Post by CD on Jun 13, 2008 12:15:25 GMT
I attended a totally fab lecture last night at the University of Stafford given to the local institute of mechanical engineers. This is my precis.
Damian Harty of Prodrive gave a lecture on motorcycle handling...
All bikes (including push bikes) steer by counter-steering. Other than very low speeds virtually nothing is done by steering towards the turn. As he put it we move the tyre contact patch under bike centre of gravity causing the bike to lean. He demonstrated balancing a broom handle on one finger. Gyro effect has almost no part in bike steering or keeping it straight.
Steering flutter is caused by the front tyre and wheel gyro effect constantly correcting the wheel direction. On ordinary bikes this happens at 8 to 10Hz causing the light flutter at about 40mph. Sports bikes are stiffer so they can get up to around 12Hz so they flutter at around 60. It’s harmless and simply holding the bars dampens it out. The gyro effect is slightly over-doing the correction so a heavier wheel would do it at lower speeds. Steering dampers will stop the flutter but...
Weave is where the frame and steering get into a standing wave at about 2 seconds frequency. This is caused partly by the bike acting like an articulated lorry (in small scoots the front wheel and fork can be 25% of the total vehicle weight). The other part is flexion in the frame and rear suspension. Weave can be very nasty and gets worse when the steering head bearings are sticking its also made worse with a steering damper. Weave usually sets in above 60mph. Acceleration lifts weight off the front wheel so combine that with steering flutter and a sports bike can more easily get into a tank slapper. Weave is the bike equivalent of an HGV trailer snake.
Weave however often feels worse than it really is. If the rider remains relaxed his body acts as a damper and keeps it reasonable. If the rider tenses up his damping effect is reduced so the weave gets worse. Putting cases on the back of the bike increase the pendulum effect and that makes the weave worse. So a sports bike with throw-overs worn head bearings and inexperienced rider - wobble - weave -tankslapper – crunch. Steering dampers fix the steering wobble, but make weave a lot worse.
Monoshocks it seems do not in themselves make a bike handle better. They do however need a braced swing arm to take the spring loads and this stiffens the swing arm. It’s more about fashion than handling performance. Divvies don’t have an especially braced swing arm and the spindle clamp is especially wimpish looking. They are also not especially braced about the swing arm bearings. This perhaps explains their weaving tendancy at 80+ with luggage. BTW all bikes weave less with a pillion. They increase the damping effect.
So extrapolating this, a Diversion with its modest frame technology (compared to a sports bike) will be sensitive to worn steering head bearings and swing arm bearings. It won’t be so fussy about the suspension linkage needle rollers as they are not part of the frame stiffness.
Forks. Telescopics are an engineering nightmare but have been developed to death and are cheap and good enough. Hossack type designs are undoubtedly better but have not had the development so cost more to make. With telescopic forks the tyre will grip until the rear wheel lifts. It’s this weight transfer (and over steering as the back tyre starts to slip) that tells us we are pushing too hard into a corner. A Hossack is anti-dive and does not lift the rear wheel on the brakes. It’s therefore possible to lose tyre traction before you realise you’re at the limit. BMW solved this with ABS. Prodrive have solved tele fork dive with an active ride height system that lifts the bike on the brakes. Either way technology is used to get around the system flaws. Damian Harty agreed that the Hossack used on the new BMW K1200 is a brilliant piece of kit and far superior to a tele setup. But look at the development costs. He didn’t mention that if Normal Hossack had been involved BMW could have avoided repeating the mistakes he made in the 1970s.
Last of all Damian Harty talked about the transport ideas Prodrive have looked at. Small light cars will save fuel. But the view up the road is restricted and the occupants feel vulnerable. The Sinclair C5 is an extreme example. The VW 1 litre (1 liter per 100Km) is amazing on fuel and road space, but very low and small. People like MPVs and 4x4 because they are tall and give a commanding view. Small cars can be made taller, but they are more likely to fall over in a sharp turn. The Smart is no worse than any other “city” car.
Ok, to get more vehicles on the same road width, we want tall, reasonable length, but narrow - sounds pretty much like a bike. Prodrive have a prototype car that has 4 wheels but leans like a bike. Right now its hard to drive because you have to adjust from car steering to bike steering and most folk (even bikers) lose it at 15mph. However modern electronics can solve that. Another idea is to have a narrow chassis that works like a car chassis, but tilts the passenger body under computer control.
He got me thinking... Maybe the MP3 scooter is the way of things to come. It’s got 3 wheels so less likely to fall over and while the front end won’t dive like a tele fork the two tyres grip better solving the Hossack over-braking problem. Put 4 wheels on it and we have a quad that rides like a bike. So when 2 wheelers get banned by the government we simply add two more wheels, get better grip, solve the snaking problems and carry on as before. Yay!
