Building a drag bike

[FiReSTaRT]

I do and you're fortunate that the box still hasn't made it into the basement (renovating the house for sale, so all non-essentials are getting packed up and put away). I'll dig them up tomorrow, and seriously, you can borrow my 1/2" drive torque wrench.. I won't be using it until spring, even though things might change next season (the garage @ the new house can fit 3 cars easily and has a proper heating setup)

[NEVER2OLD]

Have you decided on gearing Brian?? The mods you have done are certianly gonna help you get alot deeper into the 10's ....Huge potential there with seat time!!

[Brian P]

I think the sprocket that I ordered is in, I just haven't picked it up yet - hopefully this coming week. I'm going with stock front sprocket and +6 on the rear sprocket - I prefer to do ratio changes with the rear sprocket as much as possible. This should be in the ballpark for now.

With 17/45 gearing and 190/55 tire, it will do 149 mph at 11,700 rpm (peak power RPM) in 5th. With a 190/50 tire it's 144 mph at 11,700 rpm. For a stock bike with absolutely no power adders (haven't even changed the exhaust system) that should be more than enough and there's still another gear and 1000 rpm more before the rev limiter. It will probably be able to take a little more gearing than that, but I think I'll have my hands full enough as it is.

I actually made a little forward progress ... the LED license plate bolts that I ordered two months ago showed up in the mail (after having filed a dispute with Ebay and getting my money back!!) and they're installed and wired. This is a "legality" thing rather than a "performance" thing, but it's still one less thing on the to-do list.

[dricked]

Is it just personal prefrence or is there another reason why you prefer to change the rear and leave the front?

[frekeyguy]

Is it just personal prefrence or is there another reason why you prefer to change the rear and leave the front?

lessens chain wear for one.
for a drag race application, you can finely tweek the rpm that you want to go through the traps at. "With 17/45 gearing and 190/55 tire, it will do 149 mph at 11,700 rpm (peak power RPM) in 5th. With a 190/50 tire it's 144 mph at 11,700 rpm."

[Brian P]

Is it just personal prefrence or is there another reason why you prefer to change the rear and leave the front?

Less chain-pull force for a given amount of acceleration. Less chain-pull means (A) chain less likely to break, (B) less force on the output bearing of the transmission ($$$$$ if you ever break one of those, I've had it happen on my roadrace bike), (C) the chain is less likely to ride on the plastic guides of the swingarm and even if it does touch, it will do so with much less force.

[canadian john]

Looks good Brian, I would have put adjustable lowering links and a airshifter..........just more money.lol

See you at the drags............

[Brian P]

Always more things that can be done at a later date LOL ...

[PurplePeopleEater]

Hey Brian, it's been a long time Phil aka Syscrush on the CBR125 board here. That setup looks pretty rad, looking forward to seeing how it works out for you and how deep you end up immersing yourself in drag racing.

[NEVER2OLD]

Brian is HOOKED.....Its addictive as h e double hockey sticks...lol....I will put money on it that he will be spending alot more before the year is over on other upgrades!!!!!!!!

[Brian P]

We'll see how it goes ...

No pictures, but I got the correct sprocket, installed the chain, and the bike is on its wheels again. I had to customize the slot where the rear brake caliper mounts a little ... welcome to aftermarket stuff. The ride height guess looks like it came out pretty close. If it had a 190/50 rear tire (OEM size) it would probably be bang on going by appearance. With the 190/55 it looks a little high in the tail, but it did before all the suspension fiddling, too.

What it really needs, is a test ride.

It sure looks tough with this setup, though.

Still need to install a front end strap kit, but that's easy stuff that can wait.

[Brian P]

Better lighting ... here is how it stands right now. Really close, and good enough for a test ride (when the weather permits - and I am really interested in seeing how this works)





For now, I corrected the nose-down attitude by adding more front preload. At this point, overall it is only slightly lowered from stock, and that's intentional. I want to keep this streetable, and I know that extreme lowering will make a bike handle like a school bus (and cost cornering clearance and give no clearance underneath for going over bumps - and the first thing that hits the ground is the oil drain plug on these bikes ...). Also, with the ride height as it is right now, with me on it, the swingarm pivot is roughly 3 inches higher than the rear axle. Crunching the numbers (as engineers are apt to do) this still gives decent anti-squat geometry. It isn't as much antisquat as you would want for a roadrace bike, but this model has the swingarm pivot too low for that even in stock form. A good many slammed drag bikes have practically no down-angle on the swingarm at all ... which leads to pro-squat, which leads to people using solid links to replace the shock and/or using brutally stiff spring and damping rates ... not good for a street bike.

