Paraphrasing……
If equal sized tyres at equal pressure give neutral handling then changing to a wider, softer walled front tyre at a low pressure and a narrower, stiffer walled rear tyre at a higher pressure will reduce understeer (and may make the bike too responsive).
Is that correct?
No. "Neutral handling" is a dangerous misconception -- a neutral bike would be a lethal bike because you don't know which way it will break. What you mean to say is "If equal sized tyres at equal pressure give PREDICTABLE, SAFE REACTIONS TO STEERING INPUTS, then" etc.
It is worth emphasizing the important implication, that roadholding/handling aren't just theoretical results of tyre choices, but in the
first instance a matter of the rider's safety and security on the bike, which become more important on foreign roads of unknown quality, at higher speeds (25mph is more than enough to do you serious harm), and as the load on the bike rises as it normally does in touring. An understeering bike is safer than an oversteering bike (and a neutral bike is NOT WANTED anywhere except at the very top of road racing).
My cycle speeds are much lower than Andre’s, max 25mph (40kmh), normally 12 - 16mph, and I rarely corner fast.
Thanks for the giggle. I actually ride slower than you do, on average 15kph/10mph, but I'm rarely on the level for any distance, so on the downhills I can get up to 55kph, though I'm now of an age where a broken hip could easily be a death warrant, and will almost certainly be the end of cycling, so every year I back off 5kph on the steepest descents, and stay an inch or two further away from the edge of the tarmac, which is also generally the edge of the deep ditch beside the lane.
My logic is that the softer / larger front tyre will give a little suspension over bumps while the tougher Marathon Plus on the rear will be more puncture resistant.
In my experience the Schwalbe Big Apple and the Schwalbe Marathon Plus (both with the best compound and the toughest anti-intrusion band) are equally puncture proof, each pair succumbing to two punctures over the course of about 11,000km. In theory, the Plus should be more puncture-resistant because of that ghetto-style anti-broken bottle sidewall where the Big Apple has a soft, thin, very flexible skein that looks like it could be broken through by exhaling near it.
Could Andre expand on his second to last paragraph, copied below, as I seem to be changing my bike in the opposite way!
‘If we're talking about touring with any kind of a load, even just a saddle bag, never mind the loads we see the real transcontinental tourers on the forum carry, it seems to me obvious the fat tyre had better go on the back’
I think you've not only answered yourself already...
I am interested in a softer 2.00” tyre on the front with the existing 1.75 Marathon Plus on the rear for touring on roads that may be rough in Sri Lanka this coming winter. My all up weight with food and water will be 105 - 110kg.
My logic is that the softer / larger front tyre will give a little suspension over bumps while the tougher Marathon Plus on the rear will be more puncture resistant - and will be dragged over any bumps. Repairing a rear wheel puncture is always a bit more work than a front one and rear wheel punctures seem more frequent, hence the Marathon Plus on the rear.
The slightly larger / softer front tyre should also be better at going over any sand I hit, while the heavier loaded and narrower rear tyre will did a groove and slow me down and help me stay upright.
... but also explained why I referred to your experience.
You may remember we arrived here via me explaining that this sort of engineering thought exercise is an art. It's actually worse than that in this particular instance because tyres, whether on cars or or bikes, are infuriatingly non-intuitive in response, and frustratingly resistant to reduction to algorithms. However, there's a small relief here because we can just assume that all the other design factors on your bike are near-optimal, because the bike is a Thorn, known to be properly designed as a conservative tourer. So my first approximation, in a theoretical discussion that newbies may read and try to apply, was aimed at a standard, normative case, an answer that would endanger no one.
Add your bike load, and your experience, and we now have another condition, a loaded touring setup which you found satisfactory. The fat, low pressure tyre at the front has plenty of grip and a large contact patch to turn the greater weight, but will be slowed a little by it. The narrow rear tyre, even with greater pressure, will also squash out a bit more under the luggage mass and slow down any tendency to dangerous oversteer further. Looks to me like the load actively countered the "wrong way round" fitment. Also, your conservative incremental style of developing your bike is inherently safer than making large changes.
Something else to consider: A load in panniers and a rack bag provides surface towards the back of the bike for sidewinds to push against. This can be a good thing because bike stability is best served by the Center of (Aerodynamic) Pressure behind the Centre of Gravity of the bike with luggage and rider in place.
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