Damian Harty of Prodrive gave a lecture on motorcycle handling...
All bikes (including push bikes) steer by counter-steering. Other than very low speeds virtually nothing is done by steering towards the turn. As he put it we move the tyre contact patch under bike centre of gravity causing the bike to lean. He demonstrated balancing a broom handle on one finger. Gyro effect has almost no part in bike steering or keeping it straight.
Steering flutter is caused by the front tyre and wheel gyro effect constantly correcting the wheel direction. On ordinary bikes this happens at 8 to 10Hz causing the light flutter at about 40mph. Sports bikes are stiffer so they can get up to around 12Hz so they flutter at around 60. It’s harmless and simply holding the bars dampens it out. The gyro effect is slightly over-doing the correction so a heavier wheel would do it at lower speeds. Steering dampers will stop the flutter but...
Weave is where the frame and steering get into a standing wave at about 2 seconds frequency. This is caused partly by the bike acting like an articulated lorry (in small scoots the front wheel and fork can be 25% of the total vehicle weight). The other part is flexion in the frame and rear suspension. Weave can be very nasty and gets worse when the steering head bearings are sticking its also made worse with a steering damper. Weave usually sets in above 60mph. Acceleration lifts weight off the front wheel so combine that with steering flutter and a sports bike can more easily get into a tank slapper. Weave is the bike equivalent of an HGV trailer snake.
Weave however often feels worse than it really is. If the rider remains relaxed his body acts as a damper and keeps it reasonable. If the rider tenses up his damping effect is reduced so the weave gets worse. Putting cases on the back of the bike increase the pendulum effect and that makes the weave worse. So a sports bike with throw-overs worn head bearings and inexperienced rider - wobble - weave -tankslapper – crunch. Steering dampers fix the steering wobble, but make weave a lot worse.
Monoshocks it seems do not in themselves make a bike handle better. They do however need a braced swing arm to take the spring loads and this stiffens the swing arm. It’s more about fashion than handling performance. Divvies don’t have an especially braced swing arm and the spindle clamp is especially wimpish looking. They are also not especially braced about the swing arm bearings. This perhaps explains their weaving tendancy at 80+ with luggage. BTW all bikes weave less with a pillion. They increase the damping effect.
So extrapolating this, a Diversion with its modest frame technology (compared to a sports bike) will be sensitive to worn steering head bearings and swing arm bearings. It won’t be so fussy about the suspension linkage needle rollers as they are not part of the frame stiffness.
Forks. Telescopics are an engineering nightmare but have been developed to death and are cheap and good enough. Hossack type designs are undoubtedly better but have not had the development so cost more to make. With telescopic forks the tyre will grip until the rear wheel lifts. It’s this weight transfer (and over steering as the back tyre starts to slip) that tells us we are pushing too hard into a corner. A Hossack is anti-dive and does not lift the rear wheel on the brakes. It’s therefore possible to lose tyre traction before you realise you’re at the limit. BMW solved this with ABS. Prodrive have solved tele fork dive with an active ride height system that lifts the bike on the brakes. Either way technology is used to get around the system flaws. Damian Harty agreed that the Hossack used on the new BMW K1200 is a brilliant piece of kit and far superior to a tele setup. But look at the development costs. He didn’t mention that if Normal Hossack had been involved BMW could have avoided repeating the mistakes he made in the 1970s.
Last of all Damian Harty talked about the transport ideas Prodrive have looked at. Small light cars will save fuel. But the view up the road is restricted and the occupants feel vulnerable. The Sinclair C5 is an extreme example. The VW 1 litre (1 liter per 100Km) is amazing on fuel and road space, but very low and small. People like MPVs and 4x4 because they are tall and give a commanding view. Small cars can be made taller, but they are more likely to fall over in a sharp turn. The Smart is no worse than any other “city” car.
Ok, to get more vehicles on the same road width, we want tall, reasonable length, but narrow - sounds pretty much like a bike. Prodrive have a prototype car that has 4 wheels but leans like a bike. Right now its hard to drive because you have to adjust from car steering to bike steering and most folk (even bikers) lose it at 15mph. However modern electronics can solve that. Another idea is to have a narrow chassis that works like a car chassis, but tilts the passenger body under computer control.
He got me thinking... Maybe the MP3 scooter is the way of things to come. It’s got 3 wheels so less likely to fall over and while the front end won’t dive like a tele fork the two tyres grip better solving the Hossack over-braking problem. Put 4 wheels on it and we have a quad that rides like a bike. So when 2 wheelers get banned by the government we simply add two more wheels, get better grip, solve the snaking problems and carry on as before. Yay!
