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Hi
I'm after some advice on what issues I might come across fitting a 16" rear wheel on 68 bonneville,
it depends on what you want to do with the bike. relacing the original hub to a 16 lowers an all ready low gear ratio. It raises your RPMs, at highway speed. That was the thing to do back in the 70s and 80s, and I have relaced quite a few, and used harley rear rims on others.

I don't think it is a good idea now though.
 

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Aside from the ratio change it's not that big a deal. You have to watch out for tire width.
Too wide can be problematic. Here's mine as I bought it with 16" Harley
the chainguard had to be removed to fit that and I think it rubbed the chain
 

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relacing the original hub to a 16 lowers an all ready low gear ratio. It raises your RPMs, at highway speed.
This doesn't add up.A 4.10 tyre has a 3.5" profile.
18" rim +3.5"+3.5"= 25" diameter for standard rim/tyre.

The 16" rim would need a tyre profile 4.5" to have the same tyre outside diameter.For a tyre 5" wide,that's a 90% profile.

At 100% profile,the 16" rim +5" tyre would be 1" bigger diameter than standard.That would give you 104 mph compared to 100 mph standard,at the same rpm.It's still less than a 1 tooth change at the gearbox sprocket on a 650.

I do prefer using an 18" sportster rim with 4.25" tyre,rather than a 16" rim with grossly wide tyre.None of this is done for performance;it's either for economic reasons or appearance.

*You get less traction with a wide tyre made of the same rubber compound,because the tyre runs colder.
*You have more unsprung weight for the suspension to deal with,so you lose some roadholding.
*There is more flywheel inertia effect with a heavier wheel,so it won't accelerate/stop as easily.You need more power and more brakes to get back where you started from.

You will get more tyre life from a wide tyre,at the expense of performance.

To have the best combination of steering and wheel alignment,the sprocket centreline is 2-15/16" from the rim centreline.With a chain width of 0.820" at the pins,to have 1/16" clearance between chain and tyre the max tyre width is 4.95".
At 1/16" clearance and a perfectly straight rim lacing,the chain is still likely to touch the tyre occasionally.
 

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This doesn't add up.A 4.10 tyre has a 3.5" profile.
18" rim +3.5"+3.5"= 25" diameter for standard rim/tyre.

The 16" rim would need a tyre profile 4.5" to have the same tyre outside diameter.For a tyre 5" wide,that's a 90% profile.

At 100% profile,the 16" rim +5" tyre would be 1" bigger diameter than standard.That would give you 104 mph compared to 100 mph standard,at the same rpm.It's still less than a 1 tooth change at the gearbox sprocket on a 650.

I do prefer using an 18" sportster rim with 4.25" tyre,rather than a 16" rim with grossly wide tyre.None of this is done for performance;it's either for economic reasons or appearance.

*You get less traction with a wide tyre made of the same rubber compound,because the tyre runs colder.
*You have more unsprung weight for the suspension to deal with,so you lose some roadholding.
*There is more flywheel inertia effect with a heavier wheel,so it won't accelerate/stop as easily.You need more power and more brakes to get back where you started from.

You will get more tyre life from a wide tyre,at the expense of performance.

To have the best combination of steering and wheel alignment,the sprocket centreline is 2-15/16" from the rim centreline.With a chain width of 0.820" at the pins,to have 1/16" clearance between chain and tyre the max tyre width is 4.95".
At 1/16" clearance and a perfectly straight rim lacing,the chain is still likely to touch the tyre occasionally.
did you factor in the sportster sprocket size? Smallest chain drive harley sprocket is a 48 tooth (highest ratio). 51 is stock for a sportster, and 49 is common on some big twin models BUT if you are using the older hubs big twin and sporty sprockets are not interchangeable. The number of bolts is different That is a gear ration drop from a 68 trump. it is possible to hog out the holes in a sporty hub and force fit a triumph brake drum and sprocket, IF you are a complete fool. If not, you will realize that this is a recipe for an off center and out of balance hub...48 tooth sprocket is disc brake only, so I am reasonably sure that is not what will have on your bike, unless you are planning serious mods.

