Road racing setups versus Dirt

Joined
Aug 7, 2003
Messages
22
Likes
0
#1
Hi all
I heard a couple of guy's talking the other day about harshness on the track. Well to make the story shorter, I thought they were talking MX but they were talking about road racing, I was completey amazed by this,( cause I only ride dirt) being these guy's are riding on pavement. ( no breaking bumps ect.)
Is there simlarities in Road and MX setups, If so, what are they? or how are they different?

Bob
 

svi

Member
Joined
Dec 7, 2000
Messages
126
Likes
0
#2
The damping requirements are pretty much the same, the bumps are smaller but you are hitting them at a much higher speed. The damping also plays a more significant part in controlling the geometry of the bike.
Springs are at least twice the rate of off road springs and set up with a lot less sag.
On the bigger bikes chassis and fork flex play a significant part in the handling of a bike.
Good tyres are essential to good suspension, you can easily knock half a second off a lap just by fitting fresh rubber, the handling of a bike can change considerably going that half a second quicker so to get good consistant test results you've got to keep fitting new tyres, a front is normally good for an hour or so but rears tend to go off after half an hour depending on the track and tyre compound. Very Expensive!!!!!!!!
The good thing is that the track is always consistant, you don't start the day with a nicely graded track and finish up with a narly rutted one, the bad thing is it's not normally possible to build a track on your own land to go testing on.
 
Joined
Nov 8, 2003
Messages
200
Likes
0
#3
Well put SVI
The dynamics involved in roadracing are not as apparent to the bystander as they observe a rider in action. The wheels don't tend to leave the ground when they impact a bump. Loss of traction is not so obvious unless its a "heart in mouth" moment and G forces play a huge part. The bikes are set up on the firm side to assist with cornering stability, braking and accelleration. But, set them up too firm and traction begins to suffer. If you are not working with data aquisition equipment then rider feedback becomes paramount. Unfortunately tyres and geometry also play such a large part in the handling charachteristics. Yet because the suspension is the main tuning variable, riders tend to blame that first which can hamper your fault finding efforts.Some riders prefer a bit more feel than others so they know what's going on with their bike and it's relationship with the track surface. As SVI will tell you , it's a whole new world dealing with road bikes. There are subtleties, intracacies and nuances that involve a greater level of understanding and finesse. The rider/tuner relationship is a lot closer in roadracing. Mistakes are just too costly.
 
Joined
Aug 7, 2003
Messages
22
Likes
0
#4
Thanks Guy's

So in terms of dampaning requirements, we would need more LS dampaning (control) than high.
I also see these bikes have pre-load adjusters. How does this effect the handling of the bike or is this just rider preference
 

svi

Member
Joined
Dec 7, 2000
Messages
126
Likes
0
#5
Preload can be added to the forks to prevent bottoming, adding oil can often make the forks harsh in the bottom of the stroke, or it can be taken off to use more stroke.
Preload can also be used to control the pitching of the bike under acceleration or braking.
Preload on the rear is an important adjustment in the search for grip.
 

Jeremy Wilkey

Owner, MX-Tech
Joined
Jan 28, 2000
Messages
1,453
Likes
0
#6
Interesting topic and one that has some interesting observations and comments to be made about how it relates to the other topics..

As many have pointed out it the whole topic is very different. In off-road we worry about bumps and jumps, in Road road we have to get our head around that.. Its about Chassis and geometry before anything else..

We had the whole debate about piston design on the other string the other day. If you look at the leaders in Road Race suspension (IMO) Ohlins they are using incredibly small orficing and preloaded midvalves.. While this would be suicide on offroad the concept lends itself to the environment of road racing where maintaining geometry is so important. As deflection begins the orficing is so small that the shims don't regulate area and then you get a very digressive characteristic. Honda tried this with its Showa forks and shocks.. I don't think its as effective as the Ohlins stuff but clearly this has been the concept behind the designs.. Only as would be expected the result and extent of development is compromised in the Showa as a production system.

BR,
Jer
 
Joined
Nov 8, 2003
Messages
200
Likes
0
#7
Jeremy
You make mention of the Ohlins pistons having such tiny holes which, according to your previous argument should mean they have a relatively light shimstack. Particularly if the shims themselves aren't regulating the port area. But in fact the opposite is true and Ohlins employ some rather burly shimstacks to back up those restrictive pistons. The fact is Ohlins direct a greater amount of fluid through the adjuster circuit than most other manufacturers leaving a smaller amount to be regulated by the valve itself. Showa have tried the small port pistons with limited success and I believe it is the smaller amount of fluid that is bypassed that makes the difference. You have a greater amount of external adjustability with an Ohlins fork than almost any other. The level of adjustability from a Showa fork could best be described as fine tuning. But, replace that piston and accurately revalve to the riders requirements and fine tuning may be all you need.
Terry
 

