How to tune a bike's roadholding & handling to be sharper

[Andre Jute]

Let's discuss pressures.
More in the rear?

If a loaded bike is unbalanced, tyre pressure differentials won't fix the problem and could as easily aggravate it. You need to stop and redistribute the load.

On an unloaded bike, except for the rider, substantially higher pressure in the rear than the front may sharpen up the handling of your bike, especially if you start with a well-pressured front tyre.

But I must tell you that I wouldn't dream of doing it. My daily bike has a very long wheelbase and 60mm fat tackles operated at the minimum pressure that doesn't lead to constant fishbites, and the behaviour may seem sluggish to someone used to a short wheelbase, thin-high-pressure-tired road bike. It's like moving from a little English sports car to a V12 Mercedes. I have a good deal of experience developing racing car suspensions, in which the tyre and its pressure is your first line of defence, and I know for a fact that a bit of understeer is its own very handy safety margin, especially when the rider doesn't want a broken hip. The faster my bike goes, alas only on the downhills these days, the more valuable the security that flows from the understeer becomes. Rough roads? The roads here are an embarrassment, and the lanes more so, but nothing throws my bike off line, though road bikers who arrive at the bottom of the hill considerably after me are  white around the gills and worried about the integrity of their bikes.

Setting the bike up razor-sharp, even with big boots, is no problem, practically speaking. But the desirability of such an action is in deep doubt. And most of the modern touring tyres that find favour with forum members (all the ones with Marathon in the name, for starters) are intended to be used as balloons or semi-balloons, with low tyre pressures. Pumping them harder to get twitchy handling seems to me counterproductive.

The behaviour of a sharp bike at high speed won't impress you. First of all, it's not a relaxing touring ride, it's a Sundays-only speed-freak bike. You have to be on top of a nervous bike all the time or it will throw you into the gorse, because a quick steering is a positive feedback system in which whatever goes wrong gets magnified, and even a twig on the road can throw you off, whereas my bike rides right across potholes that will bend the rims on racing bikes. A neutral-handling bike (roadholding is the bike keeping the rubber on the road, handling is its ability to recover from upsets) is even worse, in that you don't know which way it will break, and half the time you will be bracing in the wrong direction, and make things worse, perhaps to take a spill.

You can try this for yourself. If you're following the official recommendations of low pressure encapsulated in an essay by Mr Blance a couple of years ago, and you have good wide tyres, pump the front tyre to about 20psi under its limit, and the rear tyre to only 5psi under its limit. Notice how much of your comfort these changes have consumed. Ride down a hill with sweeping curves at a moderate pace, riding the brakes if necessary. Notice how the bike wants to turn to the insides of corners, as if the road is much more steeply cambered than you can see is the reality, and that you have to expend considerably more effort to keep the bike tracking with the road, and out of the ditch. You can aggravate this effect by putting a smoothly worn tyre on the back to give it even better grip. You can detune the effect too, by letting air our of the rear tyre to bring the tyre pressures closer together.

I take no responsibility for any outcomes of such experiments by any readers. Read the bit again where I say, "I wouldn't dream of doing it."

[Andre Jute]

So , just to clarify , you recommended keeping both front and back at the same pressure as it is safer and gives better handling ?

It's more subtle than that.

For a start, if you inflate to the same pressure front and rear, in motion with the rider aboard, the rear tyre is more heavily pressured by about 50% on average because the rider's weight is not evenly distributed over the front and rear tyres. This does no harm at normal bicycle speeds*, but it is another reason not to mess around with pressures because in effect any extra pressure a cyclist puts in at the back is on top of a 50% differential** already. Talk of the last straw on the camel's back! You can study this by noting the rear tyre's pressure with you off the bike, and getting someone to take it with you sitting on the bike. Do the same for the front tyre and you'll see its pressure increases too, but not by as much as the rear.

