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Ironhead Sportster Motorcycle Talk (1957-1985) For all those that wanna talk about Ironhead Sportster Motorcycles

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  #11  
Old 15th October 2021
Bluto Bluto is offline
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Join Date: Jan 2018
Location: California
Posts: 369
Sportster/Buell Model: XL1200R Roadster
Sportster/Buell Year: 2008
Other Motorcycle Model: Norton 850 Commando
Other Motorcycle Year: 1973
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Quote:
Originally Posted by ZTatZAU View Post
I think the K81 was quite popular on Nortons. The 3.60 tire was likely stock width for the front.

Thanks for that! I went to the same K81 page for Dunlop USA, and they only show the one 4.10 for 19" rims. The USA chart has more info than yours though; namely, overall width in inches to two decimal places which is my main concern. The 4.10-19 shows overall width at 4.17" which makes me curious about the overall width of the 3.60-19. Perhaps it is 3.75"? The 4.25-18 shows 4.37" wide! So the inch spec is not always accurate.

Likewise, Im finding not all 100 tires are 3.94" wide and not all 90s are 3.54" wide.

ZT
In some cases the .10, .60 etc refers to the aspect ratio/low profile, not the tire width. K81's are indeed popular on Norton's and I've used them on my Commando but something to be aware of is that they track rain grooves, bridge gratings, etc. far worse than any other tire in my experience so they are a poor choice if you ride much on rain grooved freeways.

https://www.denniskirk.com/learn/tire-sizes-explained
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  #12  
Old 15th October 2021
The Doctor71 The Doctor71 is offline
Senior Chief Master Mechanic
 
Join Date: Mar 2010
Location: Toronto, Ontario
Posts: 956
Sportster/Buell Model: xlch
Sportster/Buell Year: 1971
Sportster/Buell Model #2: xlh
Sportster/Buell Year #2: 1974
Other Motorcycle Model: xl1200cp
Other Motorcycle Year: 2011
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Quote:
Originally Posted by ZTatZAU View Post
..... Would you feel the same about 3/32" where shown at the sidewall?
3/32" should be fine.

Be wary of the clearance at the outside mounting point on your fender, where the rivets are. I suspect it will be tight there. A 3.9" wide tire should be fine. Anything wider and I suspect one is looking at a problem.

Quote:
Originally Posted by ZTatZAU View Post
.....Is this tapered flange stock to provide additional side clearance at the tread?
Yes it's stock. And yes it was probably done to provide the side clearance.
An observation (you can draw your own conclusion):
  • the OEM fenders on '71 and '72, do not have that tapered flange.
  • coincident with the change in front forks in '73 and '74 in seems the factory introduced this tapered flange.
Quote:
Originally Posted by ZTatZAU View Post
Or did a PO taper it down with a grinder or a file to fit the 4.10-19 tire up front?
The taper you describe was factory done on my 1974 XLH. I imagine this applies to 1973 as well due to the front fork change.

Quote:
Originally Posted by ZTatZAU View Post
..... Do you normally ride two up? Are you a big guy?
Maybe 10-20% of the time is two up. Not sure how one defines big (lol) but I am 225lbs.

Quote:
Originally Posted by ZTatZAU View Post
..... So the inch spec is not always accurate.

Likewise, Im finding not all 100 tires are 3.94" wide and not all 90s are 3.54" wide.
you're catching on. At best, I look at that tire width inch spec as a "ballpark" spec. You gotta read the manufacturer's specs carefully. For example, an Avon Roadrider 90/90-19 is spec'd at 3.9" wide not 3.54" wide. Comparing sizes between tire manufacturers is a recipe for disaster.
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  #13  
Old 15th October 2021
ZTatZAU ZTatZAU is offline
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Quote:
Originally Posted by Bluto View Post
In some cases the .10, .60 etc refers to the aspect ratio/low profile, not the tire width. K81's are indeed popular on Norton's and I've used them on my Commando but something to be aware of is that they track rain grooves, bridge gratings, etc. far worse than any other tire in my experience so they are a poor choice if you ride much on rain grooved freeways.

https://www.denniskirk.com/learn/tire-sizes-explained
Thanks Bluto! I've been all through that DK tire-size page and several other resources. All the new knowledge allowed me to determine that you can't always trust the tire specs published on the popular sites. DK says the overall width of the K81 4.25/84-18 is 4.25" as you would think (by the book). But the Dunlop K81 page says the overall width is 4.37". So you gotta' be careful.

Speaking of careful, I broke my steel grate bridge cherry back in the 1960's. Always loved the challenge and used to go looking for them on my old Royal Enfield & BSA Lightning. Can't remember if they had K81s but probably something similar.

Quote:
Originally Posted by The Doctor71 View Post
3/32" should be fine.
Be wary of the clearance at the outside mounting point on your fender, where the rivets are. I suspect it will be tight there. A 3.9" wide tire should be fine. Anything wider and I suspect one is looking at a problem.
There's plenty of room at the fender's rivets and the ends of the fender struts. My only concern is the 3/32" (or so) clearance between the sidewall and tapered flange on the fork supports. And, BTW, to be clear... That's the old K81 4.10H-19 still mounted in the picture I posted above. It measures ever so slightly more than 4" wide at the outboard tread and sidewall. If the new K81 4.10 isn't any wider than this old one, I should be OK.

Quote:
Originally Posted by The Doctor71 View Post
Yes it's stock. And yes it was probably done to provide the side clearance.
An observation (you can draw your own conclusion):
  • the OEM fenders on '71 and '72, do not have that tapered flange.
  • coincident with the change in front forks in '73 and '74 in seems the factory introduced this tapered flange.
The taper you describe was factory done on my 1974 XLH. I imagine this applies to 1973 as well due to the front fork change.
Thanks Doctor! I appreciate the historical info.

Quote:
Originally Posted by The Doctor71 View Post
Maybe 10-20% of the time is two up. Not sure how one defines big (lol) but I am 225lbs.
I only asked because you mentioned stock or close to stock tires rubbing. I've only noticed this when two up. I usually run about 200 lb in the winter but once it warms up and I get off my a$$, I get back down to 175-180.

Quote:
Originally Posted by The Doctor71 View Post
you're catching on... You gotta read the manufacturer's specs carefully.... Comparing sizes between tire manufacturers is a recipe for disaster.
So I've discovered!

Thanks again for your thoughtful replies! ZT
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