Low prof ile Toyo T1-R and T1-S - tyre pressures?

[rolling_thunder]

Guys,

My Chim is running a mix of Toyo T1-R and T1-S fitted to 18" wheels (previous owners fault!), tyres are the low profile 225/40/18.

Reading the bible it mentions tyre pressures should be low 20's psi.

So i checked mine and they are 22 rear 20 front which is about right by the bible

But is this the best tyre pressure for low profile tyres?

Thanks

RT

[tvrbob]

Can't comment on the effect of 18" rims other than to say you need lower pressures to compensate for the lost sidewall. On the other hand Toyo T1-R & T1-S are less rigid than the Bridgestones that were used to set the pressures quoted in the users manual. The user manual says 22/24 and most people find that Toyos need to be 25/27(ish) to make them work like the old Bridgestones. To be honest I think 20psi sounds too low. If I had to guess I'd say 22/25 would be a reasonable compromise because the lost sidewall means you need lower pressure than standard but the Toyo tyre need higher pressure standard.

[Pupp]

I specifically asked the Toyo tech guys and was told 28 ish. There's a thread soewhere that a search should find

[tvrbob]

Pupp said:

I specifically asked the Toyo tech guys and was told 28 ish. There's a thread soewhere that a search should find

Don't put 28psi in the fronts. First wet or green road and you'll understeer straight into the bushes. 28psi in the rears is possible but I only go that high for the track.

By the way Toyo only had a supply contract with TVR, not a development contract. I'm surprised that they are willing to declare pressures for a chassis they have never tested, especially one with non-standard wheel size.

[rolling_thunder]

Thanks guys, i ran a search across the forums and it looks like a number of Cerbera folk are running 24lbs on each corner when using 18" wheels, although to be honest there is a quite a range all the way up to 28

Given that the car doesnt weigh much I think 28 is probably a little high but I will try adding a could of lbs and see if there is any difference.

Cheers

RT

[Tahiti450]

I run 40 profile T1Rs 17 inch front and 18 inch rear (since 2003). Tend to run at around 25 front and 28 rear. In the dry - on smooth surfaces (e.g track day / sprint) I add a couple of PSI - but check the pressures hot. Dry grip improves with higher presures up to a point, probably no more than 32-33 PSI hot. Changing the front / rear distribution will affect the balance noticably. For road use stay in the 20's but IMHO not less than 24 partly because the Toyos have a soft sidewall (single ply) and being lower profile need a bit more pressure to avoid excessive sidewall / tread distortion.

[Pupp]

tvrbob said:

Pupp said:

I specifically asked the Toyo tech guys and was told 28 ish. There's a thread soewhere that a search should find

Don't put 28psi in the fronts. First wet or green road and you'll understeer straight into the bushes. 28psi in the rears is possible but I only go that high for the track.

By the way Toyo only had a supply contract with TVR, not a development contract. I'm surprised that they are willing to declare pressures for a chassis they have never tested, especially one with non-standard wheel size.

Why not? They have experience of their product on a very broad cross-section of vehicles in all sorts of sizes. The Q was in the context of someone (can't remember who) stating they had experienced enough sidewall distortion to suffer rim-strike with their SP12s on the 22/24 or whatever recommended on fitment.... Tuscans etc on 18" spiders run higher I believe. BTW, I've been using 27 in the fronts for 18 months or more, all weathers and have never once experienced understeer, maybe your geometery is out?

[tvrbob]

Pupp said:

maybe your geometery is out?

What? These higher pressures have nothing to do with good suspension alignment. The higher pressures people are suggesting are to prevent the rim hitting the inner tyre not to provide good chassis dynamics. Stop fitting bad boy rims, problem solved.

The simple fact is that bigger rims reduce the contribution that the tyre sidewall provides to the overall suspension. This causes the front wheels to suffer greater bump steer and tram lining. The physics have been described on PH before. Obviously higher pressures will increase the probability of under steer, to say otherwise is nonsense. If you wish to reproduce the chassis dynamics of the original suspension, wheel & tyre set-up then you have to compensate for lost tyre flexing. Pumping more pressure in to stop the rims hitting the deck is just proof that you do not have sufficient tyre wall height.

[Pupp]

tvrbob said:

Pupp said:

maybe your geometery is out?

What? These higher pressures have nothing to do with good suspension alignment. The higher pressures people are suggesting are to prevent the rim hitting the inner tyre not to provide good chassis dynamics. Stop fitting bad boy rims, problem solved.

The simple fact is that bigger rims reduce the contribution that the tyre sidewall provides to the overall suspension. This causes the front wheels to suffer greater bump steer and tram lining. The physics have been described on PH before. Obviously higher pressures will increase the probability of under steer, to say otherwise is nonsense. If you wish to reproduce the chassis dynamics of the original suspension, wheel & tyre set-up then you have to compensate for lost tyre flexing. Pumping more pressure in to stop the rims hitting the deck is just proof that you do not have sufficient tyre wall height.

Well that's a bit more of a considered response than your original knee-jerk 'you'll be in the hedge...' nonsense, so thanks for that.

Trouble is, you're assuming anyone moving to 17/18s is wanting to replicate original handling... I actually wanted something rather less wallowy and more direct. Something where the suspension was doing the suspending and where the tyres were doing the, errr, tyre-ing. Having changed so much, speaking to the tyre manufacturer's own technical guys seemed a reasonable move (well more reasonable than slavish adherence to the original recommendation 'because that's what TVR said', anyhow), and I was just passing on the benefit of my experience having actually done the change and lived with it if that's ok. By the way, I suffer negligible tramlining, far less than the car used to be plagued with on the oh so brilliant OEM profiles (what were they again, there were so many variants thrown on by the factory over the years? ) and I personally have never suffered any problems with the rims grounding... I just followed up because someone else said thay had. We're only talking 4 psi difference FFS, not 4Bar.

Oh, and who's to say the offsets on the original wheels are what the chassis designer actually intended?

[GTvr]

Got new wheels a few of weeks ago, after finally deciding to go to 17" all round.
The cars handling feels so much better and direct with no tramlining or loss to ride quality.

The tyre shop recommended the toyos to be +2 from original tyre pressures front & back.

I was out early on sunday in the torrential downpour, I was a bit worried as this was the first time in the the wet with the new wheels/tyres and didn't know how they would handle but the car behaved perfectly.

[johnbear]

I used to run 24 psi on the front of tyhe my Cerb with toyo's / SP12's and was always getting rim strike. Didn't realise this was a problem others had.