With additional load on the bike, say for touring, you might make a deliberate differential in pressure from front to rear to account for the luggage distribution. A method that has some currency in cycling circles is a pressure that shows a 15% of tyre height drop between the rim and floor. There's a long thread, or several, on the forum if you want to go looking for them, in which this 15% is discussed in depth.

*But it likely accounts for a many of the pileups in bike racing at professional speeds on ultra-short wheelbases and thin tyres, because the short wheelbase and ultralight frame means that a greater proportion of the rider's weight is on the rear wheel, all of these factors tending to oversteer. Check the photo on the cover of this http://coolmainpress.com/BICYCLETourofIrelandwithLance&Andre.pdf -- the rider is braking hard as he comes up to a T-junction, but instead of doing the easy thing, sitting up to let the air slow him down, he's leaning forward to put more weight on the front wheel to give it traction so the rear wheel doesn't overtake it and he washes out -- on this day literally. Check the tension in his leg muscles when he's not even pedalling; he knows that skinky little bike wants to go the wrong way, left, down-camber, because the oversteering rear wheel multiplies any turning force the road or the rider puts into the front wheel.

**Roadholding/handling response isn't linear, nor is it proportionate. In a textbook I wrote about 34 years ago, DESIGNING AND BUILDING SPECIAL CARS, there were three small illustrations of two-dimensional graphs without any numbers, just these irregular S-shapes to demonstrate that elements in automobile suspension don't work in ways lending themselves to rational mathematical formulation. Bill Towns, the designer of several iconic Aston Martins, used to call them "the tears of hysteresis".

[leftpoole]

I think that on this particular Forum it is 'talk' about Touring bikes.
The above direction seems to me to be directly 'learned' or discerned from a very technical version of a top class track racers .
Tyres should be pumped up hard-to first of all help prevent punc****s and secondly to help the rider inasmuch that a harder tyre = a quicker roll of the tyres/wheels, loaded or not. Unless cycling across the beach!

Please......

[ourclarioncall]

Thanks Andre, intriguing stuff !

[B cereus]

It seems to me that tyre pressures will always reflect the weight that the tyre supports. The main reason that I run a greater pressure in the rear tyre is to reflect the extra weight on the rear wheel. The downside of course, apart from any handling problems, is a decrease in comfort. An alternative approach that I have also used is to run a fatter tyre on the rear, but run at roughly the same pressure as the front tyre.

How best to determine the optimum pressure?  Well certainly hard enough to avoid snakebite punctures, beyond that I think it's down to personal preferences and individual circumstances.

I must admit that I do base my tyre pressures on Frank Berto's  recommendations for a 15% tyre drop. For me it represents the best compromise between comfort and rolling resistance. I used to have a link to a pdf of Berto's original paper in Bicycle Quarterly magazine but here's a link to an article that reproduces his original graphs.

https://www.renehersecycles.com/tire-pressure-take-home/


And here's another link to one of a number of on line calculators that use Berto's original calculations.

https://roubert.name/joakim/pressure/


Incidentally Berto allows for an assumption for the weight distribution between the front and rear wheel, and the general consensus is that its somewhere between 40/60% and 45/55%. When I first adopted this method of determining tyre pressure I verified these figures using bathroom scales.

[PH]

It seems to me that tyre pressures will always reflect the weight that the tyre supports. The main reason that I run a greater pressure in the rear tyre is to reflect the extra weight on the rear wheel.

I've just posted some thoughts with crossover on the other thread, I didn't know there were two running, I'm easily confused these days.
One point I made is that weight distribution is fluid - uphill, downhill, accelerating, breaking, cruising, pushing hard...

[Andre Jute]

I must admit that I do base my tyre pressures on Frank Berto's  recommendations for a 15% tyre drop. For me it represents the best compromise between comfort and rolling resistance. I used to have a link to a pdf of Berto's original paper in Bicycle Quarterly magazine but here's a link to an article that reproduces his original graphs.

https://www.renehersecycles.com/tire-pressure-take-home/


And here's another link to one of a number of on line calculators that use Berto's original calculations.

https://roubert.name/joakim/pressure/

Grand stuff, B cereus. Thanks for the links.

