What are the correct tire pressures for a 1979 R100RT?

[Archie]

I have two questions relating to tire pressures:

1. What is the correct tire pressure I should use on my tires for riding solo with little to no luggage? My owner’s manual says 27lbs front and 29lbs rear, but Snobum says pump modern day tires a lot higher than that – 34 and 38. What is the correct tire pressure?

2. Do you think that having tire pressures of 34lbs in front and 38lbs in the rear, will raise the bike higher off the ground, hence explaining the problem I am having in getting my feet to touch the ground? Or has it got something to do with the shocks?

Here is some brief background.

I have a 1979 BMW R100RT. I recently put on new tires on the front and back. Prior to replacing the tires, I had a Metzler Marathon 100/90 – 19 on the front and a Metzler ME88 120/90 – 18 on the rear. I just replaced the tires with Shinko tires –100/90 -19 on the front and 120/90 – 18 on the rear, i.o.w. the same sizes as the Metzler’s front and back. I also replaced the rear shocks and springs with new Progressive Suspension shocks – model 12 that is 13.5 inches, the BMW OEM size.

Snobum says that we should pump up modern-day tires higher than what is stated in the BMW owner’s manual or what is posted under the seat. He says to use at least 34lbs in front and 36-38lbs in the rear. I followed his advice and inflated the tires to these amounts.

After I got the bike back from the bike shop with the new tires and shocks, the first thing I noticed was that the bike seems to be sitting higher up than before. Prior to the upgrades, when I would put my bike up on the mainstand, I was able to gently rock it back and forth on the stand. The rear wheel was on the ground and the front wheel was slightly off the ground. If I tipped the bike forward, the front wheel would be on the ground and the rear wheel would be off the ground by just a small amount. This allowed me to “swing” the bike around in my garage on the mainstand basically 180 degrees – that’s how balanced it was. I’m sure though that all BMW’s of that vintage are like this. But since the upgrades, I am unable to do this. The bike will no longer “rock” back and forth at all.

In addition, I am finding it a lot harder to get the bike on and off the mainstand, its harder to push it around in the garage for some reason, and most importantly, my feet are no longer touching the ground as easily as they did before – I now am having to tiptoe on the bike – only my toes touch the ground - which makes getting on and off the bike with the stock side-stand kind of precarious!

I look forward to the advice of the experienced members on this forum!Thank you in advance for your replies to this post!

[Deleted User 62]

Is it possible the rear shocks are adjusted to the two up riding position? Worth a check.

[SteveD]

Go the higher pressures.
Yes it will feel slightly higher in the seat.

[Airbear]

In addition, I am finding it a lot harder to get the bike on and off the mainstand, its harder to push it around in the garage for some reason ...

This has me puzzled. I wonder if there is more drag because something is wrong. I'd put the bike on the centre stand on a thin block of wood and try turning the wheels. The rear wheel will feel heavy to turn - there is a lot of heavy metal to rotate in the drive-train. The front wheel may also have a little drag from the disk pads - this is normal. But, if there is excessive drag, there may be some other problem. There are horror stories about tyre changing people tightening axle nuts to extreme torques and damaging wheel bearings. It's worth checking.

[sprints@pldi.net]

I ignore the BMW recommendations and go with the recommendation molded on the tire itself. If these modern tires were available when your bike was built years ago then BMW would have recommended higher pressure also - tire technology changed over time - Rod

[Garnet]

True tire sizes vary a lot from One tire manufacture to the next. Diameters can change more than 10mm and still have the same sizes marked on the size.

90/90-19 and 110/90-18 are usually closer to 3.25-19 and 4.00-18, but not always.

The springs on you old shock where probably sagged from age and now with new springs you are probably closer to correct ride height. You should check your sag to make sure that you have the correct springs. For the street you are looking for 25 to 33% of spring travel with rider.

[khittner1]

Your new rear shocks aren't as sacked as the ones you replaced, so it's no surprise that it's standing a bit taller. This sort of transition is a regular feature on my bike; I can stand flat-footed on my RT when I first get on it, but after the Nivomats pump up, I'm on my tiptoes.

[Garnet]

As for tire pressure, it is up to you. First of all it must be safe. Start with recommended pressures and go for a long enough ride to heat up the tires. Check the tires hot. If the pressure has risen more than about 3 lbs, add a couple of PSI and repeat. This will get you into the minimum safe pressures for your tire size and load. Then you need to adjust for handling. This is more subjective and you should do some more reading on the subject.

[Roy Gavin]

Forget the figures on the sidewall, they are maximum pressures, not the recommended running pressure.

The best pressures for you depend on the riding you intend. For an interstate run on paved free-ways 34/38 might feel good, but it probably wont if you intend a tootle around back lanes on bumpy spray seal - remember that tires are part of the suspension system on our old low tech bikes.

I would start with the old pressures and do a simple finger tip temperature test on the tire after a run, you dont want, or need much,if any, change in temp on a DOT approved road tire.

If the tires feel hot to touch after a run just increase the pressure a couple of pounds at a time until they just start to warm a little.

Race tires need need much higher temps, of course, and are cooked in tire warmers before use, but fitting tyre warmers at traffic light stops isn't really practical for most folks, which is why heat sensitive race tires are not approved for road use.

[barryh]

Do Shinko recommend pressures ? If they do they will be higher than the old original figures.

Tire pressure would make only a very small difference to ride height but the tire dimensions could be a several mm different even though they are nominally the same size. If you have changed worn tires for new there could also be up to 6mm increase in height.

Your ride height and ease of centre stand issues seem to be in conflict. When the ride height is increased the centre stand usually becomes easier to use not harder and except for differences in rear shock weight increasing the ride height shouldn't have affected the balance. Unless the springs are identical in rate and pre-load, shocks can be the same length but have different ride height. In fact changing the make of shock is more likely than not to change the ride height at least a little.

Maybe your increased ride height is a cumulative effect from new tires and shocks.