plus the older high profile 16 inch tires are becoming obsolete, or sold for novelty bikes (rat rods) so they are more expensive. Standard 16s now have an 80% aspect ratio, so to get the taller sidewall you are looking for, you have to go wider...and that is not an option on a stock trump. The old 410x16 tire size is now a front tire profile...


edit... Shrugger..just a little piece of advise... the fiberglass sportster rear fender you are using is not supposed to be drilled for a Pillion pad. Especially not one that requires 4 bolts. You may not want to carry a passenger. IF you do I would keep an eye on that. They were not really well made , and age has probably not improved the structural integrity
 

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did you factor in the sportster sprocket size? Smallest chain drive harley sprocket is a 48 tooth (highest ratio). 51 is stock for a sportster, and 49 is common on some big twin models BUT if you are using the older hubs big twin and sporty sprockets are not interchangeable.

...48 tooth sprocket is disc brake only, so I am reasonably sure that is not what will have on your bike, unless you are planning serious mods.

Standard 16s now have an 80% aspect ratio, so to get the taller sidewall you are looking for, you have to go wider...and that is not an option on a stock trump. The old 410x16 tire size is now a front tire profile...
I've seen plenty of 16" rear rims on Triumphs.They were always laced to a Triumph hub.No-one was using a Harley hub and sprocket.

Most used a tyre somewhere close to 5" width.At 80% profile the outside diameter of the tyre would then be 1" less than a factory-fitted 18 X 4.10 tyre.
In that case,the 4% effective gearing reduction would require one extra tooth at the gearbox sprocket to bring speed/rpm back to normal.
 

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I do prefer using an 18" sportster rim with 4.25" tyre,rather than a 16" rim with grossly wide tyre.
I've seen plenty of 16" rear rims on Triumphs.They were always laced to a Triumph hub.No-one was using a Harley hub and sprocket.

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Yeah..OK... SURE

Ok, so you prefer a sportster rim, which is an 18 inch rims, because you would gain so much over the 18 inch triumph rim, because???

and you cannot dictate the sidewall height of a tire just from the aspect ratio, without establishing width. ...Sure, you can claim whatever you want, but you cannot fit a 160 into a bonny. 130 is more reasonable, and that is a front tire profile.

and I do not know where you are from that people only relace triumph hubs to harley doughnuts, but I have seen countless triumphs with harley rear wheels. I have even built them that way. That used to be standard practice because the brakes were better on the harleys then the mechanical brakes on the brit bikes (especially if you use the hydraulic brakes) . With just a few moments on a lathe, you could make the honda disc brake front end fit the old triumphs as well. ..just like this one, that I built 20+ years ago


Once I even turned a harley servicart rear end upsidedown, so it could be driven from the left side, and built my own frame to run it with a trumpet engine... that one had 13 inch jensen healey mag wheels on it. There is no standard for customizing, unless you lack ability or imagination
 

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The Sportster 18" rim is only preferable to a 16" rim,not necessarily preferable to a standard rim and certainly not an 18" aluminium rim.
For those who want a "fatter" appearance on the rear wheel without so many chain clearance,effective gearing and road-holding issues,the Sportster rim on Triumph hub is a sensible alternative.

I reamed out the lower yokes on a '70 Triumph to fit CB 750 stanchions and disc,and turned a taper on the stanchions to suit the top yokes.Custom disc carrier and it all fitted.BSA 18" rim and the standard Honda spokes from the 19" rim,crossed over more instead of shortened (stronger spoke pattern).Handling didn't feel right until I shortened it an inch,and pre-unit internal springs gave an ideal spring rate.
That was back about '74,and I can't say I've found a better front end.
 

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I ran a cross 4 spoke pattern, exclusively, from the early 90s until about 5 years ago. Now I have an 80 spoke tubeless rim on the front of my daily rider, and a forged aluminum rear wheel.
 