svi

Member
Joined
Dec 7, 2000
Messages
126
Likes
0
#8
Jer,
I don't understand your thoughts on Ohlins pistons being digressive. I have always found them to be exactly the opposite being very linear with a very steep increase in damping force. They often lock up over bumps hard on the brakes, speak to any rider using Ohlins forks and their biggest complaint will always be chatter on the brakes.
I think the vast majority of people have now gone away from the preload midvalve as it makes the forks very harsh, until the pressure in the cartridge is sufficent to lift the valve. From feedback I have had from riders it would appear that although the compression bleed and piston give a rapid rise in cartridge pressure it seems you still have to compress the whole charge further to get the valve to lift which gives you a similar sensation to a very high oil level in a bottomed out set of forks.
The forks become very snatchy, they are very rigid then suddenly they let go.
Terry, you're right about the bleed with the options that are available in different size bleed orifices and needle tapers.
 
Joined
Nov 8, 2003
Messages
200
Likes
0
#9
SVI
What exactly does your work entail. Obviously you have a road background. Are you involved at factory level or helping the ever struggling privateer? I appreciate your comments. They're clear and concise. Perhaps we can share ideas my email is racetech@pnc.com.au
Regards
Terry Hay
 

Jeremy Wilkey

Owner, MX-Tech
Joined
Jan 28, 2000
Messages
1,453
Likes
0
#11
Guys many interesting pieces here.. I don't claim to know a whole lot about street bikes, only I'm progressively learning through some very smart and capable road tuners.. So that being said I can only comment on some points.. Small orficing does not cause cavitations in and of itself in practice that I'm aware of.. Of coarse you could look at it in a more specific way and say that yes it could "if" we had a situation where the pressure in the cartridge was so much larger than the "force of air spring and rod pressure (Static measurement/ verse dynamic measurement) was less than the instant and dynamic cartridge side that if you had a net pressure drop enough across the valve you could get cavitations due to a reduction in that pressure. (a specific pressure/vapor pressure) type equation.. Very intriguing.. To further go along with your point cavitations always occurs in a suspension system, we know that from some papers SVI found on the SAE site, the issues with the "preconditioning" of shock fluid in vacuum systems, and some mysteries reviled to me in testing different piston materials...

So I guess this means that cavitations occur as a function of pressure drop across the valve shim boundary, and other variables such as fluid, and internal pressure not so much by the piston orficing itself.. Although radical designs could in theory add to the effect..


As for the shim Stacks, Why wouldn’t a road race bike have a burly shim stack? Secondly more oil flow through a clicker does allow for more adjustment, but what happens to the rest of the oil, and subsequent speed / force relationship?



BR,
Jer
 
Joined
Nov 8, 2003
Messages
200
Likes
0
#12
Jeremy
Obviously if I'm making a comment it is in context with the topic. The Ohlins shimstack I refer to, is in comparison to other roadrace setups.
As you will find their shimstacks on their dirtbike forks substantial compared to other dirt forks.
I have to disagree on the Ohlins leadership issue. I don't think the Showa guys would be prepared to accept that either. Ohlins make a nice product and make hop-up options easy for roadracers. But this is when you are looking for an alternative to stock suspension. Fortunately for Ohlins, the factory Showa gear has extremely limited accessability.
 

svi

Member
Joined
Dec 7, 2000
Messages
126
Likes
0
#13
Podia,
Theoretically small orifices could cause cavitation in forks, if you were to make the by-pass passages small enough in the pistons then you would get cavitation.
Some of the factory Ohlins specifications I have seen for midvalves must be on the verge cavitation, the bleed that occurs through the rebound needle on the compression stroke must just be sufficient to prevent it.
Rather than having a tuneable midvalve set up I think that Kayaba get the rebound piston to contribute to the overall damping coefficient by making the by-pass ports so small that they create damping at high speeds.
 
Joined
Aug 27, 2002
Messages
301
Likes
0
#14
SVI

What are some of the listings under the SAE site. From my understanding there is some good info there, but I have been unable to find it. Now I see that they have changed there site, so now it does not list the papers or articles . Any info that you can a assist with would be greatly appriciated.

thanks
Russ
 

svi

Member
Joined
Dec 7, 2000
Messages
126
Likes
0
#15
type in whatever topic you are looking for in the search box. If you type in motorcycle suspension, this is the list you get.
http://www.sae.org/servlets/SiteSearch?charset=iso-8859-1&ht=0&qp=&col=portal&qs=&sae_qt1=&qc=&pw=100%2525&ws=0&la=en&qm=0&st=1&nh=25&lk=1&rf=0&oq=&rq=0&si=1&ql=&jsp_name=simplesearch.jsp&qt=motorcycle+suspension.

To read anything you do have to buy it though.