CT70 12" rims vs. larger 10" tires

Discussion in 'Modifications' started by MobileAZN, Nov 3, 2018.

  1. MobileAZN

    MobileAZN New Member

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    I need new rims for my build regardless of how I go, so I'm wondering what are the pros and cons of going with 12" rims vs. going with 10" rims again with larger tires.

    I currently have the stock swingarm, and have thought about a new one, however there is nothing wrong with my current swingarm. Is this something else I should also consider? I'm pretty settled on putting a 150cc motor into my build. Will a longer swingarm help anything other than tire selection?
     
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  3. racerx

    racerx Administrator
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    Overall tire diameter is limited by the amount of cubic real estate available. Up front, you'll eventually run out of fender clearance. Out back, you'll lose chain adjustment range. 4.00-10 Trailwings are ~18.5" tall, the most that can be stuffed into a stock bike is ~20". Going with an extended length swingarm might restore the lost adjustment range but create other issues. +2CM is enough to necessitate repositioning the fender, to prevent tire contact. Beyond that length, aesthetics become an issue...the rear wheel will look more like a foreign object, being dragged behind the bike imo. Going from a ~56" tire circumference to ~60-61" also affects gearing. Sprocket combos can be changed. However, if you run a mechanical speedometer, you'll get input error with both indicated speed and odometer readings; there's no way to compensate for that...other than a programmable electronic speedometer.

    The biggest difference between a 10" tire and a 12" is going to be sidewall height. The overall trend, with cars & bikes, has been toward shorter sidewalls. Consider that with tires this short, the sidewalls do a lot of work...protecting the rims, not to mention the rest of the chassis, from road impacts and cushioning the ride quality. They are part of the suspension, Giving up 1" of sidewall doesn't sound like much. Proportionally, that's 25% less air cushion. In terms of performance, negligible on these bikes; it's more about aesthetics. If your local roads are well-maintained and you like the look, you should be able to get by with the shorter tires. Here, in the upper Midwest, I'd be thinking kidney belt and a good chance of a damaged rim.

    If your plans include 70mph power, then I'd strongly suggest suspension & brake upgrades to balance the mechanical stresses that accompany higher speeds and increase more rapidly with each additional mph. Speed is fun...when it can be controlled.
     
  4. MobileAZN

    MobileAZN New Member

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    Thank for that info. The entire thing is getting an upgrade, as all I got with the"bike" is the frame and attached swingarm. I got a front and rear hub, but no brake clusters (basically half of what makes a hub) and bent forks from different bikes. So, the only truly salvageable parts are the frame, which needs a ton of work, and the swingarm. For as much as I have to do to the hubs, I can buy new ones for a little less than new bearings and seals.

    So the plan is to fix up the frame. I'd love to keep the stock swingarm, as she's in great shape, but if I need to swap it for the tires or rims, then I will since so much is shot anyway. I don't mind the 10" rims, as there are some sweet looking tires out there for them that ultimately look the same as the 12" rims with thinner tires. The 12" rims just looked a little nicer to me, giving more "air" and space in the wheel area vs looking so filled in and solid.

    I've found plenty of threads on max tire sizes for each, so won't rehash that. I've just seen some places talk about stability of a longer swingarm at higher speeds, but nothing definitive. I don't plan on this being a speed demon, but more of a daily commuter. I decided on the 150cc so I could have a little more get up and go off the line to keep up with traffic acceleration. If they are stable at normal city speeds (15-50mph) then that's good for me. I can always upgrade.

    Thanks!
     
  5. racerx

    racerx Administrator
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    That's a great basis for a custom build, imho...taking a seriously incomplete bike that was left for dead and transforming it into something great. Much as I appreciate purist-correct restorations and "only original once" survivors, not every bike is going to end up as one of these. It's impractical and would get boring. The contrast keeps things interesting. And, more importantly, you're putting another bike back "on the road", literally in this case, keeping out of the scrap metal supply.

    You didn't say what model/year you have. All things the same, I prefer K1-later. The K0 "pogo stick" front end is kinda lame for road use. And, the K0 swingarm is more flexible than the K1-later, which has fully-boxed construction. No worries, if you have a K0, the frame and swingarm are okay, just check the swingarm for straightness and the frame for stress cracking along the "triangle" above the upper motor mounts. If you find hairline cracks, weld them before painting and you're on your way. That front end, however, doesn't have much in the way of shock absorber action.

    The stock drum brakes, in tip-top condition, are effective up to about 55mph. I prefer an hydraulic disc brake for the front, lots more stopping power...potentially too much. That said, I ran drum brakes on a 60mph bike and they worked well enough, just required a little more/different rider technique. Engine-braking down to 50-55mph, before grabbing a handful of front brake is key.

