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WheelsAmpTires

Sparrow >> Service >> Mechanical >> Wheels and Tires

Intro

Over their production run, Sparrows were equipped with two types of wheels and tires. The early models were equipped with "Skinny" wheels, while the later models were equipped with the smaller diameter 13-inch rims.

Late model 13-inch wheels

Due to center of gravity concerns, and the availability of replacement parts, Sparrows were later built with three matching 13-inch wheels and a modified suspension system and swing arm. The original wheels were made by Primax, size 13x5.5JJ with 4x100 bolt pattern and 38mm offset. Original tires were 155R13, which is a size designation called European Metric. Although you can change the wheels and tires on the Sparrow, wheel and tire replacements are limited by width since it's a tight fit into the fender and front wheel panels.

As of December 2017, the 155R13 tires are no longer readily available; however, a similar size (155/80R13) is a suitable replacement. According to www.moderntiredealer.com (accessed Dec 7, 2017),

Another online resource www.sumitomotire.com (accessed Dec 7, 2017), confirmed this assertion regarding the aspect ratio stating:

The 155/80R13 tires have an aspect ratio of 80, which makes them about 3.1mm smaller radius than the original tires. The overall height would be 6.2mm (approx 1/4") shorter than the original 155R13 tires. Original 155R13 tire manufacturers included:

Photo of the exterior of the Primax rim:

Photo of the interior of the Primax rim:

Early model 16-inch "Skinny" Wheels

The original tire size (of those weird tandem tires CM used) was 125/85-16. My rear (original tandem) tire looked like this:

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Long story short. I looked for a replacement tire everywhere. The only tires I found that would fit the 16” rim were:

So I began to think about smaller rims with taller tires. I learned more about rims than I ever wanted to know. If someone else is looking for an alternative to the original Sparrow tall tire, I hope my experiences can shorten their “learning curve.”

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.

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, which turned out to be 117/109.

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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.

The OD of the original tire only has about an inch in clearance in front of it, so I really don't want a larger OD.

I measured that the widest tire that would fit the wheel well would be about 6.” The rim would have to be about 4½" to 5½” wide.

The problem is, modern tires are much wider than older vehicle styles. The smallest width commonly available (in regular old chain tire stores) in 15” is about 155/80, which is about 6.1” wide...a little bit over the (admittedly somewhat arbitrary) 6" limit. Plus, when you try to fit a wide tire up into the wheel well and onto the hub, fitment clearance becomes an issue...you have to be able to tilt it enough to get it past the hub and brake calipers. The comparison looks like this:

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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. I ran the numbers on those tires, and they didn't look bad:

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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 ½“ thinner than the 145, 1½“ smaller in diameter than the Sparrow, and has a load rating of 805 lbs. and a speed rating of 118mph.

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They are both designed for 4-5½” wide rims. I also found some even narrower 125R15 car tires on EBay. That's about the smallest you can use on 5" rims...the tires themselves are 5" wide.

Now, where to find 15x5 rims? Not easy.

I found a website called http://www.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 the correct (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.

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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?

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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.

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So I got a pair with a 56.1mm bore, and literally just tapped them on over the nut with a hammer. Little bit of a press fit. They are just soft aluminum, after all.

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I put the wheel on, and it fits well. The original Sparrow lug nuts only went on about 5 turns…hmmm…enough? 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. More on that in a minute.

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.

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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 5½“ 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 buy 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 ½” 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.

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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.

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In hindsight, I might have gone with the 155/80R15. About ¼” 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, and certainly better than the bare black steel Toyota rim. Especially since 80+% of the wheel is covered by the skirt, anyway.

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Getting to the steering rack

1. Remove steering column.... a big job.

2. Remove tie rod ends at ball joints

3. unscrew the joints. mark thread count for re-install

4. unbolt the rack

5. wiggly it out...it was wiggle, wiggle, fuss , fuss, pop,, how did I do that?

6. service and re-install.