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For those of you that don’t want to read my dissertation below, here is the recipe for putting 15” wheels and tires in place of the original 16’s on the rear:

Nissan Versa 15x5.5 wheels with bolt holes bored to 5/8” available from auto parts store or junkyard, $50 - $80 each. Be careful to get the 15x5.5”, Nissan also put 15x6 wheels on the Versa; They are too wide and the offset is too high. Check Wheel-Size.com before you shop for the wheel.

OR if you want a little narrower wheel, Toyota Prius C 15x5 4x100 wheels with the hub bored out to at least 56.5mm, and bolt holes bored to 5/8”, available from auto parts store or junkyard, $50 - $80 each. Be careful to get the 15”, Toyota also put 16x6.5 wheels on the Prius C. They are too wide and the offset is too high. Check Wheel-Size.com before you shop for the wheel.

If you’re putting them on the front, you may not have to bore out the hub. 125R15, 135R15, 145R15, or 155R15 tires, available online or from VW air-cooled custom shops, about $120 each. OR 155/80R15, about Ľ” wider than the above, more commonly available, and about $100 each. Check the width (mo more than 6.1) and OD (no more than 25”) before you choose.

2) 5mm or (1) 10mm 4x100 wheel spacer(s) with a 56.5mm bore, $60 online or at rice-rocket performance shops. 7/16-20 ET (extended thread) lug nuts, available at almost any local independent tire shop, about $2 each.

DEALING WITH TALL, SKINNY TIRES

If someone is looking for an alternative to the original Sparrow tall tire, I hope my research and experiences can shorten someone’s “learning curve.” The original tire size (of those weird tandem tires CM used) was 125/85 x 16. My rear (original tandem) tire looked like this:

6591

Long story short. I looked for a replacement tire everywhere. The only tires I found that would fit the 16” rim were space-saver spares (very low profile, constructed with few plies to be cheap, and only go up to 50 miles) and motorcycle tires (not a good option on flat-riding (non-leaning) vehicles like ours...expensive, and they wear too quickly) I even looked for motorcycle sidecar tires…nothing. A few classic/antique tires were close, but not close enough for me. So I began to think about smaller rims with taller tires. I learned more about rims than I ever wanted to know. Our bolt pattern is 4x100, (4 lugs on a 100mm bolt circle.) Tire width is about 4.6 inches. Outside tire diameter is about 25”. I never did take the old tire off the rim, but based on the measurements I took, I believe it to be 3” in width. I found a tire calculator at https://www.tacomaworld.com/tirecalc which is tremendously helpful. It calculates all of the above based on just the standard tire size inputs. It’s not 100% accurate; diameters seem to vary a little from actual, and the actual installed tire width depends on the rim width. But it’ll get you in the ballpark. 6611

As it turns out, adjusting size to get the actual Sparrow diameter and width, the tire calculator believes that our tires are not 125/85, but 117/98. So that’s what I used to calculate the new tire diameter and width, on a 15” rim. 6687

To fit the wheel well, the tire can’t be too wide or have too large a diameter. In fact, I have learned in other Sparrow Group posts that a little smaller diameter might be better for CG and stability. I measured that the widest tire that would fit the wheel well would be about 6.” And the rim would have to be about 4 ˝ to 5 ˝” wide. So in the calculator I played with aspect ratios for 15” tires until I came up with 149/77 R 15 for the new tire. The problem is, new tires are much wider than antique ones. The smallest width commonly available in 15” is about 155/80, which is about 6.1” wide. Awfully close to the limit…in fact, just a little bit over it. Plus, when you try to fit the tire/wheel onto the hub, clearance becomes an issue…you have to be able to tilt it enough to get it over the hub and brake calipers. Then I remembered that old air-cooled VW tires were tall and skinny. Sure enough, a site called JBugs has extra skinny tires for guys that want to lower their old beetles. https://www.jbugs.com/category/vw-tires.html They happen to have a 135R15 and a 145R15. For about $120. They also happen to be about 35 miles from me, so I drove up there and bought the 145R15. The (measured mounted) outside diameter is 1” smaller than the Sparrow tire, the load rating is 908 lbs, and the speed rating is 118mph. Mounted width is about 5.9, 1.3” wider than the original Sparrow tire. The 135 is about ˝“ thinner than the 145, 1˝“ smaller in diameter than the Sparrow, and has a load rating of 805 lbs. and a speed rating of 118mph. 6649 6650 They are both designed for 4-5˝” wide rims. Now, where to find those? Not easy. I found a website called wheel-size.com. Long story short, after a LOT of combing through the data, I found that the Toyota Prius C has 15x5 rims with a 4x100 bolt pattern. So I bought one at a junkyard for $50. The center bore of the Prius C wheel is 54mm. I discovered that the hub nut sticking out needs a wheel center bore of 56.5mm. So I had to have a machine shop bore it out to 57mm. 6850

