90-18 rear). And all I've been able to find so far is a Bridgestone BT45. Anyone have experience with this tire? Or any other vintage tire applications? I hope to use it on vintage track days, so I'm looking for the tire with the greatest traction, rather than longevity.
Patrick Loewen
Canada

The BT45, while a good tire, is not your only option for an older sportbike with bias ply tires on narrow rims. A good choice for your Interceptor would be the Avon Roadrider, which is available in the correct sizes. Additionally, both front and rear Roadriders are V-rated (149 mph), whereas most other manufacturers offer H-rated (130 mph) tires (or a mix of H- and V-rated) in those sizes. The Avons are also available in slightly wider versions for both front and rear, so you can experiment somewhat. If you try wider tires, check for clearance between the front tire and the fender, and the rear tire and the chain and swingarm. Other tire options for the baby Interceptor (as it was affectionately called back in the day) are the Continental TKV Ultra and Pirelli Sport Demon, which are also available in the correct size.

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