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That's right the Harley wheel is laced to a Triumph spool hub.
I've since removed all that crap, and many many dollars later
it's beginning to look like a Bonnie again with WM3 18"
 

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Great thread and I will be looking at these same issues as I build my Classic Bobber but I am using a 70's generic rear hard tail section which has plenty of room for my needs. Here is a pic of the rim/hub combo I will be using although I might change the tire to a more rounded and maybe different profile. This frame you see it in will not be used. I have a better one. :)
 

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That is a harley sportster hub, laced to a big twin 16 inch harley outer, safety bead, rim. (ALL mechanical brake sporty rims came with 18 inch doughnuts).
go back and look at the pic I posted of my old yellow chopper. I used the same combination, and gear ratio was way too low. If you run the stock triumph final drive sprocket, and that rear hub, you will be rebuilding your engine soon. It is a lot of fun, in town, but RPMs are too high at highway speeds. The old engines were not designed for that, and metal fatigue in a 40+ year old engine will not assist you

IF you are insistent on doing that, you do need to go with a taller drive sprocket on the transmission
 

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Thanks. Well then just might have to change the trans gear. No big deal we have access to lots of parts.

So how much bigger gear you recommend on the trans?

That is a harley sportster hub, laced to a big twin 16 inch harley outer, safety bead, rim. (ALL mechanical brake sporty rims came with 18 inch doughnuts).
go back and look at the pic I posted of my old yellow chopper. I used the same combination, and gear ratio was way too low. If you run the stock triumph final drive sprocket, and that rear hub, you will be rebuilding your engine soon. It is a lot of fun, in town, but RPMs are too high at highway speeds. The old engines were not designed for that, and metal fatigue in a 40+ year old engine will not assist you

IF you are insistent on doing that, you do need to go with a taller drive sprocket on the transmission
 

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Ok I went and looked at this back hub and rim out in my shed where it is parked and how about going with a smaller brake drum/gear set up off another Harley instead since this entire unit is actually bolted to the hub itself?

Just asking.

That is a harley sportster hub, laced to a big twin 16 inch harley outer, safety bead, rim. (ALL mechanical brake sporty rims came with 18 inch doughnuts).

IF you are insistent on doing that, you do need to go with a taller drive sprocket on the transmission
 

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there isn't a smaller one.
sportsters have a very similar brake drum/sprocket to a triumph, and if you hog out the holes you can even mismatch drums (but to not try this at home...fit is horrible...this should ONLY be done by a machinist)

harley big twin drums only have 5 bolts holding them on, but they are larger, hardened steel bolts
at the time mechanical brake drums were used, the rear sprocket size was set. standard transmission drive sprockets were 17 tooth for a 45 inch flathead/servicart. 21 was for a sportster, and 23 was for a big twin

later more options became available, but you would have to go to a disc brake to take advantage of them


IF you were to go with a later style rear wheel, you could have a disc brake, and even sportsters went with the 5 bolt pattern. There are a lot more options... and if you are going to weld on a rear section anyway (I am assuming you are welding it, since the bolt on ones suck)



this is bringing back memories...last time I dealt with a rigid triumph, I laced an old pre unit sprung hub to a 16 inch rim
 

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Nope going to stay with my bolt on as is. My budget is tight until October. I'll talk to my fabricator/builder and we will see what we can come up with. Thanks for the help.
 

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Another reason for my questions on this gear issue are my riding plans are around town but I am also a member of the PGR which might require me to run outside of the county and in fact several counties over for those "Confirmed Missions" for Vet Services. Gotta get my baby built and tuned right.
 

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MY 71 T120 had a 16" on the rear...width problems, no guard. I have a rear hub (nicely polished) with spoke head holes opened up for fatter spokes...does any one need it ? willing to sell really cheap ...just sitting on my shelf...email me ...
 

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Just counted the teeth on my rear gear and there are 46 and on my parts engine it has a 20 tooth gear. Have not counted the teeth on the gear of the engine in my other storage I plan on using but at least if that gear is smaller I have a bigger spare to install and it looks like the rear gear might be able to go down to a 45 or 44 if they go that small. Also there is plenty of room to change the height of the tire to help compensate if needed plus have plenty of room for the fender.
 
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