I doubt it'll make much difference. 90/90 is the old 3.50 section, so your front wheel is now an inch bigger in diameter (21 + 2 x 3.5). Your 21 x 3.00 is already an inch larger than my 20 x 3.0 (or, same thing, 19 x 3.50) and there are people who swear by the 21" front. Will another inch make a difference? I doubt it. If the Vincent's steering geometry is so sensitive that carrying the front wheel spindle 1/2" higher leads to disaster, then we're all doomed anyway!!! Doomed I tell you!!! Doomed!!!
It might feel a bit heavier because you've got more rubber on the road, but personally the 3.00 section seems to me to have a somewhat tenuous hold on the road and more would be better. (What works, but would require rebuilding my front wheel is to use a 20" WM2 rim, which spreads the skinny tyre out a bit compared with the WM1. I know someone who has done this and he reports nothing but good.)
What needs also to be taken into consideration is that the only similarity between the tyres fitted at Stevenage and the tyres you can buy today is that both are black and round. The rubber on my 3.00 x 20 and 110/90 (i.e. 4.10) x 19 road tyres is as good as race tyres were in the 1970's, and light years better than the bakelite ones fitted in the 1950's. And race tyres today - well, be careful touching them with dry hands. They might have to be surgically removed.
I wouldn't worry too much about fitting decent brakes to a Vincent. Rudges have the forks Brampton are based on, standard is a 9" drum, and I've seen them converted to 2ls. I've never heard of Rudge forks breaking. I've got PV 2ls brakes, and what they give me that I want above all is an instant response. Ultimately however they're still the equivalent of a single 7" brake with a 2" wide lining, and that's scarcely enough to frighten the horses. As standard Rudges had a 9" brake with an 1 1/2" wide lining from about 1932.
It might feel a bit heavier because you've got more rubber on the road, but personally the 3.00 section seems to me to have a somewhat tenuous hold on the road and more would be better. (What works, but would require rebuilding my front wheel is to use a 20" WM2 rim, which spreads the skinny tyre out a bit compared with the WM1. I know someone who has done this and he reports nothing but good.)
What needs also to be taken into consideration is that the only similarity between the tyres fitted at Stevenage and the tyres you can buy today is that both are black and round. The rubber on my 3.00 x 20 and 110/90 (i.e. 4.10) x 19 road tyres is as good as race tyres were in the 1970's, and light years better than the bakelite ones fitted in the 1950's. And race tyres today - well, be careful touching them with dry hands. They might have to be surgically removed.
I wouldn't worry too much about fitting decent brakes to a Vincent. Rudges have the forks Brampton are based on, standard is a 9" drum, and I've seen them converted to 2ls. I've never heard of Rudge forks breaking. I've got PV 2ls brakes, and what they give me that I want above all is an instant response. Ultimately however they're still the equivalent of a single 7" brake with a 2" wide lining, and that's scarcely enough to frighten the horses. As standard Rudges had a 9" brake with an 1 1/2" wide lining from about 1932.
