Wheels, Tyres and Dynamic Considerations
I've made 2 fundamental changes to the bike that needed planning and decision making.
Front Wheel
Fundamental to this bike is the USD fork swap. Bringing across the R6 forks makes major changes to the geometry (covered elsewhere), but as a consequence the front wheel size is vastly different to the original. The R6 front wheel is 120/70-17; the XV wheel is 100/90-19.
As you can see above, the R6 wheel is much smaller in diameter, but much wider. This should give the front a little more stability, but this will be dominated by the sharper rake and reduced trail due to the suspension changes. I'm also using the front fender from the R6, so increasing the diameter of the front tyre is a little more challenging than many of the builds online that run no fender at all. Doing that in Australia is likely to earn you a defect notice in no time at all.
Rear Wheel
The 5 spoke rear from the Gen 2 wheel is a much better visual match to the R6 wheel and to some extent the decision to change the rear is based on styling alone. The Gen 1 wheel size is 130/90-16 and the Gen 2 is 140/90-15.
The Gen 2 wheel is significantly wider than the Gen 1 (increasing stability), with rolling diameter on marginally smaller (giving a very slightly lower gearing - better acceleration at the expense of top speed).
As the changes to steering geometry are going to significantly sharpen the handling, increasing width of the wheels shouldn't cause the bike to become a whale through the corners. Note that increasing width on the rear is not required for traction; with the relatively low power this bike is putting down on the road, anything more than a 150 section would be overkill, and even the original 130 is sufficient (assuming the tyres are new and not designed to last for 100000ks)
Tyre Construction
There are 2 basic tyre constructions that are widely used by all tyre manufacturers; radial and bias ply. Generally, radial is used for sportsbikes and bias is used for cruisers/tourers. It is very uncommon for the two types to be mixed on one bike, and where it is (e.g. Triumph Thruxton) the rear is radial and the front is bias.
The wheel setup I'm using on the XV does not lend itself to an easy choice on this. The front is a sportsbike profile wheel and the rear a cruiser, meaning most tyres available are radial front and bias rear.
J&P Cycles (www.jpcycles.com) has a really good search function that enabled me to narrow down my choice and although you can't search by construction, each tyre has the construction in the data sheet.
Tyre Combinations
As there are no radial tyres available for the rear (that I could find anyway), all of the options are bias (including front).
Many of the tyre options were eliminated due to their reputation for durability over dry and wet grip. I'm happy to replace tyres regularly, but I am not happy to have the front wash out or the rear let go unpredictably.
Pirelli MT66
These tyres are regularly used on cafe racers, however there is no 120/70-17 available, closest is 120/90-17. The diameter of the MT66 is 2 inches larger than stock, which would create additional work in modifying the R6 fender to fit (trimming and adjusting the mounting holes).
Pirelli Sport Demon
These tyres are almost the exactly opposite of the MT66, the front stock size is available but the rear is 2.2 inches smaller in diameter than the stock rear. What the Sport Demon has going for it is a much better reputation for grip (both wet and dry). The smaller diameter should provide greater acceleration at the expense of top speed.
Avon AV72/AM26
There is no way to get a matching set of tyres from Avon, they simply don't make suitable sizes. Both tyres would seem to be a solid choice (the bike currently has the AV72 on it, but The tread patterns are visibly different and I'd prefer a matching set.
Dunlop D404
Stock tyres on many bikes, and are often commented on as being bettered by another (any other) tyre. Also doesn't have the stock front size and I would choose the MT66 over this.
Michelin Commander 2
These are the incumbent standard for many cruisers and have to be considered. They are designed for a much heavier bike than what this will be, so may be difficult to get up to temperature. Would be a solid choice, but I'm hoping to find a sportier tyre for mine.
Metzeler M888
I almost bought these, great reviews for handling on a range of bikes and seemed to be an improvement on the M880 which has been a very good cruiser tyre for many years. These a very expensive though, so I'm taking a shot on another.
The winner: Pirelli Sport Demon
The 140/70-17 is 10% smaller than the original XV750 Gen 1 wheel and 8.8% smaller than the Gen 2 wheel. This is a significant difference and will no doubt affect the handling of the bike. As a combination with the R6 front, the picture becomes a little clearer.
The rear will be 3.8% smaller than the front, which although certainly not a sports bike ratio, is not uncommon for cruisers. As an example, the 2015 883 Sportster has a front to rear ratio of -2.1% and the 2015 Yamaha Bolt is -2.4%.
The reduction in diameter front and rear with this tyre combination is 63.4mm in the front and 62.8 on the rear, so although the smaller diameter rear seems an odd choice, it maintains the original stance of the bike and should allow the changes to the suspension geometry to be more predictable.
Given the good reviews for traction that these tyres have and the relatively cheap price I'm going to try them out and will report back later.
Resources:
Robinson, John. Motorcycle Tuning: Chassis. Oxford; Butterworth-Heinemann. 1994
(books.google.com.au/books?isbn=075061840X)
Tyre Size Comparison Calculator (www.tacomaworld.com/tirecalc)
Front Wheel
Fundamental to this bike is the USD fork swap. Bringing across the R6 forks makes major changes to the geometry (covered elsewhere), but as a consequence the front wheel size is vastly different to the original. The R6 front wheel is 120/70-17; the XV wheel is 100/90-19.
