Rear tire sizes for a 1973 750 Commando.

Each turn of the rear wheel turns the speedo cable inner an exact number of turns. Change the diameter of the tire and clearly you upset the cable turns count. The speedo head can be thought of a voltmeter - spin a magnet in a coil of wire and a voltage is produced - spin faster and more voltage is produced. Calibrate accurately and you'll have an accurate speed if the tire diameter is correct. They are only accurate at the speed calibrated but that's the same for most speedos.

Due to a bunch of factors, especially how many satellites captured, GPS is not always accurate. You must know the distance to at least two satellites and the speed of light to have any hope of speed accuracy and three gives flat-line accuracy in most cases while four allows altitude changes to be accounted for. And you must have fast enough hardware to do the calculations in real-time. Most of the time, you're simply averaging previous readings.
 
Not sure exactly how much the 400/19 raised the bike but the rear tire hits the ground when up on the center stand. Contact is minimal, I can still rotate the wheel. I'm thinking id the front it replaced with an Avon 400/19 will I be creating any future clearance problems. Will the tire actually fit? There's not a whole lot of room between the tire and the front fender. Also, raising the front even slightly will lower the rear a bit.

Right??
 
If your stops on the centre stand or the pivot holes etc on the cradle are worn then if fixed that will raise the bike and restore ground clearance when on the stand.

Raising the front of the bike with a larger front tyre will get both wheels touching the floor when on the centre stand.
 
TomU said:
The Norton speedo is worthless. If you really want to know how fast you are going, you need GPS

I always wonder if the crap gauge will get you off a speeding ticket. I have not tested my theory, but i digress off a conversation about tire sizes
Click to expand...
Since fitting the Speedo drive to front wheel with an almost straight cable my original Smith's speedometer is absolutely spot on no wavering of the hand either
I adjusted it to give exact speed rather than the normal few % extra
Checked against my satnav on level road
 
The 400/19 is big!
 

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TomU said:
The Norton speedo is worthless. If you really want to know how fast you are going, you need GPS

“I always wonder if the crap gauge will get you off a speeding ticket. I have not tested my theory, but i digress off a conversation about tire sizes”
Click to expand...
Its been a while since I was in a California traffic court, but I remember people coming in with documentation showing that their speedometer was inaccurate and pleading for mercy. Invariably the judge would state that California law does not require you to have a speedometer, but it does require you to go the speed limit.
 
MichaelB said:
I probably have some of this wrong but I was told way back when that the 4:10 was a cross between the 350 and the 400.
The width of the 400 but the lower profile.
Think of it as low profile 400 or wide 3:50.
It was also developed by Dunlop special for the Commando as the K81 / TT100.
Click to expand...
You are correct in that the .10 suffix means it is a low profile tire and does not mean that it is 4.1 inches wide, the aspect ratio is between 84 and 90% (per an Avon PDF I downloaded years ago)
here is a quote from it...the tires101forconsumers.pdf is no longer available online unfortunately (though if anyone wants a copy I'd be happy to email one, there is a LOT of good info in the PDF)


"If the last two digits in the width marking are a .10 or a .60 as in 4.10-18 or 4.60-18 then the aspect ratio of the tire is between 80% and 94% - a low profile tire. Therefore, when you see a 4.10, 4.25, 5.10, 5.60, and so on it means the tires have a low aspect ratio.
Many people assume that a 4.00-18 is smaller than a 4.10-18, but that's not true. The 4.10 merely has a lower aspect ratio. Some people say they want to put the biggest tire on the back of their bike that will fit between the swing arm. Now if you notice that you have a 4.00 and buy a 4.10 you're actually going to be getting a lower, but not wider, tire than you already have. It can be very confusing and a cross reference chart of all the various sizing methods is very helpful."
 
Last edited:
bluto said:
You are correct in that the .10 suffix means it is a low profile tire and does not mean that it is 4.1 inches wide, the aspect ratio is between 84 and 90% (per an Avon PDF I downloaded years ago)
here is a quote from it...the tires101forconsumers.pdf is no longer available online unfortunately (though if anyone wants a copy I'd be happy to email one, there is a LOT of good info in the PDF)


"If the last two digits in the width marking are a .10 or a .60 as in 4.10-18 or 4.60-18 then the aspect ratio of the tire is between 80% and 94% - a low profile tire. Therefore, when you see a 4.10, 4.25, 5.10, 5.60, and so on it means the tires have a low aspect ratio.
Many people assume that a 4.00-18 is smaller than a 4.10-18, but that's not true. The 4.10 merely has a lower aspect ratio. Some people say they want to put the biggest tire on the back of their bike that will fit between the swing arm. Now if you notice that you have a 4.00 and buy a 4.10 you're actually going to be getting a lower, but not wider, tire than you already have. It can be very confusing and a cross reference chart of all the various sizing methods is very helpful."
Click to expand...
I never knew that !

On top of this, another point for the OP is to say that you cannot compare sizes of different types of tyre. Even tyres labelled as the same size, that are of different types (ie different make or model) can be very different in real dimensions.
 
Mark Savage said:
Nor me.
I do know my rear 4.00 x 18 R/R has gone horrid @ 5,500 miles with tread left though. Too many straight roads I reckon.
I've already got a replacement ready to go on but will try the BT46s next I reckon as I've always got on with Bridgestones, admittedly on modern bikes.
Click to expand...
Hi Mark, I’ve started running them F & R with good results.
 
I had a 400-18 SM on my Series 2 Interceptor last few years. I took it off this season with 5k on
it and it could easily go another 1k no problem. The only tyre that you can get any wear out of.
It was always known as the Slip Master back in the day but it never gave me a problem no doubt
due to modern tyre compounds. The profile is decidedly square but again it handled ok . Now run
a Avon RR and it handles better and is confidence inspiring. I doubt it will go much better than
3k though.
 
Onder said:
I had a 400-18 SM on my Series 2 Interceptor last few years. I took it off this season with 5k on
it and it could easily go another 1k no problem. The only tyre that you can get any wear out of.
It was always known as the Slip Master back in the day but it never gave me a problem no doubt
due to modern tyre compounds. The profile is decidedly square but again it handled ok . Now run
a Avon RR and it handles better and is confidence inspiring. I doubt it will go much better than
3k though.
Click to expand...
Which RR you got?

if you’ve an 18 rear you should use the dedicated rear tyre which has a LOT more tread depth. Universal fitment (ie can go in either front or rear) have a lot less tread depth… so do a lot less miles.
 
bit of a delay on your query..but attached is the answer
 

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