Perverse cycling myths, some of them as old as the Dunlop balloon, would be a rich field for a sociologist trying to make a name for himself. He could start with the apparently logical but altogether wrongheaded idea that hard-inflated tyres roll faster and more efficiently.

PH, add to your list of variants to allow for the fact that because a bicycle is dynamic, it generates its own loads, of which not the least is the heat of friction expanding the air in the tube, so increasing the pressure.

***

Some, like B cereus and Dan, will give the time to arrange tyre pressure scientifically. For the rest of us the key thing is to choose tyres with very flexible sidewalls, then to pump them up  to moderate pressure -- 15% rim drop takes automatic account of the weight distribution, but the same pressure front and rear as long as you choose a safe number above pinch flats is good enough for government work -- and if you're uncomfortably aware of road irregularities to let out some air in small steps.

[Andre Jute]

While here I was limiting myself to a tiny corner of a very complicated, multifaceted subject (altering the handling of your bike without tools except a pump), if you're willing to spend the money and do some spannering, and you have the right mates to help you think the whole business through (you'll find them here), you can tune the bike's handling by simply changing the fork. Dan gave some examples nine or ten years ago when he made experiments on his Sherpa, and with his usual thoroughness condensed his experience into an inspiring post. Start here and follow the link in red at the bottom of the page for more:
http://thorncyclesforum.co.uk/index.php?topic=4245.msg19567#msg19567

[B cereus]

Andre

The relationship between pressure and rolling resistance is fairly widely accepted but the best we can do to demonstrate it is on a laboratory test rig. All other things being equal, increasing pressure does reduce rolling resistance. The 15% figure is not arbitrary but was chosen by Berto because increasing pressure beyond this point gains very little in reduced rolling resistance. 

In the real world of course things are not as simple as this, it's much more difficult to both measure and predict rolling resistance, and a tyre at lower pressure may well roll over road surface irregularities better than the same tyre at higher pressure.

Juggling tyre pressure to optimise the many variables becomes something of a black art. My  view is that Berto's method at least provides a consistently reproducible reference point from which to  begin ones own experiments.

A welcome development in recent years has been the greater availability of lightweight supple walled tyres  in widths greater than 28mm. In theory such tyres might be expected to roll less well than an equivalent narrow tyre run at a higher pressure. In practice the difference is at best minimal and the benefit of the extra comfort of the wider tyre is more than worth it.

[JohnR]

We shouldn't overlook the Thorn Bible http://www.sjscycles.com/thornpdf/thorn_mega_brochure.pdf as it sets out the thinking behind the Thorn bikes. See, for example, pages 8 and 30.

I feel that Thorn got my Mercury just right with easy predictable handling throughout the speed range when there's me plus a bag on the rack. I've not tried carrying a heavy load and it's very unlikely that I will try using some panniers on the front.

[PH]

I feel that Thorn got my Mercury just right with easy predictable handling throughout the speed range when there's me plus a bag on the rack. I've not tried carrying a heavy load and it's very unlikely that I will try using some panniers on the front.

I also have a Mercury, I also think it handles wonderfully. 
But the wheel size and tyres on my Mercury may be very different to another, which seems to blow the theorizing out the water.  The ride experience will be considerably different, more than you can fine tune out, yet if we were to swap bikes for ten miles, we'd both prefer our own!  (I've swapped bikes on club rides to have a go on others and both parties always prefer their own) As I said in the sister thread, the science only plays a small part, and to repeat the advice, best thing to do is play with it and find what you like.  But good luck to anyone who can work out why they like it! Or describe it - sharp, sweet, twitchy, neutral, fast.. it's all subjective and one man's sharp will be another man's twitchy.
I don't understand it, I make no claim to, my scepticism isn't that the science is wrong, but that it's often irrelevant. Too often we work backwards - starting with the conclusion. IMO learning what you like is way more important than trying to understand why you like it.