Next problem: offset. That is the distance between the center of the rim, and the surface of the wheel that touches the hub. Super important when you’re installing the center wheel of a 3-wheel vehicle…you want it to be centered, right? 6851 I measured the offset of the original wheel at about 29mm. The Toyota Prius C wheel lists a 39mm offset. Turns out that rice-rocket shops have something called wheel spacers. A company called Sparco makes a 5mm aluminum spacer with a 4x100 bolt pattern and a 56.5mm bore, Part No. 051STB02, and they sell them in pairs, giving me the 10mm of extra offset I need. Great…until I found out that nobody stocks them and they are on an 8-week backorder from Sparco. 6684

So I got a pair with a 56.1mm bore, and literally just tapped them on over the nut with a hammer. They are just soft aluminum, after all. 6732 Put the wheel on, it fits well. The original Sparrow lug nuts only went on about 5 turns…hmmm…enough? Hmmm… I read that thread engagement should be at least the diameter of the stud. The lug nuts are 7/16-20. 7/16 x 20 = 8.75 turns minimum. Not good. I found a really nice set of hubcaps that look very much like the original chrome wheels of the Sparrow (yeah, I know, some people think they’re ugly. Maybe so, but unless I’m changing all three wheels (not yet) then I just want to match the existing one as closely as possible.) They are called baby moons, and they’re all over EBay, cheap. So I tried to put my baby moon on the Toyota wheel and…the center of the wheel sticks out too far. They won’t go on. 6678

6646 I could have and should have been satisfied with no hubcap, but…I’m an engineer. We we fix problems. There’s always a way. It’s all about the math. There’s nothing worse than a semi-retired engineer with some time and money on his hands. So back to the wheel search…the tires will fit a 5˝“ rim. After more searching, I found one: Nissan Versa has a 15x5.5 rim with a 4x100 bolt pattern, and an offset of 40mm. Close enough. So I get another wheel from the junkyard, and put it on. The baby moon fits. And now the Sparrow lug nuts go on…only about three turns. The original Nissan lug nuts only go on about ONE turn. Yikes! So after chewing on it a few days, trying to figure out how to replace the studs, or weld another ˝” onto the ends of the studs and file and thread the weld, I learned that there are such things as extended thread lug nuts. They actually have about 3/8 extra thread that projects into the wheel. The lug nuts are 7/16-20. 3/8 extra thread at 20 TPI is another 7.5 turns, giving me over ten turns. 7/16 x 20 = 9 turns. Plenty. 6922 The OD of the extra thread projection is 5/8,” so I had to bore out the lug holes about 2mm more. Then I chose to use a 120-degree countersink to put a little angled face on the hole (and thus more friction keeping it on.) Works like a charm. I’ve got a good 1” of clearance all around the tire, to the swing arm and to the upper wheel well. 6734 6846 In hindsight, I might have gone with the 155/80R15. About Ľ” wider than the tire I used, about the same diameter as the old tire, a little cheaper (about $100) and more commonly available. But I’m fine with what I got. And I think the hubcap looks good, very close to the original (especially since 80% of the wheel is covered by the skirt, anyway. 6680 6912 6735