Wheel change from 16" HPCs to 15" 10-spoke

[mudguard]

I've seen a number of discussions on BC about changing from HPC wheels to a smaller size and thought I would chime in with my recent experience.

I've driven the car for over 8k on the HPCs and ZZ3s - which are an old set nearing the end of their useful life - purely on roads and also touring 2-up The majority of opinion seems to be that the wheels, whilst visually attractive, are big and heavy for the car and the tyres hard.

This being my first Seven I wouldn't know any difference, but in comparison with observations from other Seveners driving I've been finding the back end a bit skittish, and the ride quite jolting on bumpy roads. 

I was considering a change to 15" wheels when I bought the car, but now I've done it, thanks to BC for-sale and another club member. They are currently shod with some 45-profile CR500s, at 16psi, oldish but in good condition and comfortably legal tread.

I know everyone has their own preferences and driving style, but for me, at least, the effect of the change is very positive - and certainly not small, immediately noticeable just from some bumps a few metres from my driveway.

The suspension now seems to absorb the bumps much more, and the ride is much less harsh. The steering is about 30% lighter, and the car feels much more responsive. Cornering, the rear end moves around much less and it feels much more glued to the road.

Overall the car feels much more comfortable, confidence-inspiring and safer. Now I'm slightly sad that I've spent a year driving around in (effectively) lead boots and a car that was suffering due to having (IMO) the wrong wheels. But it was still a fantastic year!

I have adjustable platform suspension but haven't tinkered yet so I've lost about 2cm of sump clearance from what was about 9cm (unladen). No sump strikes yet but I'll probably want to recover at least some of the lost clearance, either through the suspension, or a new set of 50 profile tyres, or a mixture. I thought I might have to go to 50 profiles anyway to get a softer ride, but it's not the case. If I adjust the suspension I was planning to do an equal number of turns on all four corners to preserve the rake. Speedo over-reading slightly which I know can be corrected with the pot inside the unit.

I was intending to keep hold of the HPCs but there's little point now as I won't be going back. BC for-sale beckons but I'll have the aging paint tidied up first.

Thanks to everybody who pointed me in the direction of better wheels - you know who you are!

[Wrightpayne]

Just check your clearance between the front tyre and wingstay - I think some people have had rubbing issues when cornering on 50 series tyres though this maybe with the 13/14" stays rather than bigger ones used with HPC wheels.

In fact the clearance may be too much leaving a big gap between tyre / wing. Perhaps review when you're got 50 series tyres on.

I had the same stepped change moving from Michelin pileups to toyo r888s - enjoy!​

Ian

[mudguard]

I definitely have (very) big wings!

Clearance now is around 12mm at the worst point, previously was around 4mm. 

Clearly the smaller wheel doesn't "fill" the wing in the same way, but then you could argue that the HPC wheels "fill" the wing a little too much! To my eyes at least, it doesn't look ridiculous. 

Are there any non-aesthetic concerns with the wing being the larger size and the wheel being smaller? i.e. am I liable to reach orbit?

[David Brown]

When adjusting your suspension you will have to measure the height front and rear and keep the difference the same as front suspension unit is at an angle where as the rears are vertical. If you just do it by counting the turns the back will move more than the front. The rake (difference in height front to rear should be about 15mm measured under the rear of the front wishbone vs under the chassis at the front of the rear wing). Does that poor explanation make sense?

[Wrightpayne]

For road use I think you'll be fine - on race cars the bigger the gap the more lift is generated at speed. I now run 13" wheels with 13/14" wingstays and the gap is more than I'd like but have the option of 70 series tyres which can cause rub issues on the race wingstays. Its a compromise I'm happy with.

So, next debate is which 15" 50 series tyres? Toyo's ? I think the OEM fitted CR500s are pretty good from the feedback on the forum.

Ian

 

[mudguard]

Good point about counting turns - thanks David. But it looks to me to be the other way around - turns would have more effect on the front. e.g. assuming lower wishbone parallel, front damper sitting at 45 degrees with a length of 30cm, then if adding 1cm to the rear needs 10 turns, only 7.2 turns would be needed on the front.

That's if my trig is accurate! Better to measure as you say.