Hello Barry.
For a Triumph Toledo or Dolomite, I would personally regard 55-Series tyres as ultra-low-profile; especially considering that their suspensions were designed to function with 80-Series or 70-Series tyres; unlike the MGF or MGTF.
The vertical stiffness of the tyres, combines in series with the vertical stiffness of the helical coil springs, so if the tyre stiffness is increased, the overall stiffness of the suspension is increased, resulting in a harsher ride on uneven surfaces and possible skittishness on bumpy bends; particularly in relation to the rear suspension, which has a high un-sprung weight, owing to the heavy live axle. Hence, I would be wary of fitting such low-profile tyres (which might prove to be an expensive experiment), unless I could first assess the effect, by driving or travelling on such roads, in an existing car which had already been modified in this way.
Steve LaFerre, "Bottoming Out: With Aspect Ratios, How Low is Really Too Low?”, Tire Review, 22nd May 2008.
http://www.tirereview.com/bottoming-out ... y-too-low/
Unless one can obtain 55-Series tyres in a suitably LOW speed rating (even the S, T & H speed-ratings of 113, 118 & 130 mph are grossly over-rated for a Triumph 1500TC; especially as the national maximum speed limit is 70 mph), the additional internal reinforcement associated with a high-speed, high-performance tyre, will increase rolling resistance and consequently fuel-consumption rate as well; which would conflict with my philosophy of life.
Colin Chapman, the brains behind Lotus racing cars and sports cars, worked on the principle that good handling in a road-going sports car, was best achieved by using soft springs, firm damping and appropriate choice of anti-roll bar stiffness. The original 1960s vintage Lotus Elan, with its narrow, high-profile tires, is arguably one of the best-handling sports cars made; which in many ways was the inspiration for the Mazda MX5.
I had suspected that your wheel & tyre combination, might be MGF or MGTF, 6 x 15 inch “Minilite style” 8-spoke alloy wheels & 185/55 R15 tyres. How much clearance do you have between the tyres and the wheel arches, when the car is loaded to full capacity with four hefty adults and/or luggage? Any tendency for the tyres to rub against any part of the bodywork or suspension, when the rear axle is hard against the bump-stop, would probably constitute an MOT failure for safety reasons. Even though these 40+ year old cars are exempt from MOT testing, they must at all times, when used or parked on the public highway, be capable of passing an MOT inspection!
In the following topic thread, “Cliftyhanger” wrote:
Quote:
I have the 15" MGF alloys with 185/55/15 tyres, and I had to trim the rear arches back to nothing.
Board index » The Triumph Dolomite Club » Dolomite-related [Start here!] » MG-Rover-Austin Maestro or Montego alloy wheels for Triumph Toledo & Dolomite
https://forum.triumphdolomite.co.uk/vie ... =4&t=34572
https://forum.triumphdolomite.co.uk/vie ... 72#p319956
From what I have read on the subject of wheels and tyres, one of the tyre-industry’s “rules of thumb”, indicates that fitting the same individual tyre to a 6 inch wide wheel rim, will increase the tyre’s actual section-width by approximately 1/5-inch (i.e. 5•08 mm) compared to what it would be on a 5½-inch wide wheel rim, so would bring a tyre’s sidewall about 2½ mm closer to the adjacent surfaces of the wheel arches.
Although I have yet to receive confirmation from any sources, I suspect that the MGF & MGTF, 6 x 15 inch alloy wheels are all of 28 mm offset; which is something I hope various parties will confirm or refute, either by reference to what is printed on the wheels or by actual measurements & calculation, which is quite easy to do.
Having a wheel of 5½ inch rim-width with a 31 mm offset, would by the tyre-industry’s “rule of thumb”, give about 5½ mm more outboard clearance between the tyre sidewall and the rear wheel arch, than would a wheel of 6 inch rim-width with a 28 mm offset.
Two of my
seven second-hand MG 2•0 Maestro, 5½ x 15 inch, lattice-style alloy wheels (31 mm offset) came already fitted with 185/55 R15 tyres (V speed rating of more than 130 mph – ridiculous!), which I have trial-fitted on my 1974 Triumph Toledo 1300 “HL Special”, resulting in more than adequate inboard & outboard clearance, between the tyre sidewalls and the rear wheel arches!