As you can see above, the R6 wheel is much smaller in diameter, but much wider. This should give the front a little more stability, but this will be dominated by the sharper rake and reduced trail due to the suspension changes. I'm also using the front fender from the R6, so increasing the diameter of the front tyre is a little more challenging than many of the builds online that run no fender at all. Doing that in Australia is likely to earn you a defect notice in no time at all.
Rear Wheel
The 5 spoke rear from the Gen 2 wheel is a much better visual match to the R6 wheel and to some extent the decision to change the rear is based on styling alone. The Gen 1 wheel size is 130/90-16 and the Gen 2 is 140/90-15.
The Gen 2 wheel is significantly wider than the Gen 1 (increasing stability), with rolling diameter on marginally smaller (giving a very slightly lower gearing - better acceleration at the expense of top speed).
As the changes to steering geometry are going to significantly sharpen the handling, increasing width of the wheels shouldn't cause the bike to become a whale through the corners. Note that increasing width on the rear is not required for traction; with the relatively low power this bike is putting down on the road, anything more than a 150 section would be overkill, and even the original 130 is sufficient (assuming the tyres are new and not designed to last for 100000ks)
Tyre Construction
There are 2 basic tyre constructions that are widely used by all tyre manufacturers; radial and bias ply. Generally, radial is used for sportsbikes and bias is used for cruisers/tourers. It is very uncommon for the two types to be mixed on one bike, and where it is (e.g. Triumph Thruxton) the rear is radial and the front is bias.
The wheel setup I'm using on the XV does not lend itself to an easy choice on this. The front is a sportsbike profile wheel and the rear a cruiser, meaning most tyres available are radial front and bias rear.
J&P Cycles (www.jpcycles.com) has a really good search function that enabled me to narrow down my choice and although you can't search by construction, each tyre has the construction in the data sheet.
Tyre Combinations
As there are no radial tyres available for the rear (that I could find anyway), all of the options are bias (including front).
| Brand | Spec | Size | Brand | Spec | Size |
| Pirelli | MT66 | 140/90B15 | Pirelli | MT66 | 120/90B17 |
| Pirelli | Sport Demon | 140/70B15 | Pirelli | Sport Demon | 120/70B17 |
| Avon | AV72 | 140/90B15 | Avon | AM26 | 120/70B17 |
| Dunlop | D404 | 140/90B15 | Dunlop | D404 | 120/90B17 |
| Michelin | Commander 2 | 140/90B15 | Michelin | Pilot Activ | 120/70B17 |
| Metzeler | M888 | 140/90B15 | Metzeler | M888 | 120/70B17 |
Many of the tyre options were eliminated due to their reputation for durability over dry and wet grip. I'm happy to replace tyres regularly, but I am not happy to have the front wash out or the rear let go unpredictably.
Pirelli MT66
These tyres are regularly used on cafe racers, however there is no 120/70-17 available, closest is 120/90-17. The diameter of the MT66 is 2 inches larger than stock, which would create additional work in modifying the R6 fender to fit (trimming and adjusting the mounting holes).
Pirelli Sport Demon
These tyres are almost the exactly opposite of the MT66, the front stock size is available but the rear is 2.2 inches smaller in diameter than the stock rear. What the Sport Demon has going for it is a much better reputation for grip (both wet and dry). The smaller diameter should provide greater acceleration at the expense of top speed.
Avon AV72/AM26
There is no way to get a matching set of tyres from Avon, they simply don't make suitable sizes. Both tyres would seem to be a solid choice (the bike currently has the AV72 on it, but The tread patterns are visibly different and I'd prefer a matching set.
Dunlop D404
Stock tyres on many bikes, and are often commented on as being bettered by another (any other) tyre. Also doesn't have the stock front size and I would choose the MT66 over this.
Michelin Commander 2
These are the incumbent standard for many cruisers and have to be considered. They are designed for a much heavier bike than what this will be, so may be difficult to get up to temperature. Would be a solid choice, but I'm hoping to find a sportier tyre for mine.
Metzeler M888
I almost bought these, great reviews for handling on a range of bikes and seemed to be an improvement on the M880 which has been a very good cruiser tyre for many years. These a very expensive though, so I'm taking a shot on another.
The winner: Pirelli Sport Demon
The 140/70-17 is 10% smaller than the original XV750 Gen 1 wheel and 8.8% smaller than the Gen 2 wheel. This is a significant difference and will no doubt affect the handling of the bike. As a combination with the R6 front, the picture becomes a little clearer.
The rear will be 3.8% smaller than the front, which although certainly not a sports bike ratio, is not uncommon for cruisers. As an example, the 2015 883 Sportster has a front to rear ratio of -2.1% and the 2015 Yamaha Bolt is -2.4%.
The reduction in diameter front and rear with this tyre combination is 63.4mm in the front and 62.8 on the rear, so although the smaller diameter rear seems an odd choice, it maintains the original stance of the bike and should allow the changes to the suspension geometry to be more predictable.
Given the good reviews for traction that these tyres have and the relatively cheap price I'm going to try them out and will report back later.
Resources:
Robinson, John. Motorcycle Tuning: Chassis. Oxford; Butterworth-Heinemann. 1994
(books.google.com.au/books?isbn=075061840X)
Tyre Size Comparison Calculator (www.tacomaworld.com/tirecalc)