Front Tyre

ILLF8ED · Aug 13, 2022

Question…if the contact area of the two sizes is the same why do they feel so much different?

SteveA · Aug 13, 2022

Fast Eddie said:
What are you matching it up to on the rear?

The problem IMO with the 90/90 is the section, not the width, it is a very low profile. So it might not suit your choice of rear. And IMHO they just look ‘wrong’ (I said IMHO… not looking for an argument here) !

The 90/90 has a diameter of 25.5”

The recommended (by Avon) 100/90 is 26.4”

That’s a big difference.

If you want narrow without the low profile, the 3.25 19 is a better choice with a 26.1” diameter.

I run a 100 / 90 / 19 front on the Commando with a 4.00 / 18 rear. I like the bigger contact patch of the 100 (even if that’s just psychological) and this combo gives the same diameters front and rear.

On two other bikes I run the 3.25 / 19 with the 4.00 / 18 and, honestly, that set up seems just as good.

I might try a 3.25 /19 on the Commando one day, but thus far I’ve been loath to tamper with what works !

Indeed, but the lower profile might be considered a good thing by someone like....me!

Lowering the front a touch also helps the steering, though some might find it twitchy, but some find Commandos twitchy anyway.

But, the emphasis on tail up nose down is coming to you from someone who has 2" longer than original shocks and a 130/70 tyre on the rear of a long lazy Rickman....race bike...
horses for courses....

SteveA · Aug 13, 2022

marshg246 said:
IMHO when you put tires on rims smaller than the tires were designed for you are punished in two ways. 1) PITA getting them on. 2) Tire wear due to them not being shaped as designed once on and inflated. Also, I wonder if an Avon 100/90 properly inflated actually has more contact area than a 90/90 properly inflated since the 100/90 will be rounder than designed.

I haven't a clue why Norton used WM2 rim front and rear. It certainly wasn't what others I know about were doing and AFAIK, not what Dunlop was recommending.

More money saving, one rim size, one tyre size.....bean counters rule....

marshg246 · Aug 13, 2022

Fast Eddie said:
Good question. I know we’re not tyre scientists and this is perhaps rather crude, but…

I’ve just mounted a new front 100 /90 on my 2.5 rim.

If you post some shots of one on the stock rim, showing the profile as best you can, I’ll try and match the shots and perhaps we can see what the difference looks like?

Very difficult to get pictures that show what eyes see - tried to get the sidewall bulge but never could get a good angle. The picture is of a 750, on the center stand, 100/90/19 tire on a WM2 rim inflated to 32psi. At its widest point, the tire measures 100.03mm. The tire has less than 10 miles on it. The camera is on the floor so it's the best angle I could get of the contact patch. Laying on the floor with the side of my head on floor, it's clear that there's very little contact. Of course, it will increase with the bike off the stand and a rider on.

Have an almost new Avon Super Venom 100/90/19 installed on a 2.5 rim, inflated to 32psi - not on a bike. The sidewalls are not nearly as bulged. It measures 105.44mm at its widest point. Of course, different tire but same manufacturer.

Fast Eddie · Aug 14, 2022

Hi Greg,
100 / 90 19 on a 2.5“ rim at 28 PSI.
I guess we can file there under ‘inconclusive‘ !

triumph2 · Aug 15, 2022

I have a pair of mk2 Roadriders on my MK III. 100/90/19 on the rear and 90/90/19 on the front on stock rims. I find the handling and grip to be excellent though it is twitchy at speeds over 70mph on the highway.

Fast Eddie · Aug 15, 2022

triumph2 said:
I have a pair of mk2 Roadriders on my MK III. 100/90/19 on the rear and 90/90/19 on the front on stock rims. I find the handling and grip to be excellent though it is twitchy at speeds over 70mph on the highway.

My guess is that cos of the reduced diameter, not the width.

You’re effectively running an inch smaller wheel in the front.

That drops the front, changes rake, trail, etc.

But it is only a guess.

Front Tyre

ILLF8ED

SteveA

SteveA

marshg246

Fast Eddie

triumph2

Fast Eddie

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