For reason of overall effective engine-gearing, reduced rolling-resistance (by virtue of both optimum aspect-ratio and lower speed rating – typically a T rating of 118 mph, which is still grossly over rated), improved ride comfort and more progressive loss of grip approaching the limit, I feel inclined to use 185/65 R15 tyres, which are readily available at competitive prices; including the impressive looking, Finnish made, Nokian all-weather tyre, that is also “snowflake-rated” for low-temperature winter use.
Jim Kerr, "Auto Tech: The science of tire-testing", Canadian Driver, 10th September 2003
https://www.canadiandriver.com/2003/09/ ... esting.htm
Julian Edgar, Ultimate DIY Automotive Modification Tool-Kit, Part 6, “Getting a handle on ride and handling”, DIY Tech Features Section, Autospeed.com, Issue 529, 5th May 2009.
http://www.autospeed.com/cms/A_111151/a ... larArticle
To date, I have still yet to obtain definitive information about the wheel-offsets of the factory-standard steel wheels for the Triumph Toledo 1300 & 1500 [4 x 13 inch], Triumph 1500TC [size unknown] or Triumph Dolomite 1300, 1500, 1500HL, 1850 & 1850HL [4½ x 13 inch]. I suspect they are probably close to that of the 5½ x 13 inch Triumph Dolomite Sprint alloy wheels (35 mm offset), but that is purely conjecture on my part.
Since before my father bought our ex-demonstrator cum dealership runabout, 1974 Triumph Toledo 1300, in May 1975, it has NOT been equipped with the factory-standard 4 x 13 inch steel wheels [of as yet unkown offset] & 155 SR13 radial-ply tyres.
To date, I have run the car with 5½ x 13 inch alloy wheels of two different wheel offsets (i.e. 21 mm & 35 mm) and two different tyre sizes (i.e. 185/70 R13 & 175 SR13 = circa 175/80 R13 ~ 175/83 R13) in three combinations. You might want to read the following posts to discover what disconcerting and potentially dangerous steering characteristics I encountered with one of these combinations!
Board index » The Triumph Dolomite Club » Restoration Projects » Triumph Toledo - Wellington New Zealand
https://forum.triumphdolomite.co.uk/vie ... 64#p322664
https://forum.triumphdolomite.co.uk/vie ... 78#p338378
https://forum.triumphdolomite.co.uk/vie ... 79#p338579
Alistair Cox in New Zealand has yet to report back on his experiences with new 5 x 13 inch “Minilite style” alloy wheels (20 mm offset) & 175/70 R13 tyres fitted to his Triumph Toledo 1500. I very much hope he doesn’t encounter the same weird steering characteristics that I did, with 5½ x 13 inch Cosmic alloy wheels (21 mm offset) & 185/70 R13 tyres, when negotiating tight bends under power! With 6 x 15 inch MGF alloy wheels (28 mm offset?) & 185/55 R15 tyres, I probably would not expect the outcome to be quite so extreme, but please be wary!
185/65 R15 – 1952•4 mm circumference
175 R13 => 175/83 R13 – 1949•9 mm circumference (5•65% larger than 155/83 R13)
175 R13 => 175/82 R13 – 1939•0 mm circumference (5•62% larger than 155/82 R13)
175 R13 => 175/80 R13 – 1917•0 mm circumference (5•54% larger than 155/80 R13)
185/70 R13 – 1851•0 mm circumference
155 R13 => 155/83 R13 – 1845•6 mm circumference
185/55 R15 – 1836•2 mm circumference
155 R13 => 155/82 R13 – 1835•9 mm circumference
155 R13 => 155/80 R13 – 1816•4 mm circumference
175/70 R13 – 1807•0 mm circumference
_________________
Regards.
Nigel A. Skeet
Independent tutor of mathematics, physics, technology & engineering, for secondary, tertiary, further & higher education.
https://www.linkedin.com/profile/view?id=308177758
Upgraded 1974 Triumph Toledo 1300 (Toledo / Dolomite HL / Sprint hybrid)
Onetime member + magazine editor & technical editor of Volkswagen Type 2 Owners' Club