Little details: LED license plate bolts, has a DOT-marked red rear reflector (not the stock one - but nevertheless, it's there, and it's DOT marked), plenty of room for suspension travel front and rear, the axle nut is secured with a hair-pin cotter (for quicker tire changes) and the cotter is safety-wired to the axle adjustment bolts (so that it doesn't get lost), stock rear chain guard is trimmed for tire clearance.

The chain that I got came with a clip-type master link, which apparently everyone uses for drag racing, but in my roadracing background, those are not good. The clip is on there facing the correct direction and the clip is sunk into JB Weld and I safety-wired around that link to hold the clip in place before the JB Weld cured, so the safety wire is in the JB Weld, too. I think that's sufficient redundancy.

Wheelbase as shown is 61 inches.

Stock final drive ratio is 17-39 (2.294:1), what's on there is 17-45 (2.647:1) and I've adjusted the Speedohealer to correctly compensate for this and for the 7% error programmed into the stock speedometer.

That this still looks very much like a stock bike, and has all the CMVSS and HTA required lights and reflectors and they all work, and still has a chain guard, and will still sound like a stock bike owing to the stock muffler, and still retains the stock catalytic converter ... is purely intentional.

Obviously this is not a wild enough setup for someone running Pro Stock or whatever, but it should be fun to play with as is ...

[PurplePeopleEater]

Hey, I recognize that rec room. Congrats on having the only house out in the 'burbs that I'm jealous of.

The bike really is looking nice and mean - I like seeing the methodical approach you're taking and look forward to seeing some timeslips and race reports!

BTW - can we see a pic of your overkill safetywire & JB Weld clip link?

[FiReSTaRT]

Hey Brian,
Sorry I couldn't find the bolts.. The guys brought me the wrong tote from Orangeville and I didn't wanna bug them to go through the rest of them until they find the right one. I'm guessing that Murphy will help me find'em about a week before the street test.

[Brian P]

No worries. The ones I had on order showed up in the mail ... more than 2 months after being ordered and a month after Paypal refunded the charge! They're on the bike right now, although not visible in the photos.

Somewhere out there, someone in China is probably moaning the loss of their $8. If he finds me and asks for it, I'll pay it.

[NEVER2OLD]

After reading all your engineering approaches to this project and our fellow racer Mike,I have decided to leave the front sprocket on my zx14 at a stock at 17 and go from 41 on the back to 46....Alot easier on the chain and output shaft...I have a steep learning curve the same as you brian...lol

[Brian P]

Test ride ... Success!

Quickshifter works, speedohealer works, and most importantly ... the steering response seems pretty neutral during normal riding. THAT was one of the big objectives with this. Riding around normally, one would be hard pressed to tell that this is a stretched bike - it all feels normal and works fine. Probably in a back-to-back comparison with a stock one, the steering would feel a little slower and the ride a little smoother (the longer arm gives more leverage to the rear shock) - if anything, the latter is a *good* thing for normal day-to-day riding on the bumpy roads around here.

I am totally good with it at this point. I'm not touching a thing. There's one more to-do item - a front end strap - but that's an easy install and won't affect how the bike works for normal daily riding.

[FiReSTaRT]

S-weet! I hope you took video

[Freestyle72]

Test ride ... Success!

Quickshifter works, speedohealer works, and most importantly ... the steering response seems pretty neutral during normal riding. THAT was one of the big objectives with this. Riding around normally, one would be hard pressed to tell that this is a stretched bike - it all feels normal and works fine. Probably in a back-to-back comparison with a stock one, the steering would feel a little slower and the ride a little smoother (the longer arm gives more leverage to the rear shock) - if anything, the latter is a *good* thing for normal day-to-day riding on the bumpy roads around here.

I am totally good with it at this point. I'm not touching a thing. There's one more to-do item - a front end strap - but that's an easy install and won't affect how the bike works for normal daily riding.

What is a front end strap? And how does it work? And what does it accomplish?

[Brian P]

It's a set of brackets that attach to the front brake calipers, then a strap (like a tie-down strap) goes from one side, up over the triple clamps to the other side. When you go to the drags, you tighten that strap, which holds the front end slightly compressed and prevents the forks from extending when you launch hard off the line. That keeps the center of gravity down, which helps stop the bike from doing a wheelie. For street riding, you just disconnect the strap to allow normal suspension movement. More on this later.