    Pretty much every other front end will perform better than stock K0, even the cheapie Chinese inverted forks...which include a disc brake. You definitely want upgraded rear shocks. The originals were quality parts, but too weakly sprung.

    Your take on horsepower, I agree with. That displacement should work very well, as you expect. The one annoying thing may be a tendency toward being slightly "wheelie happy" on the 1-2 upshift, under full throttle. Again, a little bit of rider technique can go a long way. This is where an extended swingarm might help. I've remained on the Honda + JDM side of the fence. That limits displacement, mostly below 125cc and chassis parts to G`Craft, Takegawa, Kitaco and Over Racing, none of which is cheap. Were all things the same, I'd be running 150cc+ in at least one bike. In terms of dollars per horsepower, Chinese engines top the list. I also understand your take on the aesthetics of the taller wheels/shorter sidewalls combo. It has a big impact on the visual "weight" and "balance". If you happen to prefer that, remember...this is your bike, you're the one paying for stuff and you're the one who will live with the results of the choices. So build it as you see fit. The worst that might happen is that you decide that you don't like something, encounter parts failures, or damage, in which case you'd either replace or change something(s). Custom machinery is never truly finished, the intervals between changes/upgrades just grow longer. That's a part of the hobby, too, an important one.

    FYI, the true dividing line, IMO, for these bikes is right around 60mph. Above that speed, things change dramatically and logarithmically. A reasonably well-balanced CT70 can cruise along happily at a "natural" speed in the 45-55mph range, which is very practical anyplace but on a limited-access freeway.
     
  6. MobileAZN

    MobileAZN New Member

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    Yeah, that's one reason I picked this up. I've built other bikes, and currently ride my '80 CX500 (my avatar is mid-engine rebuild). I grabbed this 1977 CT70 for $60. She's super rough. I've already cut out the rear rust and am fabbing a new rear side for it. Once that's all welded in, going for the Jungle Grey/Casper clear powder coat (think Nardo grey) and Chartreuse with some black.

    Going concourse or resto wasn't ever on the list for this, as there was too much out there to do and it's too fargone for a resto to be practical for me. If I was gonna spend the money, I'll be making her custom.

    Inverted fork set is definitely going on the front. I'd say the only decent part I have from the triple is the bottom section of the triple and stem. I don't have a top part. I've never liked the original shocks, so those are for sure getting an upgrade. I like the remote reservoir ones I've used on other builds. No wiring, switches, levers. I did get one bent handlebar. Basically, the ONLY truly good part that I could use without fixing or mods is that rear swingarm HAHAHAHA.

    Here are some photos of what I got. This is it other than what I mentioned above, bent footpeg mounting bracket, and a cracked headlight bucket. But hey, a frame for $60, which I did the paperwork, payed my $27 and got a title, too. :)


    download_20180620_150425.jpg IMG_20180623_114651.jpg IMG_20180904_172044.jpg image-20180920_155627.jpg IMG_20180907_131259.jpg IMG_20181011_185139.jpg IMG_20181017_172746.jpg

    Thanks for the input! It's all gonna help as I collect all the parts for this build. I'm hoping this build gets done by spring, but as with most projects, money flow will be the deciding factor on speed of build.
     
  7. racerx

    racerx Administrator
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    That's some serious metal fab. Given the as-received condition of this frame, I'd have done the same thing. No doubt, this one's not purist-resto material. That interior heat shield isn't critical thin gauge metal was used originally. That little section of wheel arch, however, is structural. Since it's impractical trying to replicate the rolled beading, might be a good idea to use somewhat thicker sheetmetal instead. I'm talking about ~0.090"-ish. If your welding and metalfinishing skills are on par with what I think I'm seeing here, the lower wheel arch should be very clean once it's done. If you're going full custom, may as well enjoy all of the advantages that come with it. There are a LOT of spotwelds, stamping artifacts and the vintage stick welding wasn't exactly art. Smoothing all of these results in a very clean finished frame that takes on a subtle, sculptural, appearance. Ask me how I know this:whistle:

    Bent footrest crossbars respond favorably to "the blue wrench". Kinda curious as to how you could have gotten a cracked HL shell; the `77 model had a steel HL shell and seperate, round, speedometer. Sounds like a P.O. threw a K0 HL assembly, or part of one, at this abused/neglected machine.
     
  8. MobileAZN

    MobileAZN New Member

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    Yeah, I'm using a slightly thicker steel in this area, and I'm backing it with thick bar cross bracing. That's the plan anyway. Yeah, the headlight shell looks like it took a serious hit, and he probably threw it in knowing I probably wasn't going to use it anyway. Chances are that's how I got most of what's in here. He has 3 others, and I'm working to get at least one more (proving difficult). The other one he will probably part with is in the same condition. He will probably keep the 2 better ones (more complete).
     

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