Author Topic: Is a 4.25 rear tyre too big?  (Read 1094 times)

Offline SteveW

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Is a 4.25 rear tyre too big?
« on: January 25, 2018, 08:37:32 PM »
Just getting my CB550 ready for its MOT, noticed that the previous owner has put a 4.25 wide rear tyre on it.

Seems fit ok, haven't ridden the bike yet due to knackered carbs, electrics etc etc.

Is 4.25 too wide?, I will probably be changing tyres soon, should i go for a 4.0?
1974 CB550 K0
1980 CB650
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1966 Lambretta LI150 Series 3
1981 RD350LC
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Offline SteveD CB500K0

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Re: Is a 4.25 rear tyre too big?
« Reply #1 on: January 25, 2018, 08:47:54 PM »
Always had 425/85 Tyres on my 500s.


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Offline hairygit

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Re: Is a 4.25 rear tyre too big?
« Reply #2 on: January 25, 2018, 08:48:14 PM »
Fitting a 4.25 tyre on the rear was a popular choice back in the day. Don't recall anyone having issues with it, and it raises the gearing very slightly and makes the bike fractionally taller, but it won't be a problem unless you're a shortarse like Mrs Hairy, or a certain member on here with a green 750!

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Offline SteveW

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Re: Is a 4.25 rear tyre too big?
« Reply #3 on: January 25, 2018, 08:53:57 PM »
Thanks guys  :)
1974 CB550 K0
1980 CB650
2000 CBR929RR Fireblade
1966 Lambretta LI150 Series 3
1981 RD350LC
1972 Raleigh Chopper
1974 Raleigh Tomahawk
2011 Henry Hoover

Offline Johnwebley

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Re: Is a 4.25 rear tyre too big?
« Reply #4 on: January 25, 2018, 09:36:59 PM »
I fit a 4.00
That is already 2 sizes bigger than the 3.50 standard for the 500

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Offline Bryanj

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Re: Is a 4.25 rear tyre too big?
« Reply #5 on: January 26, 2018, 07:14:29 AM »
4.00 x 18 is English size and 4,25/85 x 18 Metrisish and both sizes are really the same (550 came with 400 x 18)

Offline matthewmosse

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Re: Is a 4.25 rear tyre too big?
« Reply #6 on: January 26, 2018, 08:14:47 AM »
I think the widest tyre I had was a 500/15 that I picked up on a custom rim, that just about fitted cold but rubbed the torque arm sometimes on the move. I suspect going that wide on a stock rim would bunch the tyre up and not be safe....
Got a 500/4 with rust and a sidecar and loadsa bits. nice and original and been round the clock

Offline Moorey

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Re: Is a 4.25 rear tyre too big?
« Reply #7 on: January 26, 2018, 09:52:33 AM »
4.00 x 18 is English size and 4,25/85 x 18 Metrisish and both sizes are really the same (550 came with 400 x 18)

Where is the Metrisish bit, isn't 4.25 = 4.1/4 inch 85 is tyre wall size as a % of tyre width and 18 the wheel diameter in inches.  So isn't  a 4.25 wider than a 4.00 .  Sometimes another 1/4" can make all the difference or am I wrong.

Offline MrDavo

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Re: Is a 4.25 rear tyre too big?
« Reply #8 on: January 26, 2018, 11:09:14 AM »
The early CB750 I had in the 70's came with an oversize back tyre, 4.25 TT100 I think, coupled with Girling shocks that may have been on the long side (I didn't know any better to find out / go back to standard) it snaked like a pig (if pigs can snake) at high speeds, I got rid after a couple of near terminal experiences, it put me off big Hondas for years.

The K1 I have now has bog stock tyre and shock sizes, and doesn't wobble at all.
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Offline K2-K6

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Re: Is a 4.25 rear tyre too big?
« Reply #9 on: January 26, 2018, 11:21:06 AM »
Isn't 4.25  decimal expression of inches, so a sort of hybrid rather than fractions of an inch as would be usual?

So decimal being base ten does make it metrinch-ish by combining the two  :)

It's a real hotchpotch even now with tyres all still having imperial sized rim diameter, plus metric width,  plus % aspect ratio as moorey states.

Offline Seabeowner

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Re: Is a 4.25 rear tyre too big?
« Reply #10 on: January 26, 2018, 04:05:18 PM »
Yes and the rim is 1.85" wide just add to the mix.
Always used 4.25/85 in years of old. The 85% profile coincidentally means the gearing is virtually unchanged,
Standard tyre was 3.50 on 500, 3.75 on 550.
Phil
1971  CB500K0  Candy Jade Green or Candy Gold
1973  CB500K1  Candy Ruby Red
1975  CB550F1   Shiny Orange
1978  CB550K     Excel Black

Offline Tomb

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Re: Is a 4.25 rear tyre too big?
« Reply #11 on: January 26, 2018, 04:14:05 PM »
I use a 120/90 rear on my 500, it works really well - if you want to go totally metric.

Its close but clear of the torque arm though I am using a Dresda box section arm if that makes any difference.
Tom
'73 CB550 with CB500 engine café racer
'62 CB77 Sprinter
'70 CD175
'78 CB550 with sidecar
'80 Z50R
And a load of old Yamaha 1100's

Offline peterengland

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Re: Is a 4.25 rear tyre too big?
« Reply #12 on: January 26, 2018, 05:57:22 PM »
No, it's fine
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Offline Green1

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Re: Is a 4.25 rear tyre too big?
« Reply #13 on: January 26, 2018, 07:12:47 PM »
Just to let you know I have a 400x18 on my 550 and 750 and can reach the floor easily even though I am a wee bit shorter then you freakishly tall people.
Super Moto boots help a little on the 750  ;)
Current bikes
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Offline SteveD CB500K0

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Re: Is a 4.25 rear tyre too big?
« Reply #14 on: January 26, 2018, 07:35:08 PM »
Do these “super” boots have high heels then?


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